Track quiz
The ten rules first. Then the quiz.
| Idea | The rule |
|---|---|
| Work | Work = force × distance moved along the force. If there is no movement, the work is zero. Work is negative when the force points against the movement. |
| Two kinds | With no friction, work on an object equals its energy gain. Moving objects have kinetic energy, ½ m v². Raised objects have potential energy, m g h. So do stretched springs and bands. |
| Energy changes form | Energy changes from one form to another. It is not created or destroyed. If only gravity acts, PE + KE stays the same. |
| Friction | Friction and air resistance turn mechanical energy mostly into heat. The mechanical energy falls. The total energy does not. |
| Power | Power is the rate of doing work. Power = work ÷ time. The unit is the watt. |
| A machine | A simple machine changes the size or direction of the force. It does not reduce the work. Advantage = load ÷ effort. |
| Three machines | A fixed pulley only changes the direction of the force. A lever balances when effort × effort arm = load × load arm. With no friction, a ramp's advantage is its length ÷ its height. |
| Nothing is free | A machine cannot give out more work than it takes in. Friction turns some of the work into heat. |
| Stored energy | A stretched or squeezed springy object stores potential energy. Held still, it has no kinetic energy. |
| Work from a graph | Work is the area under the force–displacement graph. For a steady force, the area is force × displacement. |
You push hard on a parked truck. It does not move. What work do you do on it?
A 15 N force moves a cart 6 m along the force. How much work is done?
A brake pad grips a spinning wheel. What is the work of the brake force?
The speed of a cycle halves. Its kinetic energy becomes…
A 3 kg brick is 4 m up. Take g = 10. What is its PE?
A ball falls. Its PE drops by 30 J. Only gravity acts. What happens to its KE?
A pendulum swings lower and stops. Where did its energy go?
A motor does 900 J of work in 15 s. What is its power?
A ramp is 10 m long and 2 m high. Ignore friction. What is its advantage?
A band is pulled back and held still. Which energy does it have?
What does the area under a force–displacement graph give?
This is Chapter 7 once more, Exploration — Work, Energy, and Simple Machines.
Next comes Journey Inside the Atom. What is matter made of?