Question

A bullet is shot into a block of plastic. The bullet penetrates the block 0.4 m. The mass of the bullet is 11 g. It is traveling with a speed of 540 m/s before it hits the block.

(a) Use kinematic equations to find the magnitude of the acceleration on the bullet as it is penetrating the block (ignore gravity, and assume that the force on the bullet as it penetrates the block is constant).

(b) Use Newton's Second Law to find the magnitude of the force exerted on the bullet by the plastic block.

Answer 1

Part A.

Using 3rd kinematic equation:

V^2 = U^2 + 2*a*S

U = Initial Velocity = 540 m/sec

V = final velocity = 0 m/sec

S = distance penetrated = 0.4 m

So, a = acceleration = ?

a = (V^2 - U^2)/2S

a = (0^2- 540^2)/(2*0.4)

a = -364500 m/sec^2

Magnitude of acceleration = |a| = 364500 m/sec^2

Part B.

Newton's 2nd law:

F = m*a

Magnitude of force will be

|F| = m*|a|

m = mass of block = 11 gm = 11*10^-3 kg

|F| = 11*10^-3 kg*364500 m/sec^2

|F| = 4009.5 N

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