Physics: Bob can row 14 mph in still water. the total time to travel downstream and return upstream to the starting point is 4 hours. if the total distance downstream and back is 42 miles, determine the speed of the river (current speed).

Bob can row 14 mph in still water. the total time

esdancer3494

02.06.2020

7 mph

Explanation:

Let the speed of river is c mph.

The effective downstream speed = 14 + c

The effective upstream speed = 14 - c

Total time = 4 hrs

Total distance = 42 miles

Time for upstream + time for downstream = 4 hrs

21 / (14 + c) + 21 / (14 - c) = 4

21 (14 - c + 14 + c) = 4 (196 - c^2)

21 x 28 = 4 (196 - c^2)

c^2 = 196 -147 = 49

c = 7 mph

0,0(0 оценок)

yalley1982

18.07.2020

$4.2\times10^3 W$

$2.67\times 10^4 W$

Explanation:

Given:

Mass of the rider plus the bike, $m=2\times10^2 kg$

Speed of the motorbike, u= 20 m/s

Force of resistance, $F=2.1\times10^2N$

Power necessary to sustain speed can be calculated as follows:

1. When road is level

Power = force x velocity

P= F x u

$P=(2.1\times10^2N)(20 m/s) = 4.2\times10^3 W$

2. When the road is sloped at an angle 35°

An additional force will act due to component of gravity that is

$= mg sin \theta$

So,

$P = (F+mgsin\theta)u\\ P= (2.1\times10^2+2\times10^2\times 9.8 \times sin35^o)(20)=2.67\times 10^4 W$

0,0(0 оценок)