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A rifle of mass 4.2 kg fires a bullet of mass 12 g at 750 m/s…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A rifle of mass fires a bullet of mass at .
- With what speed does the rifle recoil, if it is free?
- A shooter of mass holds it tight against the shoulder, so rifle and shooter recoil together. What is their recoil speed?
Need a reminder? The key ideaShow
Conservation of momentum. If no outside force acts on a system, its total momentum stays the same:
Bodies that stick together move off with one common velocity.
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. , , , shooter .
Plan. The momentum of the bullet forward must be balanced by the momentum of whatever recoils.
Step 1. The bullet's momentum. .
Step 2. A free rifle. , backward.
Step 3. Held firmly. , backward.
Answer: (a) . (b) .
Check. Holding the rifle tight makes the recoiling mass about times bigger, so the speed is about times smaller. That is why shooters hold rifles firmly.
Watch out. Change to before you divide.
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