Question

A trolley of mass $60kg$ moves on a smooth horizontal surface and has kinetic energy $120J$.A mass of $40kg$ is lowered vertically on to the trolley. The total kinetic energy of the system after lowering the mass is:

• A. $60J$
• B. $72J$
• C. $120J$
• D. $144J$

Answer 1

The correct option is B $72J$

KE of trolley $=120=0.5m{V}^2$

$\therefore 120=0.5\times 60\times V^2$

$V=2m/s$

Using momentum conservation:

$60\times 2=100\times v$
$1.2m/s=v$

$(K.E)total=\frac{1}{2}\times 100\times (1.2{)}^2$ $=72J$