A stream of bullets from an M16 assault rifle, each with a mass of 4.00 grams, i

Question

A stream of bullets from an M16 assault rifle, each with a mass of 4.00 grams, is fired at the bottom of a steel plate with a mass of 10.0 kg. The steel plate is held in a horizontal position by two low friction bearings that constrain it from moving horizontally but which allow it to move freely vertically. If the bullets hit the bottom of the plate at near-normal incidence (perpendicular to the plate) with a speed of 904 m/s and bounce off elastically, how many bullets must be fired at the plate each second to keep it suspended?

Explanation / Answer

this is just a momentum problem.

change in momentum for one bullet = 2mv =2*0.004*904 = 7.232

force on the plate must equal mg = 10*9.8 = 98

now... force on bullets = force on plate

change in momentum of bullets = force on plate * time

number of bullets * 7.232 for each bullet = 98 * 1 second

number of bullets = 98 * 1 / 7.232 = 13.55 bullets per second

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