1. Bumper Cars 1 Bumper Cars

2. Bumper Cars 2 Introductory Question 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? 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. B. It will spin slower. C. It will spin at the same rate.

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

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

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

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

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

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

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

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

11. Bumper Cars 11 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 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 second object exerts on the first object. the first object.

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

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

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

15. Bumper Cars 15 Introductory Question (revisited) 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? 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. B. It will spin slower. C. It will spin at the same rate.

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

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

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