Question

A body is falling from a height 20 m,
mass of the body is 400 g, if 20% energy
is lost during the time of striking the
ground, with how much kinetic energy
the body will move in the upward
direction ?​

Answer 1

GiveN:-

• A body has a mass of 400g .
• 20% energy is list on striking the ground.
• It is dropped from a height of 20m.

To FinD :-

• With how much Kinetic energy will the body move up upwards.

SolutioN :-

When a body is dropped from a height h , its velocity at the ground is √2gh ,

where

• g is acclerⁿ due to gravity.
• h is height from which it is dropped.

So , here velocity of body at ground = √ [ 2 × 10 × 20] = √400 = 20m/s .

Now we know that Kinetic energy is half the product of mass and square of velocity , i.e. ½mv².

So , the Kinetic energy :

= ½ × 400g × 20m/s².

= ½ × 400/1000 kg × 20m/s × 20m/s

= 10 × 20 × 0.4 J .

= 80 J .

Now , 20% energy is lost , So 80% energy is left .

So , Energy left = 80% of 80J

= 80/100 × 80 J .

=64 Joules .

Hence 64 J of energy was left with the ball.