Question

an object of mass 4 kg dropped from a height of 80m.find the value of kinetic energy when it has covered a distance equal to 3/4 of the height.(take g=10) class 9​

Answer 1

m=4kg

h=80m

g=10m/s^2

u=mgh

u=4×10×80

u=3200 J

Answer 2

Given,

Mass(m) = 4 kg

Height(h)= 80 M

Gravity constant(g) = 10

To find,

Kinetic Energy when distance is 3/4 of the height.

Solution,

Mass of the object = 4 kg

Height = 80m

Potential energy at free fall(P.E) = mass x g x height

Potential energy(P.E)= 4 x 10 x 80

Potential energy(P.E) = 3200 J

The kinetic energy When distance equals to 3/4 of height

Distance(d) = 3/4 x 80

Distance(d) = 60 m

Kinetic energy at certain distance(K.E1) =?

According to law of conservation of energy,

K.E at free fall + P.E at free fall = K.E. at distance + P.E1 at distance

0 + 3200 J=  K.E1 + mg(3/4 of h)

3200 J = K.E1 + 4 x 10 x (3/4 x 80)

3200 J = K.E1 + 40x 60

3200 J = K.E1 + 2400J

3200 J - 2400J= K.E1

K.E= 8000J

( This will be considered as the final result. As, the initial units are in SI system, we have to use the SI units as the final answer also. And, the SI unit of kinetic energy is Joule.)

Hence, the kinetic energy or K.E  will be equal to 8000J when it has covered the distance of 3/4 of height