You lift a 7.5 kg box at a steady speed through 1.4 m.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
You lift a box at a steady speed through . Then you carry it along a level corridor, also at a steady speed. Taking :
- How much work do you do on the box while lifting it? How much does gravity do?
- How much work do you and gravity each do while you carry it along the corridor?
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.
Given. , ; lift ; carry level.
Plan. for each force on each stage.
Step 1. Lifting, you. Force up, motion up: .
Step 2. Lifting, gravity. Force down, motion up: . The net work is zero, as it must be at a steady speed.
Step 3. Carrying. Your force is up and gravity is down, but the motion is level: both are at to it, so both do zero work.
Answer: (a) You: ; gravity: . (b) Zero for both.
Check. The box's kinetic energy never changes, so the net work at every stage is zero. It is.
Watch out. Carrying the box tires you, but that is work inside your muscles. On the box itself, a vertical force does no work over a level path.
Discussion (0)
Log in to join the discussion.
Loading…