A 100 kg train traveling at 20 m/s crashes into a stationary 400 kg car.
After the collision the car and the train are stuck together. What is
their velocity.

common velocity V = 4 m/s

Explanation:

Given that a 100 kg train traveling at 20 m/s crashes into a stationary 400 kg car.

Since both of them were stuck together after collision, the collision is inelastic.

The formula for inelastic collisions is:

M1U1 + M2U2 = ( M1 + M2 )V

We are told that the second car is stationary.

Therefore, U2 = 0

This will reduce the equation to:

M1U1 = (M1 + M2)V

Where V = common final velocity

M1 = 100 kg

M2 = 400 kg

U1 = 20 m/s

U2 = 0

Substitute all the parameters into the equation

100 × 20 = ( 100 + 400 ) V

2000 = 500V

Make the V the subject of formula

V = 2000/500

V = 4 m/s