A lift of mass 840 kg hangs from its cable…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A lift of mass hangs from its cable. Taking , what is the tension in the cable when the lift
- speeds up going up, at ;
- moves up but slows down, at ?
Need a reminder? The key ideaShow
Draw a free-body diagram: the body alone, with every force on it. Then apply along the motion. For a body hanging from a rope or standing on a scale, with up positive:
What matters is the direction of the acceleration, not of the motion.
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. , .
Plan. With up positive, .
Step 1. Speeding up, going up. : .
Step 2. Slowing down, going up. The acceleration is downward, : .
Answer: (a) . (b) .
Check. At rest the tension would be . One answer is above it and one below, as expected.
Watch out. In (b) the lift moves up, but the acceleration is down. Use the sign of the acceleration, not of the motion.
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