A machine gun fires bullets of mass 25 g at 450 m/s, at a rate of 8 bullets every second.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A machine gun fires bullets of mass at , at a rate of bullets every second. What average force must the gunner apply to hold the gun still?
Need a reminder? The key ideaShow
Momentum is , a vector. A force acting for a time changes it. The change is the impulse:
Choose a positive direction and give every velocity its sign. For a force that changes with time, the impulse is the area under the force-time graph.
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. per bullet, , bullets per second.
Plan. The gun gives each bullet its momentum. By the third law the bullets push the gun back. The force is the momentum given out each second.
Step 1. One bullet. .
Step 2. Each second. .
Answer: The gunner must push forward with an average of .
Check. The units: per second is , a newton. Good.
Watch out. Change grams to kilograms first: .
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