National Association of Rocketry (NAR) Level 2 Practice Exam

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If a motor has total impulse 320 N·s and burn time 5 s, what is the average thrust?

32 N

64 N

Average thrust is found by dividing the motor’s total impulse by how long it burns. Total impulse tells you the total "push" over the entire burn, and dividing by the burn time gives the steady force that would produce that same push on average.

So, with a total impulse of 320 N·s and a burn time of 5 s, the average thrust is 320 divided by 5, which equals 64 N. This makes sense because the five-second burn over 320 N·s spreads out to about 64 N of force at any moment on average.

If you see other numbers, they come from using a different burn time: 32 N would come from dividing by 10 s, 128 N from 2.5 s, and 160 N from 2 s.

128 N

160 N