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A 3.5 kg box rests on a level table. Taking g = 9.8 m/s², find the normal reaction when nothing…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A box rests on a level table. Taking , find the normal reaction when
- nothing else acts on it;
- you press down on it with ;
- you pull straight up on it with ;
- you pull it with at above the horizontal, and it stays on the table.
Need a reminder? The key ideaShow
The normal reaction is the push of a surface, at right angles to it. It is only as big as it needs to be: find it from the forces across the surface, which balance if the body does not move off the surface.
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. , so .
Plan. Balance the vertical forces in each case: is whatever is needed.
Step 1. (a) .
Step 2. (b) .
Step 3. (c) .
Step 4. (d) The upward part of the pull is , so .
Answer: (a) . (b) . (c) . (d) .
Check. Pushing down raises ; pulling up lowers it. This matters later, because friction depends on .
Watch out. Writing out of habit gives the wrong friction in many problems. Always find from the forces.
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