A ball is dropped from rest from a height of 28 m.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A ball is dropped from rest from a height of . Ignoring air and taking , use energy to find its speed
- just before it reaches the ground;
- when it is above the ground.
Need a reminder? The key ideaShow
When only conservative forces (gravity, springs) do work, mechanical energy is conserved: . Forces that do no work, like a smooth surface's push or a string at right angles to the motion, do not spoil it.
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. , where is the distance fallen.
Step 1. (a) .
Step 2. (b) : .
Answer: (a) . (b) .
Check. The mass cancels, so a heavy ball and a light one fall alike (without air).
Watch out. Use the distance fallen, not the height above the ground.
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