Question

An object of mass 20 kg is raised to a height 5 m above the ground. What is its P.E.? If object is allowed to fall, find its K.E. just before touching the ground and also its K.E. when it is half way down. [3 MARKS]

Answer 1

Each point: 1 Mark

1. Potential energy at the height h = mgh
Where,
m = Mass of the object = 20 kg
h = Vertical displacement = 5 m
g = Acceleration due to gravity = $10m{s}^{-2}$

= 20×5×10

= 1000J;

2. When object touches to the ground complete potential energy transforms into the kinetic energy;

Kinetic energy of the object just before touching to the ground = Potential energy when object is at the top position = 1000J.

3. At half way both potential and kinetic energy will be equal.

Kinetic energy when object is half way down = $\frac{kineticenergyatthebottom}{2}$

$\frac{1000}{2}$

= 500 J

$\therefore$Kinetic energy at the half way = 500 J