Question

Two boys A and B are standing at two positions as shown in the diagram. Each step is 20 cm high and 20 cm wide. The potential energy is assumed to be zero at the ground. Consider $g=10m/s^2$ and assume that there is no loss of energy.


What is the kinetic energy of the ball at the point of release?

[1 mark]

• A. +8.5 J
• B. +6.0 J
• C. -6.0 J
• D. -8.5 J

Answer 1

The correct option is D -8.5 J

Initial position of the ball with respect to the ground,
$h_i=50+(20\times 8)=210cm=2.1m$

Final position of the ball with respect to the ground, $h_f=Levelofthethirdstepabovetheground$
$h_f=(20\times 2)=40cm=0.4m$

Change in potential energy, $\Delta P.E.=mg{h}_f-mg{h}_i=mg(h_f-h_i)$

$\Rightarrow \Delta P.E.=0.5\times 10\times (0.4-2.1)J\Rightarrow \Delta P.E.=-8.5J$