Chapter 6, Problem 61P

To determine

ToDetermine:

The total force resisting the motion.

Answer to Problem 61P

Solution:$\text{2686}\text{N}$

Explanation of Solution

Given:

Boat power rated $\text{=35}\text{hp=35×746}\text{W}$

Speed of boat $\text{=35}\text{km/h}$

  $\text{=}\frac{\text{35×1000}\text{m}}{\text{60×60 s}}$

  $\text{=9}\text{.72}\text{m/s}$

Formula used:$P=\overrightarrow{F}.\overrightarrow{V}=FV \cos \theta$

Resistive force acts opposite to Velocity so $\theta ={180}^0$

  $P=FV\cos {\left(180\right)}^{\text{0}}$

  $P=-FV$

  $F=-\left(\frac{P}{V}\right)$

Negative sign means opposite to velocity direction.

Calculation:

  $1hp=746W$

  $F=\frac{P}{V}=\frac{\text{35×746 W}}{\text{9}\text{.72 m/s}}$

  $F=\text{2686}\text{N}$

Conclusion:

When boat moves with constant velocity then frictional force oppose the motion. So to maintain constant speed, some energy is supplied.