Question

An automobile engine propels a 1000 kg car A along a leveled road at a speed of $36km{h}^{-1}$. The frictional force is 100 N. Suppose after traveling a distance of 200 m, this car collides with another stationary car B of the same mass and comes to rest. Let its engine also stop at the same time. Now, car B starts moving on the same level road without getting its engine started. Find the speed of the car B just after the collision.

• A. 36 km/h
• B. 72 km/h
• C. 18 km/h
• D. 100 km/h

Answer 1

The correct option is A 36 km/h

mass of car A $\left(m_A\right)=$mass of car B $\left(m_B\right)=$1000kg

initial velocity of car A $\left(u_A\right)=36km/h=10m/s$

initial velocity of car B $\left(u_B\right)=0m/s$

final velocity of car A $\left(v_A\right)=0m/s$

final velocity of car B $\left(v_B\right)=?$

Applying law of conservation of momentum.

$m_A{u}_A+m_B{u}_B=m_A{v}_A+m_B{v}_B$

or, $1000\times 10+1000\times 0=1000\times 0+1000\times v_B$

$v_B=10m/s=36km/h$

The speed of car B after collision is 10 m/s = 36 km/h