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| 等 级：长老 |
| 经 验 值：652 |
| 魅 力 值：518 |
| 龙 币：3958 |
| 积 分：1707.2 |
| 注册日期：2011-12-07 |
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貌似这个看得人不多，呵呵，但是我还是坚持着把其余的部分发上来吧~~这章讲的是Forces__各种力。
Forces
A force a s push,a pull, a squeeze or a twist.
When the go-kart is at the top of the hill, a pushing force starts it moving.
A pulling force helps it back up the hill.
A force can make things change direction. It can also make things change shape.
You can see pulling forces in a game of tug-of-war.
Gravity
Gravity is a force that pulls things down to the ground.
Friction
Friction is a force between surfaces that rub together. Friction slows you down. It's easier to cycle on smooth ground because there is less friction.
There is lots of friction when you ride on rough ground...so you cycle slower.
Reduce friction
To show how to reduce friction in this experiment, you will need: a heavy book and some marbles.
Place the book on a table. Try moving with one finger. 2. Place the book on some marbles. Move it again with the same finger. 3. The marbles reduce the friction, so the book moves smoothly with less effort.
A skydiver falls through the air very fast. But with a parachute, the air has a greater area to push against. The extra air resistance slows down his fall.
Air resistance Air resistance is a force that slows things in the air. To reduce air resistance, planes have a smooth, streamlined shape.
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2011-12-24 22:31:34
