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An automobile engine propels a 1000 kg car (A) along a levelled road at a speed of 36 km h–1. Find the power if the opposing frictional force is 100 N. Now, suppose after travelling a distance of 200 m, this car collides with another stationary car (B) of 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.

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m(A)= m(B) =1000 kg.           v = 36 km/h =10 m/s 

Frictional force = 100 N 

Since, the car A moves with a uniform speed, it means that the engine of car applies a force equal to the frictional force 

Power =  Force × distance / time =  F . V  

= 100 N × 10 m/s 

= 1000 W 

After collision 

mA uA + mB uB = mA vA + mB vB

1000 ×10 +1000 × 0 = 1000 × 0 + 1000 × vB 

vB = 10 m s–1

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