Question

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

• A. $1000J$
• B. $2000J$
• C. $1500J$
• D. $100J$

Answer 1

The correct option is A $1000J$

Given, $massofobject,m=40kg$ ;
$height,h=5m$ ;

Acc. due to gravity, $g=10m{s}^{-2}$
Initial potential energy at a height of $5m$,

$E_{P1}=mgh=40\times 10\times 5=2000J$

Initial kinetic energy, $E_{K1}=0$

Final potential energy at a height of $2.5m$
$E_{P2}=mg{h}^{\prime }=40\times 10\times 2.5=1000J$

Final kinetic energy, $E_{K2}=?$

By conservation of energy,
$Initialmechanicalenergy=Finalmechanicalenergy$
or $E_{P1}+E_{K1}=E_{P2}+E_{K2}$
or $2000+0=1000+E_{K2}$
or $E_{K2}=2000-1000J=1000J$