A force of 15 N pushes a door at right angles to it, at the handle, 0.85 m from the hinges…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A force of pushes a door at right angles to it, at the handle, from the hinges.
- What torque does it give about the hinges?
- What force, at right angles to the door from the hinges, gives the same torque?
Need a reminder? The key ideaShow
Torque about a point: , of size , which is the force times its moment arm (the perpendicular distance from the point to the force's line).
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. Torque is force times moment arm.
Step 1. (a) .
Step 2. (b) .
Answer: (a) . (b) , more than three times as much.
Check. The moment arm shrank by , so the force must grow by : .
Watch out. Pushing near the hinges is hard work not because the door is heavier there, but because the moment arm is short.
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