Momentum, Work, and Simple Machines

Momentum, work, and simple machines all describe how force moves things: momentum is mass times velocity, work is force times distance, and a machine trades one for the other without creating energy. The examples below work through each, with practice.

Momentum

Momentum is a measure of how hard it is to stop a moving object. It depends on mass and velocity: [ text{momentum} = text{mass} times text{velocity}. ] A heavy truck moving slowly can have as much momentum as a light car moving fast. In a collision, momentum is conserved — the total momentum before equals the total after. That is why a moving ball transfers its momentum when it hits a still ball.

Work

Work, in physics, means using a force to move an object a distance: [ text{work} = text{force} times text{distance}. ] Lifting a box does work; simply holding it still does not, because nothing moves. If you push with (10) newtons over (3) meters, you do [ 10 times 3 = 30 text{ joules} ] of work. Work is how force actually transfers energy to an object.

Simple Machines

Simple machines make work easier by changing the size or direction of a force. There are six: the lever, the pulley, the wheel and axle, the inclined plane, the wedge, and the screw. A ramp (inclined plane) lets you raise a heavy object with less force by pushing it over a longer distance. A lever lets you lift a big load with a small effort. Machines do not reduce the total work — they trade more distance for less force, which is called mechanical advantage.

Watch: A Short Video Lesson

CodeLucky walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A Routine for These Questions

1. Momentum = mass times velocity; it is conserved in collisions.
2. Work = force times distance; no movement means no work.
3. Simple machines trade distance for force (mechanical advantage).
4. Know the six: lever, pulley, wheel and axle, inclined plane, wedge, screw.

Practice

1. Write the formula for momentum.
2. A (2) kg ball moves at (5) m/s. What is its momentum?
3. Write the formula for work.
4. You push a cart with (20) N over (4) m. How much work is done?
5. Does holding a heavy box still do any work?
6. Name two simple machines.

Answers

1. (text{momentum} = text{mass} times text{velocity}).
2. (2 times 5 = 10) (kg m/s).
3. (text{work} = text{force} times text{distance}).
4. (20 times 4 = 80 text{ joules}).
5. No — nothing moves, so no work is done.
6. Any two of: lever, pulley, wheel and axle, inclined plane, wedge, screw.

Where This Fits in Your Science Prep

These ideas build on forces and energy, since work is how force transfers energy. See all topics on the Science Topics Hub.