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PHYSICS 103: Lecture 6

• Explaining MotionØ Newton’s 3 Laws Ø Applications of Laws

Agenda for Today:

Inertia

• Question: If I throw a ball in the air, what causes it to keep moving?

• Why does stamping your feet clean the snow off your shoes?

• Why does an unseatbelted driver hit the steering wheel when the car comes to a sudden stop?

A Few Questions...

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NEWTON’S LAWS OF MOTION

FFIRST IRST LLAWAW: Law of Inertia

An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an external force.

INERTIAThe quality of an object that indicates the reluctance of an object to be altered in its movement

(things are inherently “lazy”)

NEWTON’S LAWS OF MOTION

SecondSecond LLAWAW:

The net force on an object is equal to the product of that object’s mass times its acceleration. The acceleration is in the same direction as the force.

F = m . a

(Throwing a baseball is much easier than throwing a bowling ball)

(mass is in units of kilograms)

FREELY FALLING OBJECTSWhy do things fall at the same rate?

Heavy Ball Light Ball

Fheavy = mheavy aheavy Flight = mlight alight

a = g = constant = 9.81 m/s2 ~ 10 m/s2a = g = constant = 9.81 m/s2 ~ 10 m/s2

aheavy = alight

FgFg

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Force is a vectorForce is a vector:

The net force on an object is what determines the acceleration of the object

Is there a force acting on me?

Example problem: Pushing on a block

10 kg

20 N

2 N

a=?

NEWTON’S LAWS OF MOTION

ThirdThird LLAWAW:

For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object.

(For every action there is an equal but opposite reaction)

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Questions

• If you push on your friend who is running away from you with a force of 50 N, how much force will he/she exert back on you?

• You are ice skating with your identical twin (who has the same mass). You push off your twin with a force of 50 N. What will happen to you and your twin as your pushing off each other? 1 second after you let go?

• You now ice skate with a sumo wrestler and push off the sumo wrestler with a force of 50 N. What will happen to you? What will happen to the sumo wrestler?

Main Points from Today’s Lecture

• Newton’s 3 Laws of MotionØ You should understand the three laws of

motion and think of examples of them in everyday life.

Ø You should be able to calculate the net force on an object and understand that this is what determines the acceleration.

Ø You should be able to apply Newton’s laws to real problems, solving for the net force and acceleration