An 8.0 g bullet is fired into a 1.6 kg block hanging on strings, and stays inside…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
An bullet is fired into a block hanging on strings, and stays inside. The block swings up . Taking , how fast was the bullet moving?
Need a reminder? The key ideaShow
In a collision with no outside force, momentum is conserved: . In an elastic collision kinetic energy is conserved too. Head-on and elastic, with the target at rest: , .
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.
Given. , , .
Plan. Work backwards: swing, then hit.
Step 1. The swing. .
Step 2. The hit. .
Answer: About .
Check. Of the bullet's , only goes into the swing: about .
Watch out. Energy for the whole process would give , far too slow.
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