Question

At a height of 20 m above the ground, an object of mass 4.0 kg is released from rest. It is travelling at a speed 20 m/s when it hits the ground. The object does not rebound and the gravitational field strength is 10 N/kg. how much energy is converted into heat and sound on impact?
(a) 40 J (b) 80 J
(c) 800 J (d) 1600 J

Answer 1

Answer:

mass, m = 1kg

height, s = 20m

initial velocity of ball, u = 0m/s

acceleration, a = 10m/s

2

Using, v

2

= u

2

+ 2as

= 0 + 2×10 ×20

= 400

v = 20m/s

Now, let us take upward direction as positive and downward direction as negative.

Initial momentum of the ball (before striking the ground) = m×v

= 1×(-20) (velocity is downward, hence negative)

= -20kgm/s

Now, when the ball strikes and rebounds, its velocity becomes +20m/s (after striking, it moves upward, hence positive velocity)

Final momentum (after striking the ground) = m×v

= 1×(+20)

= +20kgm/s

Change in momentum = Final momentum - Initial momentum

= 20kgm/s - (-20kgm/s)

= 40kgm/s in upward direction