Chapter 11, Problem 72A

To determine

Initial speed of the ball.

Answer to Problem 72A

  $4.13\text{ m/s}$

Explanation of Solution

Given:

Mass of the ball, $m=10\text{ g}$

Height from which ball is thrown, $h=2\text{ m}$

Final speed of the ball, $v=7.5\text{ m/s}$

Formula used:

Third equation of motion:

  $v^2=u^2+2gh$

Here, v is final speed, u is initial speed, g is acceleration due to gravity and h is height.

Calculation:

Acceleration due to gravity on earth is $g=9.8{\text{ m/s}}^2$

Now, putting the value of v, g and h.

  $\begin{array}{l}{7.5}^2=u^2+2\times 9.8\times 2\\ 56.25=u^2+39.2\\ u^2=56.25-39.2\\ u^2=17.05\end{array}$

Taking square root on both sides.

  $\begin{array}{c}u=\sqrt{17.05}\\ =4.13\text{ m/s}\end{array}$

Conclusion:

Therefore, initial speed of the ball is 4.13 m/s.