Question

A boy releases a $0.5\text{kg}$ ball along the frictionless floor with the speed of $20\text{m/s}$. The ball is deflected by an obstacle on the way. After the deflection, it moves with $5\%$ of its initial kinetic energy. What is the final speed of the ball?

• A. $14.41\text{m/s}$
• B. $1.00\text{m/s}$
• C. $19.00\text{m/s}$
• D. $4.47\text{m/s}$

Answer 1

The correct option is D $4.47\text{m/s}$

Initial kinetic energy of the ball,

$K{E}_i=\frac{1}{2}m{v}^2=\frac{1}{2}\times 0.5\times {20}^2=100\text{J}$

After deflection, kinetic energy of the ball,

$K{E}_f=5\%$ of $K{E}_i$

$\Rightarrow \frac{1}{2}\times 0.5\times v_o^2=0.05\times 100=5$

$\Rightarrow v_o=2\sqrt{5}\text{m/s}=4.47\text{m/s}$