A bullet of mass 20 g is moving horizontally at a speed of 775 m/s when it strik

Question

A bullet of mass 20 g is moving horizontally at a speed of 775 m/s when it strikes a 4 kg block at rest.

(a) The block is made of wood; the bullet penetrates and stops. What is the initial speed of the bullet-block combination?

___ m/s

(b) The block is made of a very hard steel, as is the bullet, and the collision is perfectly elastic. What is the resulting velocity of the block? [Hint: Use the approximation that the mass of the bullet is much less than the mass of the block.]

____m/s in the original direction of motion of the bullet

Explanation / Answer

a)   Conservation of momentum to figure initial velocity of block-bullet system.

m1u1 + m2u2 = (m1 + m2)v

if m1 is the bullet and m2 is the block and we u2 = zero

v = m1u1 / (m1 + m2)

v = 0.020(775) / (0.02 + 4)

v = 3.86 m/s

b) v=m1u1 / (m1 + m2) =0.020(775) / (0.02 + 4) =3.875 m/s

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