Say whether each work is positive, negative or zero, and why.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Say whether each work is positive, negative or zero, and why.
- Friction from the snow on a sledge sliding down a hill.
- The floor of a lift pushing on a passenger while the lift rises.
- Gravity on an apple falling from a tree.
- Gravity on a satellite in a circular orbit.
- The string's pull on a swinging pendulum bob.
- Your hands on a ball as you catch it.
Need a reminder? The key ideaShow
The work done by a steady force is , in joules. It is positive when the force helps the motion, zero when the force is at right angles to it, and negative when it opposes it. From parts: .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. For each force, find the angle between it and the motion.
Step 1. (a) Friction acts up the hill, against the sledge's motion: negative.
Step 2. (b) The floor pushes up and the passenger moves up: positive.
Step 3. (c) Gravity down, apple moving down: positive.
Step 4. (d) Gravity points to the centre, the motion is along the circle: zero.
Step 5. (e) The string pulls along itself, at right angles to the bob's path: zero.
Step 6. (f) Your hands push back against the ball's motion to stop it: negative.
Answer: (a) negative, (b) positive, (c) positive, (d) zero, (e) zero, (f) negative.
Watch out. A normal reaction can do work. On a still floor it does none, but in a rising lift the floor moves with the push.
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