Newton’s LawsNewton’s Laws

Easy as apple pie!Easy as apple pie!

ForceForce

ForceForce a push or a pull a push or a pull Causes object to start or stop moving or Causes object to start or stop moving or

change directionchange direction

Newton’s 1Newton’s 1stst Law Law

Greeks thought that Greeks thought that the natural state of an the natural state of an object was at rest.object was at rest. ex. A ball stops rollingex. A ball stops rolling without friction an without friction an

object would never object would never stop!stop!

Newton’s 1Newton’s 1stst an an object in motion tends object in motion tends to stay in motion; an to stay in motion; an object at rest tends to object at rest tends to stay at reststay at rest

Newton’s 1Newton’s 1stst Law Law

InertiaInertia property of matter that resists a property of matter that resists a change in motionchange in motion An object with great mass has high inertiaAn object with great mass has high inertia

Newton’s 2Newton’s 2ndnd Law Law

FA = 1000 kg x 0.05 m/s/sFA =

FB = 2000 kg x 0.05 m/s/sFB =

Newton’s 2Newton’s 2ndnd Law Law

Force = mass x Force = mass x accelerationacceleration An object will An object will

only accelerate only accelerate if there is an if there is an unbalanced unbalanced forceforce

Newton’s 3Newton’s 3rdrd Law Law

For every action For every action there is and there is and equalequal and and opposite opposite reactionreaction The astronaut The astronaut

pushes on the pushes on the rock and the rock and the rock pushes on rock pushes on the astronautthe astronaut

Newton’s 3Newton’s 3rdrd Law Law

For every action For every action there is and equal there is and equal and opposite and opposite reactionreaction A bird pushes A bird pushes

down on the air down on the air and the air and the air pushes up on the pushes up on the birdbird

Newton’s 3Newton’s 3rdrd Law Law

Note: Equal and opposite forces does not result in equal acceleration if the masses are different.

Newton’s LawsNewton’s Laws

11stst Law Law:: (inertia: objects tend to do what they are (inertia: objects tend to do what they are doing)doing) cannon ball will rest until a force is put on itcannon ball will rest until a force is put on it ball will roll straight until ramp puts a force on itball will roll straight until ramp puts a force on it

22ndnd Law Law:: (f = m x a) (f = m x a) greater force put on ball accelerates it moregreater force put on ball accelerates it more greater mass of ball but greater force on watergreater mass of ball but greater force on water

33rdrd Law Law:: (every action has an equal but opposite (every action has an equal but opposite reaction)reaction) ball moves right, cannon recoils leftball moves right, cannon recoils left ball move down, water splashes upball move down, water splashes up Newton's Laws - YouTubeNewton's Laws - YouTube

GravityGravity

With the upward force of the floor equilibrium is attained and there is no motion

Without the upward force of the hand there is not equilibrium of forces and motion occurs

Gravity: air resistanceGravity: air resistance

Opposes Opposes downward downward motion of motion of falling falling objects.objects.

Larger Larger surface surface areas areas increases increases air air resistanceresistance

Which Law???Which Law???

F=ma

Inertia

Which Law???Which Law???

The pellet has entered the egg at the left side but not exited yet. One second later there was raw egg all over the work table and the backdrop.

F=ma, Inertia

Which Law???Which Law???

Action-reaction

Inertia

Which Law???Which Law???

F=ma

Inertia

Action-reaction

Which Law???Which Law???

Inertia F=ma Action-reaction

Free Body Diagrams Free Body Diagrams

A A force diagram, force diagram, which is also known as a which is also known as a free free body diagram, body diagram, is a sketch in which all the force is a sketch in which all the force vectors acting on an object are drawn with their vectors acting on an object are drawn with their initial points at the location of the object. initial points at the location of the object.

Opposition to MotionOpposition to Motion

FrictionFriction a force a force that opposes motionthat opposes motion Caused by rough Caused by rough

surfaces of surfaces of all all materialsmaterials

3 Types of Friction3 Types of Friction

1. 1. SlidingSliding when solid objects grind when solid objects grind over each otherover each other

puck and icepuck and ice

2. 2. RollingRolling wheels spinning on an wheels spinning on an axleaxle

skateboards eventually roll to a skateboards eventually roll to a stopstop

3. 3. FluidFluid liquids or gases slow the liquids or gases slow the motion of a solidmotion of a solid

wind resistancewind resistance

oil a squeaky hingeoil a squeaky hinge

pushes a surferpushes a surfer

Free-body diagramsFree-body diagrams

Free-body Free-body diagrams are diagrams are used to show the used to show the relative relative magnitude and magnitude and direction of all direction of all forces acting on forces acting on an object. an object.

This diagram This diagram shows four shows four forces acting forces acting upon an upon an object. There object. There aren’t always aren’t always four forces, four forces, For example, For example, there could be there could be one, two, or one, two, or three forces. three forces.

Problem 1Problem 1

A book is at rest on a A book is at rest on a table top. Diagram the table top. Diagram the forces acting on the book.forces acting on the book.

In this diagram, there are In this diagram, there are normal and gravitational normal and gravitational forces on the book.forces on the book.

Problem 2Problem 2

An egg is free-falling from a nest in a An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a tree. Neglect air resistance. Draw a free-body diagram showing the forces free-body diagram showing the forces involved.involved.

Gravity is the only Gravity is the only force acting on the force acting on the egg as it falls.egg as it falls.

Problem 3Problem 3

A flying squirrel is gliding (no wing flaps) A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant from a tree to the ground at constant velocity. Consider air resistance. A free velocity. Consider air resistance. A free body diagram for this situation looks like…body diagram for this situation looks like…

Gravity pulls down Gravity pulls down on the squirrel on the squirrel while air while air resistance resistance keeps the keeps the squirrel in the squirrel in the air for a while.air for a while.

Problem 4Problem 4

A rightward force is applied to a book in A rightward force is applied to a book in order to move it across a desk. Consider order to move it across a desk. Consider frictional forces. Neglect air resistance. frictional forces. Neglect air resistance. Construct a free-body diagram. Let’s see Construct a free-body diagram. Let’s see what this one looks like.what this one looks like.

Note the larger applied Note the larger applied force arrow force arrow pointing to the right since the book pointing to the right since the book is accelerating to the right. Friction is accelerating to the right. Friction force opposes the direction of force opposes the direction of motion. The force due to gravity motion. The force due to gravity and normal forces are balanced.and normal forces are balanced.

Problem 5Problem 5

A skydiver is descending with a constant A skydiver is descending with a constant velocity. Consider air resistance. Draw a velocity. Consider air resistance. Draw a free-body diagram.free-body diagram.

Gravity pulls down on Gravity pulls down on the skydiver, while the skydiver, while air resistance air resistance pushes up as she pushes up as she falls.falls.

Problem 6Problem 6

A man drags a sled across loosely packed A man drags a sled across loosely packed snow with a rightward acceleration. Draw snow with a rightward acceleration. Draw a free-body diagram.a free-body diagram.

The applied force arrow points to the right The applied force arrow points to the right and is larger than the frictional force since and is larger than the frictional force since the object is accelerating. Since the sled is the object is accelerating. Since the sled is on the ground, the normal and gravitational on the ground, the normal and gravitational force are balanced. force are balanced.

Problem 7Problem 7

A football is moving upwards toward its peak A football is moving upwards toward its peak after having been booted by the punter. after having been booted by the punter. Draw a free-body diagram. (Neglect air Draw a free-body diagram. (Neglect air friction)friction)

The force of The force of gravity is the gravity is the only force only force described. (no described. (no air resistance).air resistance).

Problem 8Problem 8

A car runs out of gas and is coasting down a A car runs out of gas and is coasting down a hill. hill.

The car is coasting The car is coasting down the hill, down the hill, there is dragging there is dragging friction of the friction of the road (left pointing road (left pointing arrow) as well as arrow) as well as gravity and gravity and normal forces, normal forces, but but no applied no applied force.force.

Net ForceNet Force

Now let’s take a look at what happens when Now let’s take a look at what happens when unbalanced forces do not become unbalanced forces do not become completely balanced (or cancelled) by completely balanced (or cancelled) by other individual forces. other individual forces.

An unbalanced forces exists when the An unbalanced forces exists when the vertical and horizontal forces do not cancel vertical and horizontal forces do not cancel each other out.each other out.

Example 1Example 1

Notice the upward Notice the upward force of 1200 force of 1200 Newtons (N) is Newtons (N) is more than gravity more than gravity (800 N). The net (800 N). The net force is 400 N up.force is 400 N up.

Example 2Example 2

Notice that while the normal force and gravitation Notice that while the normal force and gravitation forces are balanced (each are 50 N) the force of forces are balanced (each are 50 N) the force of friction results in unbalanced force on the friction results in unbalanced force on the horizontal axis. The net force is 20 N left.horizontal axis. The net force is 20 N left.

Another way to look at balances Another way to look at balances and unbalanced forcesand unbalanced forces

Balanced or unbalanced?Balanced or unbalanced?

Accelerating – Unbalanced

Constant velocity –balanced

deccelerating – Unbalanced

Balanced or Unbalanced?Balanced or Unbalanced?

Which objects are moving?Which objects are moving?