Question

A block of mass 200kg approaches a man with a velocity of10ms"! He fires bullets
each of mass 10gm with a velocity of 200 ms. The number of bullets he must fire
at it to make the block just turn back is
1) 1000
2) 100
4) 1
3) 10
​

Answer 1

Given:

A block of mass 200 kg approaches a man with velocity of 10 meters per second.

Mass of bullet = 10 gm  Velocity of each bullet = 200 meters per second.

To Find:

The number of bullets the man must fire to make the block just turn back.

Solution:

Momentum = momentum of a object is defined as the product of mass of object and its velocity.

It is a vector quantity.Its unit is $\frac{kg m}{s}$.

Initial Momentum of block = m×v = 200 (-10) = -2000 Ns.

Initial momentum of bullet = 0.010(200)= 2 Ns.

Let the number of bullets fired = n

Mass of n bullets = nm

Initial momentum of all the bullets = n (2)  Ns

Final momentum of block = 0 (The block has to just turn back.)

Final momentum of bullets = 0

Since there is no external force , momentum is conserved.

Initial momentum of system = Final momentum of system

 -2000 + 2n = 0 + 0

n = 1000

The number of bullets the man must fire so as to make the block turn back is n = 1000

Answer 2

Explanation:

the number of bullets to turn back the block is 1000