Question

An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is halfway down.

Answer 1

Step (1) Given data:

Mass of the object, $m=40kg$

Height, $h=5m$

Step (2) Finding the potential energy of the object:

Potential energy (K.E) is given by the expression,

$K.E=m\times g\times h$ where,

• $m$ = Mass of the object
• $h$ = Vertical displacement
• $g$ = Acceleration due to gravity (9.8 ms^-2)

$$\begin{gathered} P.E=40\times 9.8\times 5 \\ P.E=1960J \end{gathered}$$

Step (3) Finding the Kinetic energy of the item at halfway down:

At this time, the item has an equal quantity of potential energy and Kinetic energy.

This can be due to the law of conservation of energy. Hence, halfway down, the K.E. of the object can be calculated as

$$\begin{gathered} \frac{1960J}{2} \\ \Rightarrow 980J \end{gathered}$$

The kinetic energy of the object halfway down = 980 J.