Ared cart of mass 4 kg slides to the right on a frictionless track with a velocity of 4 m/s. it collides with the stationary blue cart of mass 1 kg. after the collision, the blue cart has a velocity of 8 m/s. calculate the velocity of the red cart after the collision.

Speed of the red cart  after collision = 2 m/s

Explanation:

Given that

Mass of red cart,m₁ = 4 kg

u₁=4 m/s

Mass of stationary blue cart ,m₂ =1 kg

u₂ = 0 m/s

After collision speed of blue cart , v₂ =8 m/s

Speed of the red cart after collision = v₁

There is no any external force on the masses that is why the linear momentum will be conserve.  

Initial linear momentum = Final linear momentum

P = m v

m₁u₁+m₂u₂ = m₁v₁+m₂v₂

4 x 4 + 1 x 0 = 4 x v₁+ 1 x 8  

v₁ =2 m/s

Speed of the red cart  after collision = 2 m/s

the answer is b, have negligible mass