A passenger of mass 64 kg stands on a scale in a lift.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A passenger of mass stands on a scale in a lift. Taking , what does the scale read when the lift
- speeds up going up, at ;
- moves at a steady speed;
- speeds up going down, at ;
- falls freely, after the cable snaps?
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. (a) .
Step 2. (b) : .
Step 3. (c) : .
Step 4. (d) : . The passenger feels weightless.
Answer: (a) . (b) . (c) . (d) .
Check. At a steady speed the reading is the true weight, whether the lift moves up or down.
Watch out. The passenger's weight does not change at all. Only the scale's push changes.
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