Question

A car of mass $400kg$ and travelling at 72 kmph crashes into a truck of mass $4000kg$ and travelling at 9 kmph, in the same direction. The car bounces back at a speed of 18 kmph. The speed of the truck after the impact is

• A. 9 kmph
• B. 18 kmph
• C. 27 kmph
• D. 36 kmph

Answer 1

Answer: (B)

18 kmph

Answer 2

By the law of conservation of linear momentum

$m_{1}u_{1} + m_{2}u_{2} = m_{1}v_{1} + m_{2}v_{2}$

⇒ $400 \times 72 + 4000 \times 9 = 400 \times ( - 18) + 4000 \times v_{2}$

⇒ $v_{2} = 18km/h$.