L14 脉冲和动量 Impulse and Momentum

Midterm Paper Retry (8:10~8:40)

Review: Homework Question: Problem 8.35

You've taken your neighbor's young child to the carnival to ride the rides. She wants to ride "The Rocket". Eight rocket-shaped cars hang by chains from the outside edge of a large steel disk. A vertical axle through the center of the ride turns the disk, causing the cars to revolve in a circle. You've just finished taking physics, so you decide to figure out the speed of the cars while you wait. You estimate that the disk is 2.5 m in diameter and the chains are 6 m long. The ride takes 10 s to reach full speed, then the cars swing out until the chains are 20 deg from vertical.
  1. Picture
  2. Free Body Diagram (including Fnet, show coordinate system)
  3. Solve: Newton's 2nd Law
  4. Assess

News: Class in computer room this afternoon. Next week no classes Relax but do homework!

Another Perspective: (Based on Newton's Laws) Momentum

$$ \mathbf { \vec a } (t) = { { \mathbf { \vec F }(t) \over m } } \rightarrow $$ $$ \Delta \mathbf { \vec v } = \int { { \mathbf { \vec F }(t) \over m }\, dt } $$ $$ \Delta \mathbf { \vec p } = \int { { \mathbf { \vec F }(t) }\, dt } \equiv \mathbf { \vec J }$$

優點:Advantage: Simplifies solutions when only final state required.

缺點:Disadvantage: Loose all time information

isolated system → Δ p = 0 (Momentum Conservation)

Impulse: Newton's 2nd Law in another form, loc

Knight Chapter 9 Impulse & Momentum Slides 1 to 35

Collision Lab
Collision Lab (Introduction to Inelastic Collisions)

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