Download ppt - Department of Physics and Applied Physics 95.141, F2010, Lecture 6 Physics I 95.141 LECTURE 6 9/22/10

Department of Physics and Applied Physics95.141, F2010, Lecture 6

Physics I95.141

LECTURE 69/22/10


Exam Prep Problem

• A rock is launched with an initial speed vo=22m/s at an angle θo=65º from the origin. Ignore air resistance.– A)(10pts) Draw a coordinate system for this problem

and show the trajectory of the rock. Give the initial velocity vector of the rock in component form.

– B) (8pts) How high does the rock go and how long does it take the rock to get to that height?

– C) (7pts) How far, horizontally, will the rock travel before it lands?


Exam Prep Problem

• A rock is launched with an initial speed vo=22m/s at an angle θo=65º from the origin. Ignore air resistance.– A)(10pts) Draw a coordinate system for this problem and show

the trajectory of the rock. Give the initial velocity vector of the rock in component form.


Exam Prep Problem

• A rock is launched with an initial speed vo=22m/s at an angle θo=65º from the origin. Ignore air resistance.– B) (8pts) How high does the rock go and how long does it take

the rock to get to that height?


Exam Prep Problem

• A rock is launched with an initial speed vo=22m/s at an angle θo=65º from the origin. Ignore air resistance.– B) (7pts) How far, horizontally, will the rock travel before it

lands?


Administrative Notes

• HW review Session Tonight– 6:30 pm, OH 218

• Structure of HW Review Sessions– Why we do it this way

– Why it is important to try

• Structure of this course– Combination of your effort/our effort

• Lecture Notes

• Practice exams/prep problems

• Attendance Policy


Outline

• Force• Newton’s 1st Law• Newton’s 2nd Law• Newton’s 3rd Law

• What do we know?– Units– Kinematic equations– Freely falling objects– Vectors– Kinematics + Vectors = Vector Kinematics– Relative motion– Projectile motion


Force

• Intuitively, we understand a Force to be a push or pull on an object– This could be a racket hitting a ball, elevator cable

pulling on a car, a locomotive pulling a train…• These are all contact forces, where the force is exerted by

one object in contact with another

– Force can also be exerted by magnets, electric fields, or gravity

• Here, Force is exerted by a field, not an object, but this is still a force!

• Force has magnitude and direction: it is a VECTOR!


Force

• In order for something at rest to start moving, a Force must be exerted on it.

• However, if something is already moving, you also need a Force acting on it to stop motion!


Inertia

• Obviously, some things are harder to get moving than others…

• If you had to choose whether to carry an ice skater or an offensive lineman up to the Dean’s office on the 5th floor: which do you think would be easier?

• We quantify inertia with a measure we call mass• SI units: mass [kg]


Newton’s 2nd Law

• If Newton’s first law tells us how an object behave with no Force, we also want to understand how an object behaves with a Force applied.

• The acceleration of an object is directly proportional to the net force applied to the object, and the constant of proportionality is mass.


Units for Force

• Using Newton’s 2nd Law, we can figure out the units of Force.


Redefine Force

• An action capable of accelerating an object

• Acceleration is a vector, as is Force. Net acceleration is in the same direction as net Force.


Example Problem

• A 1500kg car traveling at 100km/h needs to stop in 55m. What force is required?

• Draw diagram

• Determine coordinate system


Example Problem


• Knowns/Unknowns (UNITS)


Example Problem


• Choose equations and solvemx

kgm

v

final

sm

o

55

1500

8.27


Connection between Newton’s 1st, 2nd Laws

• If F=ma• And no Force is exerted on an object• How can we describe the object’s velocity?


Newton’s 2nd Law

• Do Newton’s 1st and 2nd Law work every where?– Say you are in a pickup truck that is accelerating, and

you put a frictionless block in the back.– What do you see happening to the block?


Newton’s 2nd Law

• Do Newton’s 1st and 2nd Law work every where?– Say you are in a pickup truck that is accelerating, and

you put a frictionless block in the back.– What is the Force causing this?


Newton’s 2nd Law• Do Newton’s 1st and 2nd Law work every where?

– Say you are in a pickup truck that is accelerating, and you put a frictionless block in the back.

– What does someone on the ground see?


Inertial Reference Frames

• Newton’s Laws only work in Inertial reference frames

• Inertial reference frame– One in which Newton’s Laws hold

– A Reference Frame that moves with a constant velocity

– A Reference Frame at rest


Newton’s 3rd Law of Motion

• 1st Law: Law of Inertia, things want to keep on doing what they are already doing.

• 2nd Law: Forces cause objects to accelerate, with a=F/m.

• But where do forces come from?– Forces come from other objects

• Hammer exerts a force on the nail• The Earth exerts a force on a falling ball (gravity)• Elevator cable exerts force on car…


Newton’s 3rd Law

• But when hammer hits nail, it stops very quickly.• This is deceleration, which means the nail must

also exert a Force on the hammer.• This is Newton’s 3rd Law

– Whenever one object exerts a Force on a second object, the second object exerts an equal Force in the opposite direction.

– Sometimes, you hear: “For every action, there is an equal and opposite reaction”

– Remember though, the action and reaction act on different objects!


Action/Reaction

• They actually get the Physics right here!

• For every Force exerted on an object, there is an equal and opposite force exerted by that object


Describing Forces

• When we talk about Forces, it is very important that we describe the Force as “exerted by” something and “acting on” another thing.– For instance, in the case of the ice skater, force is exerted on the

skater by the ice.– For the car, the Force making the car move is exerted by the

ground on the tires.• When we start solving problems, it is important to keep

this straight. If you are interested in the motion of an object, you only want to sum up the Forces acting on that object.

• For a person walking, the ground is what moves the person forward.

GPPG FF


The Motivator


Draw Diagram

• Want F, need to figure out acceleration– Assume

• Tf=0.25s

• M=60kg• Δx=1.8m• Δy=-1m

1.8m

1m


Projectile Motion Problem (I)

• What is initial launch velocity?


Acceleration

• If we know the initial launch velocity, then we know the final velocity after acceleration– Initial velocity of part II = final velocity of part I


What Did We Learn

• Newton’s Laws– Law of Inertia– F=ma– Action/Reaction

• Solving basic Force problems• How to describe motion of an object with Force

applied.