Bumper Cars 1 Bumper Cars Bumper Cars 2

Bumper Cars 1 Bumper Cars

Bumper Cars 2 Introductory Question n You are riding on the edge of a spinning playground merry-go-round. If you pull yourself to the center of the merry-go-round, what will happen to its rotation? A. It will spin faster. It will spin slower. It will spin at the same rate. B. C.

Bumper Cars 3 Observations about Bumper Cars Impacts alter velocities and angular velocities n Cars often appear to exchange their motions n The fullest cars are hardest to redirect n The least-full cars get slammed during collisions n

Bumper Cars 4 3 Questions about Bumper Cars Does a moving bumper carry a “force”? n Does a spinning bumper carry a “torque”? n On an uneven floor, which way does a bumper car accelerate? n

Bumper Cars 5 Question 1 Does a moving bumper carry a “force”? n Starting and stopping a bumper car seems to require the “investment” and “withdrawal” of some directed quantity of motion. What is it? n

Bumper Cars 6 Momentum A translating bumper carries momentum n Momentum is n a conserved quantity (can’t create or destroy) n a directed (vector) quantity n a measure of difficulty reaching present velocity n momentum = mass· velocity

Bumper Cars 7 Exchanging Momentum Bumper cars exchange momentum via impulses n An impulse is n the only way to transfer momentum n a directed (vector) quantity impulse = force· time n n When car 1 gives an impulse to car 2, car 2 gives an equal but oppositely directed impulse to car.

Bumper Cars 8 Head-On Collisions Cars exchange momentum via impulses n The total momentum never changes n Car with the least mass changes velocity most n The littlest riders get creamed n

Bumper Cars 9 Question 2 Does a spinning bumper carry a “torque”? n Spinning and un-spinning a bumper car seems to require the “investment” and “withdrawal” of some directed quantity of rotational motion. What is it? n

Bumper Cars 10 Angular Momentum A spinning carries angular momentum n Angular momentum is n a conserved quantity (can’t create or destroy) n a directed (vector) quantity n a measure of difficulty reaching present ang. velocity n angular momentum = rotational mass· angular velocity

Bumper Cars 11 n Newton’s Third Law of Rotational Motion For every torque that one object exerts on a second object, there is an equal but oppositely directed torque that the second object exerts on the first object.

Bumper Cars 12 Exchanging Angular Momentum Cars exchange ang. mom. via ang. impulses n An angular impulse is n the only way to transfer angular momentum n a directed (vector) quantity angular impulse = torque· time n n When car 1 gives an angular impulse to car 2, car 2 gives an equal but oppositely directed angular impulse to car 1.

Bumper Cars 13 Glancing Collisions Cars exchange ang. mom. via ang. impulses n Total angular momentum about a chosen point in space remains unchanged n Car with the smallest rotational mass about that chosen point changes angular velocity most n The littlest riders tend to get spun wildly n

Bumper Cars 14 Rotational Mass can Change Mass can’t change, so the only way an object’s velocity can change is if its momentum changes n Rotational mass can change, so an object that changes shape can change its angular velocity without changing its angular momentum n

Bumper Cars 15 Introductory Question (revisited) n You are riding on the edge of a spinning playground merry-go-round. If you pull yourself to the center of the merry-go-round, what will happen to its rotation? A. It will spin faster. It will spin slower. It will spin at the same rate. B. C.

Bumper Cars 16 Question 3 n On an uneven floor, which way does a bumper car accelerate?

Bumper Cars 17 Acceleration and Potential Energy An object accelerates in the direction that reduces its total potential energy as rapidly as possible n Forces and potential energies are related! n A car on an uneven floor accelerates in whatever direction reduces its total potential energy as rapidly as possible n

Bumper Cars 18 Summary about Bumper Cars n During collisions, bumper cars exchange momentum via impulses n angular momentum via angular impulses n n Collisions have less effect on cars with large masses n cars with large rotational masses n