## 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.

- Picture
- Free Body Diagram (including Fnet, show coordinate system)
- Solve: Newton's 2nd Law
- 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)

### Knight Chapter 9 Impulse & Momentum Slides 1 to 35