Question

A large 700 kg truck is driving with a speed of 20 m/s when it collides with a 300 kg car which is not moving. The two stick together. After the collision will the car and truck will have a speed Less than, Greater than. or Equal to 20 m/s?

Answer 1

The final speed after collision must be less than 20 m/s if momentum is to be conserved. In fact, it will be 14 m/s as explained below.

Explanation:

Before the collision, only the 700 kg truck has momentum (the car is not moving)

`m_t xx v_t = 700kg xx 20 m/s = 14 000 (kgm)/s`

After the collision, the combined mass of the two joined vehicles is 1000 kg, but the total amount of momentum must still be 14 000 kgm/s.

So the final speed must be

`(1000 kg) xx v_f = 14 000 (kgm)/s`

`v_f = 14 m/s`