I am a little confused over a problem I found whilst reading through an example A-level Mechanics exam paper.(adsbygoogle = window.adsbygoogle || []).push({});

The problem is on the Mechanics M2 paper, question 2.

This question asks for the magnitude of a resistive force that slowed a bullet, of mass 6 grams, from 400m/s to 250m/s.

If F = ma, then should this resistive force not be F = 0.006 * 150² = 135N, as the force to decelerate a mass is equal and opposite to that force required to accelerate it (ignoring air resistance, etc.)?

Should the momentum of this bullet not be P = mV = 0.006 * 400 = 2.4N ?

But the answer given in the paper is very different. The question and answer from the exam paper are:

Q. A bullet of mass 6 grams passes horizontally through a fixed, vertical board. After the bullet has traveled 2cm through the board its speed is reduced from 400m/s to 250m/s. The board exerts a constant force on the bullet.

Find, to 3 significant figures, the magnitude of this resistive force.

A. F * 0.002 = 0.5 * 0.006 (400² - 250²)

F = 14600N

The exam paper can be found here:

http://www.edexcel.org.uk/VirtualContent/83441/GCE_Mech_M1_M5_Specimen_Paper_mkscheme.pdf

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# Homework Help: Force Acting on Bullet

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