Question

An object of mass $40kg$ is raised to a height of $5m$ above the ground. What is its potential energy initially and at half way point? If the object is allowed to fall, find its kinetic energy when it is half-way down.

• A. $1960J,1000J,960J$
• B. $1960J,960J,1000J$
• C. $1960J,980J,980J$
• D. $1960J,1960J,0J$

Answer 1

The correct option is C $1960J,980J,980J$

Gravitational potential energy is given by the expression, $W=mgh$
where,
$h$ is the vertical displacement $=5m$
$m$ is the mass of the object $=40kg$
$g$ is the acceleration due to gravity = $9.8m{s}^{-2}$
$\therefore W=40\times 5\times 9.8=1960J.$
At half-way down, the potential energy of the object will be $\frac{1960}{2}=980J$.
At this point, the object has an equal amount of potential and kinetic energy.
This is due to the law of conservation of energy.
Hence, half-way down, the kinetic energy of the object will be $980J.$