Question

A car of mass 400 kg travelling at 72 km/h crashes into a truck of mass 4000 kg travelling at 9 km/h in the same direction. the car bounces back at a speed of 18 km/h. the speed of the truck after the impact is

Answer 1

In the first question, the acceleration is not zero. It maybe zero before the car hits the wall, but when it hits the wall, the car's speed goes from 50 mph to 0 mph in a very short space of time. This is a large deceleration (i.e. acceleration in the opposite direction to its movement), hence it experiences a very large force in the opposite direction of movement. This is why having a large crumple zone at the front of the car is important as it allows the car to decelerate more slowly.

In the second question, as soon as you let go of the ball, it is no longer accelerating (since there is no force acting on it). It will continue at the same speed that it had when it left your hand. Similarly, on Earth, when you throw a ball, it is travelling fastest at the instant it leaves you hand. It slows due to friction with the air and then the ground.

Answer 2

HELLO.....

Let mass of car m1= 400 kg,
Initial velocity of car u1=70km/hr =19.44 m/s
Let the mass of truck m2=4000 kg
Initial velocity of truck u2 = 9km/hr =2m/s
Final velocity of car v1=18km/hr = 5m/s
 velocity of truck v2 =?
According to law of conservation of momentum. Total momentum before collision is equal to total momentum after collision.


m1×u1+m2×u2=−m1×v1+m2×v2400×19.4+4000×2.5=−400×5+4000v24000v2 =7760+10000+2000v2=4.94m/sv2=4.94×185v2=17.784km/hr

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