Question

A 20 kg mass is thrown from the ground to a height of 50 m. (a) find the kinetic energy of the mass at this height. (b) find the potential energy of the mass at this height. (c) find the velocity of the mass when it reach the ground.

Answer 1

Answer:

(a) 0 kJ

(b) 9.81 kJ

(c) 31.32 m/s

Explanation:

(a)

From the law of conservation of energy, energy can only be transformed from one state to another. At a height of 50 m, all the kinetic energy is converted to potential energy hence KE=0

(b)

Potential energy, PE=mgh where m is the mass, g is acceleration due to gravity and h is the height

Substituting 50 m for h and 20 Kg for m, taking g as 9.81 then

PE=20*9.81*50=9810 J=9.81 kJ

(c)

Relating the equation of potential energy to the equation of kinetic energy, which is $0.5mv^{2}$

$mgh=0.5mv^{2}$ where v is the velocity of the mass

$v=\sqrt {2gh}$

Substituting 50 m for h and taking g as 9.81 then

$v=\sqrt {2*9.81*50}=31.32091953\approx 31.32 m/s$