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Work adds up along the path

Science · Energy · ages 18-19

Name ______________________ Date ____________

A force pushing at right angles to the motion does zero work.
Circle one: True False
In physics, what is work?
- Energy handed over during a displacement
- Any effort you feel, with or without motion
- Force stored inside an object at rest
A force points opposite the motion. What is the sign of its work?
- Positive
- Zero always
- Negative
What does the work energy theorem say?
- The change in kinetic energy
- Net work always equals zero
- Net work equals the change in mass
Why does stretching a spring need an integral instead of force times distance?
- Because the pull grows with the stretch
- Because the motion has no displacement
- Because work is never defined for springs
A steady 5 newton push moves a box 4 meters along the push. How many joules of work is that?
Answer: ______________
Two paths join the same start and finish, and a varying force does different work on each. What can you conclude?
- Work must be path independent here
- Work here depends on the route, so integrate along the actual path
- The work must be zero on both paths
A student says carrying a heavy bag along a level path at steady speed does work on the bag. What is wrong?
- Nothing is wrong, carrying always does work
- The lifting force is vertical while the motion is horizontal, so its work is zero
- Work can never be zero when muscles get tired

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · No part of it points along the motion, so nothing accumulates.
Energy handed over during a displacement · Work needs both a force and a displacement to carry energy across.
Negative · Opposing pushes drain energy of motion, so their work is negative.
The change in kinetic energy · Every push added along the path shows up as gained or lost energy of motion.
Because the pull grows with the stretch · A growing force has no single value to multiply, so you add up tiny steps instead.
20 · Steady force times distance gives 5 times 4, which is 20 joules.
Work here depends on the route, so integrate along the actual path · For a varying force the integral follows the route, so the actual path matters.
The lifting force is vertical while the motion is horizontal, so its work is zero · Sideways to the motion means no component along it, so the physics work is zero.