Question

A man wearing a bullet proof vest stands still on roller skates. The total mass is 80 kg . a bullet of mass 20 grams is fired at 400 m/s .it is stopped by the vest and falls to the ground . what is then the velocity of the man

Answer 1

Explanation:

m - mass of bullet = 0.02 kg

M - mass of man = 80 kg

u - initial velocity of bullet = 400 m/s

U - initial velocity of man =0 m/s

v - final velocity of bullet = 0 m/s

V - final velocity of man

Using conservation of momentum,

initial momentum = final momentum

m⋅u+M⋅U=m⋅v+M⋅V

0.02⋅400+80⋅0=0.02⋅0+80⋅V

8=80⋅V⇒V=0.1 m/s

Therefore, the man moves with a velocity of 0.1 m/s

Answer 2

Answer:

We know that,

Mass of man with vest is= 80 kg (m1)

The mass of bullet=20grams or 0.02 kg (m2)

The velocity of man is unknown, so= x m/s (v1)

The velocity of bullet is= 400m/s (v2)

So, mass x acceleration= momentum

=> m1 x v1 = m2 x v2

=> 80kg x xm/s = 0.02kg x 400m/s

=> 80 / 8 = xm/s

=> 10m/s or 0.1m/s = xm/s