Physic Chap 2 Note

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Linear Motion

Linear motion is the motion in 1 dimension (1-D) or the motion in a straight line.

Distance

1) The distance traveled by an object is the total length that is traveled by thatobject.

2) Distance is a scalar quantity.

3) The SI unit of distance is m (metre).

Displacement

1) Displacement of an object from a point of reference, O is the shortest distanceof the object from point O in a specific direction.

2) Displacement is a vector quantity.

3) The SI unit of displacement is m (metre).

Distance vs Displacement

Distance travelled = 200m

Displacement = 120 m, in the direction of Northeast

Speed

1. Speed is the rate of change in distance. It is a measure of how fast the distance

change in a movement.

2. Speed is a scalar quantity.3. The SI unit of speed is m/s (metre per second)
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Equation of Speed

Velocity

1. Velocity is define as the rate of displacement change. It is the measure of how

fast the displacement change of a moving object.2. Velocity is a vector quantity.

3. The unit of displacemnet is m/s (metre per second)
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Equation of velocity

Positive or Negative Sign of Velocity

1. In velocity, the positive/negative sign indicates direction.

2. You can take any direction as positive and the opposite as negative.

3. For a linear motion, normally we take the motion to the right as positive andhence the motion to the left as negative.

Acceleration

Acceleration is the rate of velocity change.Acceleration is a vector quantity. It is ameasure of how fast the velocity change.

Acceleration is a vector quantity.

The unit of acceleration is ms-2.
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Equation

Additional Notes

An object moves with a constant velocity if the magnitude and direction of the motion is

always constant.An object experiences changes in velocity if

the magnitude of velocity changes

the direction of the motion changes.

An object that experiences changes in velocity is said to have acceleration.

An object traveling with a constant acceleration, a, if the velocity changes at a constantrate.

Equation of Uniform Acceleration

Most of the motion problems can be solved by the following equations. Therefore, make

sure that you memorise all of them.
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How we know when to use the equation?

There are 3 types of motion:

motion with uniform velocity

motion with uniform acceleration

motion with changing acceleration

The 4 equations are used when the motion is uniform acceleration.

Motion with changing acceleration is not in SPM Physics syllabus. It will be discussed in

Form 5 add maths.

Ticker Timer

A ticker-timer consists of an electrical vibrator which vibrates 50 times per second. This

enables it to make 50 dots per second on a ticker-tape being pulled through it. The timeinterval between two adjacent dots on the ticker-tape is called one tick. One tick is equal

to 1/50 s or 0.02 s.
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Uniform Velocity

Thedistance of the dots is equally distributed.

All lengths of tape in the chart are of equal length.

The object is moving at a uniform velocity.

Uniform Acceleration

The

distance between the dots increases uniformly.

The length of the strips of tape in the chart increase uniformly.

The velocity of the object is increasing uniformly, i.e. the object is moving at a

constant acceleration.
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Uniform Deceleration

The

distance between the dots decreases uniformly.

The length of the strips of tape in the chart decreases uniformly.

The velocity of the object is decreasing uniformly, i.e. the object is decelerating

uniformly.

Finding Velocity

Velocity of a motion can be determined by using ticker tape through the following

equation:

Caution!:

t is time taken from the first dot to the last dot of the distance measured.

Example 1

Diagram 2.4 shows a strip of ticker tape that was pulled through a ticker tape timer that

vibrated at 50 times a second. What is the

a. time taken from the first dot to the last dot?

b. average velocity of the object that is represented by the ticker tape?
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Answer:

a. There are 15 ticks from the first dot to the last dot, hence

Time taken = 15 0.02s = 0.3sb. Distance travelled = 15cm

Example 1

The ticker-tape in figure above was produced by a toy car moving down a tilted runway.

If the ticker-tape timer produced 50 dots per second, find the acceleration of the toy car.

Answer:

In order to find the acceleration, we need to determine the initial velocity, the final

velocity and the time taken for the velocity change.Initial velocity,

u = \frac{s}{t} = \frac{{3cm}}

{{0.02s}} = 150cms^{ - 1}

Time taken for the velocity change,t = (0.5 + 4 + 0.5) ticks = 5 ticks

t = 5 0.02s = 0.1s

Example 2

A trolley is pushed up a slope. Diagram above shows ticker tape chart that show the

movement of the trolley. Every section of the tape contains 5 ticks. If the ticker-tape

timer produced 50 dots per second, determine the acceleration of the trolley.
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Answer:

In order to find the acceleration, we need to determine the initial velocity, the final

velocity and the time taken for the velocity change.Initial velocity,

Time taken for the velocity change,

t = (2.5 + 5 + 5 + 5 + 2.5) ticks = 40 ticks

t = 40 0.02s = 0.8s

Displacement - Time Graph

In a Displacement-Time Graph, the gradient of the graph is equal to the velocity ofmotion.

Analysing Displacement - Time Graph

Gradient = 0Hence, velocity = 0
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Gradient is constant,

hence, velocity is Uniform

Gradient is negative and constant, hence

velocity is uniform and in opposite direction

Gradient is increasing, hence velocity is

increasing.

Gradient is decreasing, hence velocity is

decreasing.
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Velocity - Time Graph

The gradient of the velocity-time gradient gives a value of the changing rate invelocity, which is the acceleration of the object.

The area below the velocity-time graph gives a value of the object's displacement.

Analysing Velocity-Time Graph

Uniform velocity

Uniform acceleration
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Increasing acceleration

Uniform deceleration

Decreasing acceleration

Converting a Velocity-Time graph to Acceleration-Time graph

In order to convert a velocity-time graph to acceleration time graph, we need to find the

gradient of the velocity time graph and plot it in the acceleration-time graph.Free fallingis a motion under gravitational force as the only force acting on the moving object. In

SPM, you need to know the graphs of free falling of the following movement

1. Launching object upward.2. Dropping Object from High Place

3. Object Falling and Bounce Back
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Launching Object Upward

Velocity-Time Graph Acceleration-Time Graph

Dropping Object from High Place
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Object Falling and Bounce Back

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Mass

Mass is defined as the amount of matter. The SI unit of mass is kilogram (kg)

Mass is a scalar quantity.

InertiaInertia is the property of a body that tends to maintain its state of motion.

Newton's First Law

In the absence of external forces, an object at rest remains at rest and an object in motion

continues in motion with a constant velocity (that is, with a constant speed in a straightline).

Jerking a Card

When the cardboard is jerked quickly, the coin will fall into the glass.

Explanation:

The inertia of the coin resists the change of its initial state, which is stationary.

As a result, the coin does not move with the cardboard and falls into the glassbecause of gravity.
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Pulling a Book

When the book is pulled out, the books on top will fall downwards.

Explanation:

Inertia tries to oppose the change to the stationary situation, that is, when the book

is pulled out, the books on top do not follow suit.

Pulling a Thread

1. Pull slowly - Thread A will snap.

Explanation:

Tension of thread A is higher than string B.

Tension at A = Weight of the load + Pulling Force

2. Yank quickly - Thread B will snap.

Explanation:

The inertia of the load prevents the force from being transmitted to thread A,hence causing thread B to snap.

Larger Mass - Greater Inertia
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Bucket filled with sand is more difficult to be moved. It's also more difficult to be

stopped from swinging.

Explanation:

Object with more mass offers a greater resistance to change from its state of

motion.

Object with larger mass has larger inertia to resist the attempt to change the stateof motion.

Empty cart is easier to be moved

An empty cart is easier to be moved compare with a cart full with load. This is because acart with larger mass has larger inertia to resist the attempt to change the state of motion.

Momentum

Momentum is defined as the product of mass and velocity.Momentum is a vector quantity. It has both magnitude and direction.

The SI unit of momentum is kgms-1
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Formula:

Example 1

A student releases a ball with mass of 2 kg from a height of 5 m from the ground. What

would be the momentum of the ball just before it hits the ground?

Answer:

In order to find the momentum, we need to know the mass and the velocity of the ball

right before it hits the ground.

It's given that the mass, m = 2kg.The velocity is not given directly. However, we can determine the velocity, v, by using

the linear equation of uniform acceleration.

This is a free falling motion,

The initial velocity, u = 0The acceleration, a = gravirational acceleration, g = 10ms-2

The dispacement, s = high = 50m.

The final velocity = ?From the equation

v2 = u2 + 2as

v2 = (0)2 + 2(10)(5)

v = 10ms

-1

The momentum,

p = mv =(2)(10) = 20 kgms-1

Principle of Conservation of Momentum

The principle of conservation of momentum states that in a system make out of objects

that react (collide or explode), the total momentum is constant if no external force is

acted upon the system.Sum of Momentum Before Reaction

= Sum of Momentum After Reaction
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Formula

Example 1 - Both Object are in the Same Direction before Collision

A Car A of mass 600 kg moving at 40 ms-1 collides with a car B of mass 800 kg moving

at 20 ms-1 in the same direction. If car B moves forwards at 30 ms-1 by the impact, what is

the velocity, v, of the car A immediately after the crash?

Answer:m1 = 600kg

m2 = 800kgu1 = 40 ms

-1

u2 = 20 ms-1

v1 = ?v2 = 30 ms

-1

According to the principle of conservation of momentum,

m1u1 + m2u2 = m1v1 + m2v2(600)(40) + (800)(20) = (600)v1 + (800)(30)40000 = 600v1 + 24000

600v1 = 16000v1 = 26.67 ms-1

Example 2 - Both Object are in opposite direction Before Collision

A 0.50kg ball traveling at 6.0 ms-1 collides head-on with a 1.0 kg ball moving in theopposite direction at a speed of 12.0 ms-1. The 0.50kg ball moves backward at 14.0 ms-1

after the collision. Find the velocity of the second ball after collision.
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Answer:

m1 = 0.5 kg

m2 = 1.0 kgu1 = 6.0 ms

-1

u2 = -12.0 ms-1

v1 = -14.0 ms-1

v2 = ?

(IMPORTANT: velocity is negative when the object move in opposite direction)

According to the principle of conservation of momentum,m1u1 + m2u2 = m1v1 + m2v2(0.5)(6) + (1.0)(-12) = (0.5)(-14) + (1.0)v2-9 = - 7 + 1v2v2 = -2 ms

-1

Explosion

Before explosion both object stick togetherand at rest.

After collision, both object move at oppositedirection.

Total Momentum before collision Is zero Total Momentum after collision :

m1v1 + m2v2

From the law of conservation of momentum:

Total Momentum Before collision = Total Momentum after collision

0 = m1v1 + m2v2

m1v1 = - m2v2

(-ve sign means opposite direction)

Examples or Application of Conservation of Momentum in Explosion

1. Fire a pistol or rifle2. Launching a rocket

3. Application in jet engine

4. Fan boat

Example 3

A man fires a rifle which has mass of 2.5 kg. If the mass of the bullet is 10 g and it

reaches a velocity of 250 m/s after shooting, what is the recoil velocity of the pistol?


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Answer

This is a typical question of explosion.

m1 = 2.5 kgm2 = 0.01 kg

u1 = 0 ms-1

u2 = 0 ms-1

v1 = ?

v2 = 250 ms-1

By using the equation of conservation of momentum principle0 = m1v1 + m2v20 = (2.5)v1 + (0.01)(250)

(2.5)v1 = -2.5v1 = -1 ms-1

Elastic Collision

Elastic collision is the collision where the kinetic energy is conserved after the collision.

Total Kinetic Energy before Collision= Total Kinetic Energy after Collision

Additional notes:

In an elastic collision, the 2 objects separated right after the collision, and

the momentum is conserved after the collision.

Total energy is conserved after the collision.

Inelastic Collision

Inelastic collision is the collision where the kinetic energy is not conserved after the

collision.Additional notes:

In a perfectly elastic collision, the 2 objects attach together after the collision,

and

the momentum is also conserved after the collision.

Total energy is conserved after the collision.

Example 1 - Perfectly Inelastic Collision

A lorry of mass 8000kg is moving with a velocity of 30 ms-1. The lorry is thenaccidentally collides with a car of mass 1500kg moving in the same direction with a

velocity of 20 ms-1. After the collision, both the vehicles attach together and move with a

speed of velocity v. Find the value of v.

Answer:

(IMPORTANT: When 2 object attach together, they move with same speed.)m1 = 8000kg

m2 = 1500kg

u1 = 30 ms-1


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u2 = 20 ms-1

v1 = v

v2 = vAccording to the principle of conservation of momentum,

m1u1 + m2u2 = m1v1 + m2v2

(8,000)(30) + (1,500)(20) = (8,000)v+ (1,500)v270,000 = 9500v

v = 28.42 ms-1

Rocket

1. 1Mixture of hydrogen and oxygen fuels burn in the combustion chamber.2. Hot gases are expelled through the exhausts at very high speed .

3. The high-speed hot gas produce a high momentum backwards.

4. By conservation of momentum, an equal and opposite momentum is produced and

acted on the rocket, pushing the rocket upwards.

Jet Engine

1. Air is taken in from the front and is compressed by the compressor.

2. Fuel is injected and burnt with the compressed air in the combustion chamber.

3. The hot gas is forced through the engine to turn the turbine blade, which turns thecompressor.

4. High-speed hot gases are ejected from the back with high momentum.

5. This produces an equal and opposite momentum to push the jet plane forward.

Force

1. A force is push or pull exerted on an object.2. Force is a vector quantity that has magnitude and direction.

3. The unit of force is Newton ( or kgms-2).

Unbalanced Force/ Resultant Force

When the forces acting on an object are not balanced, there must be a net force acting on

it. The net force is known as the unbalanced force or the resultant force.

When a force acts on an object, the effect can change the

1. size,

2. shape,3. stationary state,

4. speed and

5. direction of the object.

Newton's Second Law

The rate of change of momentum of a body is directly proportional to the resultant force

acting on the body and is in the same direction.


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Implication:

When there is resultant force acting on an object, the object will accelerate (moving

faster, moving slower or change direction).

Formula of Force

From Newton's Second Law, we can derived the equation

(IMPORTANT: F Must be the net force)

Summary of Newton's 1st Law and 2nd Law

Newton's First Law:

When there is no net force acting on an object, the object is either stationary or move

with constant speed in a straight line.

Newton's Second Law:

When there is a net force acting on an object, the object will accelerate.

Example 1

A box of mass 150kg is placed on a horizontal floor with a smooth surface; find theacceleration of the box when a 300N force is acting on the box horizontally.

Answer:

F = ma

(300) = (150)aa = 2 ms-2

Example 2

A object of mass 50kg is placed on a horizontal floor with a smooth surface. If thevelocity of the object changes from stationary to 25.0 m/s in 5 seconds when is acted by a

force, find the magnitude of the force that is acting?Answer:We know that we can find the magnitude of a force by using the formula F = ma. The

mass m is already given in the question, but the acceleration is not give directly.

We can determine the acceleration from the formula .From the formulaF = ma = (50)(5) = 250N

The force acting on the box is 250N.
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Impulse

Impulse is defined as the product of the force (F) acting on an object and the time of

action (t). Impulse exerted on an object is equal to the momentum change of the object.Impulse is a vector quantity.

Formula of impulseImpulse is the product of force and time.

Impulse = F t

Impulse = momentum changeImpulse = mv mu

Impulsive Force

Impulsive force is defined as the rate of change of momentum in a reaction.

It is a force which acts on an object for a very short interval during a collision orexplosion.

Effects of impulse vs Force

A force determines the acceleration (rate of velocity change) of an object. A

greater force produces a higher acceleration. An impulse determines the velocity change of an object. A greater impulse yield a

higher velocity change.

Examples Involving Impulsive Force

Playing football

Playing badminton Playing tennis

Playing golf

Playing baseball

Long Jump
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1. The long jump pit is filled with sand to increase the reaction time when atlete land

on it.

2. This is to reduce the impulsive force acts on the leg of the atlete becauseimpulsive force is inversely proportional to the reaction time.

High Jump

(This image is licenced under the GNU Free Document Licence. The original file is from

the Wikipedia.org.)

During a high jump, a high jumper will land on a thick, soft mattress after the

jump.

This is to increase the reaction time and hence reduces the impulsive force acting

on the high jumper.

JumpingA jumper bends his/her leg during landing. This is to increase the reaction time and hencereduce the impact of impulsive force acting on the leg of the jumper.

Crumble Zone

The crumple zone increases the reaction time of collision during an accident.

This causes the impulsive force to be reduced and hence reduces the risk of injuries.
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Seat Belt

Prevent the driver and passengers from being flung forward or thrown out of the carduring an emergency break.

Airbag

The inflated airbag during an accident acts as a cushion to lessen the impact when the

driver flings forward hitting the steering wheel or dashboard.

Head Rest

Reduce neck injury when driver and passengers are thrown backwards when the car is

banged from backward.

Windscreen

Shatter-proof glass is used so that it will not break into small pieces when broken. This

may reduce injuries caused by scattered glass.

Padded Dashboard

Cover with soft material. This may increases the reaction time and hence reduce the

impulsive force when passenger knocking on it in accident.

Collapsible Steering Columns

The steering will swing away from drivers chest during collision. This may reduce the
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impulsive force acting on the driver.

Anti-lock Braking System (ABS)

Prevent the wheels from locking when brake applied suddenly by adjusting the pressure

of the brake fluid. This can prevents the car from skidding.

Bumper

Made of elastic material so that it can increases the reaction time and hence reduces the

impulsive force caused by collision.

Passenger Safety Cell

The body of the car is made from strong, rigid stell cage.

This may prevent the car from collapsing on the passengers during a car crash.

Gravitational Field

A gravitational field as a region in which an object experiences a force due togravitational attraction.

Gravitational Field Strength

The gravitational field strength at a point in the gravitational field is the gravitationalforce acting on a mass of 1 kg placed at that point.

The unit of gravitational field strength is N/kg.

The gravitational field strength is denoted by the symbol "g".

Gravitational Field Strength Formula

Gravitational Acceleration

The gravitational acceleration is the acceleration of an object due to the pull of the

gravitational force.

The unit of gravitational acceleration is ms-2

Gravitational acceleratio is also denoted by the symbol "g".

Symbol: g

Important notes:

Gravitational acceleration does not depend on the mass of the moving object.

The magnitude of gravitational acceleration is taken to be 10ms-2.

Gravitational Field Strength vs. Gravitational Acceleration

Both the gravitational field strength and gravitational acceleration have the

symbol, g and the same value (10ms-2) on the surface of the earth.

When considering a body falling freely, the g is the gravitational acceleration.


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When considering objects at rest, g is the Earths gravitational field strength

acting on it.

Weight

The weight of an object is defined as the gravitational force acting on the object.The SI unit of weight is Newton (N)

Differences between Weight and Mass

Weight Mass

Depends on the gravitational fieldstrength

Independent from the gravitational fieldstrength

Vector quantity Scalar Quantity

Unit Newton (N) Unit: Kilogram (kg)

Free Falling

1. Free falling is a motion under force of gravity as the only force acting on themoving object.

2. Practically, free falling can only take place in vacuum.

Gravitational Acceleration

1. The gravitational acceleration is the acceleration of an object due to the pull of thegravitational force. It has the unit of ms-2

2. The symbol of gravitational acceleration is " g ".

3. Gravitational acceleration does not depend on the mass of the moving object.

4. The magnitude of gravitational acceleration is taken to be 10ms-2.

Gravitational Field Strength vs. Gravitational Acceleration

1. Both the gravitational field strength and gravitational acceleration have the

symbol, g and the same value (10ms

-2

) on the surface of the earth.2. When considering a body falling freely, the g is the gravitational acceleration.3. When considering objects at rest, g is the Earths gravitational field strength

acting on it.

Vector and Scalar Quantity

A scalar quantity is a quantity which can be fully described by magnitude only.


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A vector quantity is a quantity which is fully described by both magnitude and direction.

Vector Diagram

The arrow shows the direction of the vector.

The length representing the magnitude of the vector.

Equal Vector

Two vectors A and B may be defined to be equal if they have the same magnitude and

point in the same direction.

Case of Free Falling 1 - Falling from High Place

When an object is released from a high place,

1. its initial velocity, u = 0.

2. its acceleration is equal to the gravitational acceleration, g, which taken to be10ms-2 in SPM.

3. the displacement is the of the object when it reaches the ground is equal to theinitial height of the object, h.
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Case of Free Falling 2 - Launching Object Upward

If an object is launched up vertically,

1. the acceleration = -g (-10ms-2)

2. the velocity become zero when the object reaches the highest point.

3. the displacement of the object at highest point is equal to the vertical height ofobject, h

4. the time taken for the object to move to the maximum height = the time taken for

the object to fall from the maximum point to its initial position.

Vector Addition - Triangle Method

Join the tail of the 2nd vector to the head of the 1st vector. Normally the resultant vector

is marked with double arrow.
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Vector Addition - Parallelogram Method

Join the tail of the 2nd vector to the tail of the 1st vector. Normally the resultant vector ismarked with double arrow.

Addition of 2 Perpendicular Vectors

If 2 vectors (a and b) are perpendicular to each others, the magnitude and direction of the

resultant vector can be determined by the following equation.
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Example 2

Diagram above shows that four forces of magnitude 2N, 4N, 5N and 8N are acting onpoint O. All the forces are perpendicular to each others. What is the magnitude of the

resulatant force that acts on point O?

Answer:

The resultant force of the horizntal component = 5 - 2 = 3N to the right

The resultant force of the vertical component = 8 - 4 = 4N acting downward.Therefore, the magtitude of these 2 force components,

Vector Resolution

A vector can be resolve into 2 component which is perpendicular to each others.

Example 1
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Diagram above shows a lorry pulling a log with an iron cable. If the tension of the cable

is 3000N and the friction between the log and the ground is 500N, find the horizontal

force that acting on the log.

Answer:Horizontal component of the tension = 3000 cos30o =2598N

Friction = 500N

Resultant horizontal force = 2598N - 500N =2098N

Example 2

Diagram above shows two forces of magnitude 25N are acting on an object of mass 2kg.Find the acceleration of object P, in ms-2.

Answer:

Horizontal component of the forces = 25cos45o + 25cos45o = 35.36N

Vertical component of the forces = 25sin45o - 25sin45o = 0NThe acceleration of the object can be determined by the equation

F = ma

(35.36) = (2)aa = 17.68 ms-2
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Inclined Plane

Weight component along the plane = Wsin.

Weight component perpendicular to the plane = Wcos.

Example 3

A block of mass 2 kg is pulling along a plane by a 20N force as shown in diagram above.

Given that the fiction between block and the plane is 2N, find the magnitude of the

resultant force parallel to the plane.Answer:

First of all, let's examine all the forces or component of forces acting along the plane.The force pulling the block, F = 20N

The frictional force Ffric = 2N

The weight component along the plane = 20sin30o = 10NThe resultant force along the plane = 20 - 2 - 10 = 8N
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Vectors in Equilibrium

When 3 vectors are in equilibrium, the resultant vector = 0. After joining all the vectors

tail to head, the head of the last vector will join to the tail of the first vector.

Forces in equilibrium

Forces are in equilibrium means the resultant force in all directions are zero.

When the forces acting on an object are balanced, they cancel each other out. The net

force is zero.

Effect :

an object at rest is continuely at rest [ velocity = 0]

a moving object will move at constant velocity [ a = 0]
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Example 1

Diagram above shows a load of mass 500g is hung on a string C, which is tied to 2 otherstrings A and B. Find the tension of string A.

Answer:

Tension of string C, TC = weight of the load = 5N

All forces in the system are in equilibrium, hence

Vertical component of tension A (TA) = TCTAcos60

o = TCTA = TC/cos60

o

TA = 5/cos60o = 10N
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Work

1. Work done by a constant force is given by the product of the force and the

distance moved in the direction of the force.2. The unit of Nm(Newton metre) or J(Joule).

3. Work is a scalar quantity.

Equation of Work

When the direction of force and motion are same, = 0o, therefore cos = 1Work done,

W = F s

Example 1

A force of 50 N acts on the block at the angle shown in the diagram. The block moves a

horizontal distance of 3.0 m. Calculate the work being done by the force.

Answer:
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Work done,

W = F s cos

W = 50 3.0 cos30o = 129.9J

Example 2

Diagram above shows a 10N force is pulling a metal. The friction between the block and

the floor is 5N. If the distance travelled by the metal block is 2m, find

a. the work done by the pulling force

b. the work done by the frictional force

Asnwer:

(a) The force is in the same direction of the motion. Work done by the pulling force,

W = F s = (10)(2) = 20J

(b) The force is not in the same direction of motion, work done by the frictional force

W = F s cos180o= (5)(2)(-1) = -10J

Work Done Against the Force of Gravity

Example 3

Ranjit runs up a staircase of 35 steps. Each steps is 15cm in height. Given that Ranjit's

mass is 45kg, find the work done by Ranjit to reach the top of the staircase.
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Answer:

In this case, Ranjit does work to overcome the gravity.

Ranjit's mass = 45kgVertical height of the motion, h = 35 0.15

Gravitational field strength, g = 10 ms-2Work done, W = ?

W = mgh = (45)(10)(35 0.15) = 2362.5J

Finding Work from Force-Displacement Graph

In a Force-Displacement graph, work

done is equal to the area in between the graph and the horizontal axis.

Example 4
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The graph above shows the force acting on a trolley of 5 kg mass over a distance of 10 m.

Find the work done by the force to move the trolley.

Answer:In a Force-Displacement graph, work done is equal to the area below the graph.Therefore, work done

Energy

Energy is defined as the capacity to do work. Work is done when energy

is converted from one form to another.

Nm or Joule(J)

Gravitational Potential Energy

Gravitational potential energy is the energy stored in an object as the

result of its vertical position (i.e., height).

Formula:

Example 1

A ball of 1kg mass is droppped from a height of 4m. What is the maximum

kinetic energy possessed by the ball before it reached the ground?

Answer

According to the principle of conservation of energy, the amount of

potential energy losses is equal to the amount of kinetic energy gain.

Maximum kinetic energy

= Maximum potentila energy losses

= mgh = (1)(10)(4) = 40J

Elastic Potential Energy

Elastic potential energy is the energy stored in elastic materials as

the result of their stretching or compressing.

Formula:
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Example 2

Diagram above shows a spring with a load of mass 0.5kg. The extention

of the spring is 6cm, find the energy stored in the spring.

Answer:The energy stored in the spring is the elestic potential energy.

Kinetic Energy

Kinetic energy is the energy of motion.

Equation of Kinetic Energy

Example 1
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Relationship between Energy and Work Done

During a conversing of energy,

Amount of Work Done = Amount of Energy Converted

ExampleA trolley of 5 kg mass moving against friction of 5 N. Its velocity at A is 4ms-1 and itstops at B after 4 seconds. What is the work done to overcome the friction?

Answer:

In this case, kinetic energy is converted into heat energy due to the friction. The workdone to overcome the friction is equal to the amount of kinetic energy converted into heat

energy, hence

Power

Power is the rate at which work is done, which means how fast a work is done.

Formula:

Example 1An electric motor takes 20 s to lift a box of mass 20kg to a height of 1.5 m. Find the

amount of work done by the machine and hence find the power of the electric motor.

Answer:

Work done,

W = mgh = (20)(10)(1.5) = 300J

Efficiency

The efficiency of a device is defined as the percentage of the energy input that is

transformed into useful energy.

Example

In the example above, the input power is 100J/s, the desire output power (useful energy)

is only 75J/s, the remaining power is lost as undisire output. Therefore, the efficiency of

this machine is

75/100 x 100% = 75%
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Air Conditioner

1. Switch off the air conditioner when not in use.

2. Buy the air conditioner with suitable capacity according to the room size.3. Close all the doors and windows of the room to avoid the cool air in the room

from flowing out.

Refrigerator

1. Always remember to close the door of refrigerator.

2. Open the refrigerator only when necessarily.3. Always keep the cooling coil clean.

4. Defrost the refrigerator regularly.

5. Choose the refrigerator with capacity suitable for the family size.6. Refrigerator of large capacity is more efficient compare with refirgerator of small

capacity.

Lamp or Light Bulb

1. Use fluorecent bulb rather than incandescent bulb. Fluorescent bulbs are muchmore efficient than incandescent bulbs.

2. Use a lamp with reflector so that more light is directed towards thr desirable

place.

Washing Machine

1. Use front-loading washing machine rather than top-loading wahing machine

because it uses less water and electricity.2. Use washing machine only when you have sufficient clothes to be washed. Try to

avoid washing small amount of clothes.

Elasticity

Elasticity is the ability of a sub-stance to recover its original shape and size after

distortion.

Forces Between Atoms
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The intermolecular forces consist of an attractive force and a repulsive force.

At the equilibrium distance d, the attractive force equal to the repulsive force.

If the 2 atoms are brought closer, the repulsive force will dominate, produces a netrepulsive force between the atoms.

If the 2 atoms are brought furhter, the attractive force will dominate, produces anet attractive force between the atoms.

Graph of Forces Between 2 atoms

x0 = Equilibrium Distance

When the particles are compressed, x < x0, the attractive force between the particlesincreases.If the distance x exceeds the elastic limit, the attractive force will decreases.

Hooke's Law

Hooke's Law states that if a spring is not stretched beyond its elastic limit, the force that

acts on it is directly proportional to the extension of the spring.

Elastic Limit

The elastic limit of a spring is defined as the maximum force that can be applied to aspring such that the spring will be able to be restored to its original length when the force

is removed.

Equation derived from Hooke's Law

From Hook's Law, we can derived that
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Spring Constant

Spring constant is defined as the ratio of the force applied on a spring to the extension of

the spring.It is a measure of the stiffness of a spring or elastic object.

Graph of Streching Force - Extension

Gradient = Spring constant

Area below the graph = Work done

F-x graph and spring constant
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The higher the gradient, the greater the spring constant and the harder (stiffer) spring.

For example, the stiffness of spring A is greater than spring B.

Spring

Arrangement in series: Arrangement in parallel:

Extension = x number of springStiffness decreases

Spring constant = k/number of spring

Extension = x number of springStiffness increases

Spring constant = k number of spring
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Factors Affecting the Stiffness of Spring

Stiffer Less stiff

Material type of spring

(A steel spring is stiffer than a copper spring)

Diameter of wire of spring

(The greater the diameter of the wire, the stiffer the spring)

Diameter of the spring

(The smaller the diameter of spring, the stiffer the spring)

Length of the string

(Shorter spring is stiffer)
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Physic Chap 2 Note