Chapter 2, Problem 43P

To determine

The time between each photo flash of apple.

The number of photo flashes per second.

Answer to Problem 43P

The time between each photo flash of apple is 0.039 seconds.

Number of photo flashes per seconds is 25.

Explanation of Solution

Given:

Initial speed, $u=0\text{ m/s}$

Height between the successive apples, $h=0.75\text{ cm}$

Formula used:

The height of free- falling body any instant of time is calculated as,

  $h=ut+\frac{g{t}^2}{2}$

  $N=\frac{1}{t}$

Where, $N$ = number of flashes per second

  $u$ = initial speed

  $g$ = acceleration due to gravity

Calculation:

The interval between the any two successive flashes is calculated as,

  $\begin{array}{l}h=ut+\frac{g{t}^2}{2}\\ 0.75\times {10}^{-2}=0\times t+\frac{9.8\times t^2}{2}\\ t=\sqrt{\frac{0.75\times {10}^{-2}\times 2}{9.8}}\\ t=0.039\text{ s}\end{array}$

The number of flash per second is calculated as,

  $\begin{array}{l}N=\frac{1}{t}\\ N=\frac{1}{0.039\text{ s}}\\ N=25{\text{ s}}^{-1}\end{array}$

Conclusion:

The time between each photo flash of apple is 0.039 seconds. The number of flashes per second is 25.