A 1500 kg truck traveling at 80 km/h collides with another car of mass 1000 kg traveling at 30 km/h in the same direction. The two cars stick together after the collision. Their speed immediately after the collision is

a. 40 km/h

b. 60 km/h

c. 110 km/h

d. 55 km/h

Answer: b. 60 km/h

Explanation:

Momentum = mass * velocity

Before collision,

Momentum of truck is

1500 * 80 = 120000 kg. km/h

Momentum of car is

1000 * 30 = 30000 kg. km/h

Since they are moving in the same direction, total momentum before collision is

120000 + 30000 = 150000 kg. km/h

After Collision, the two cars stick together after the collision. It means that they have a common velocity, V km/h. Therefore, momentum after collision is

V (1500 + 1000)

= 2500V kg. km/h

The law of conservation of momentum states that the total momentum of two objects before collision = the total momentum of the two objects after collision. Therefore

2500V = 150000

V = 150000/2500

V = 60 km/h