19. A crew team rows a boat at a rate of 20 km/h in still water. Beginning at t

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A crew team rows a boat at a rate of 20km/h in still water.Beginning at time =0minutes, the team rows for 30minutes up a river(against the current).The speed of the current is 1.5 km/h.How long will it take the crew team to row down the river(with the current)to return to thestarting point?

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20. If it were possible for a rocket to travel at the speed of light (3.0 x 108 m/s), about how long would it take to reach this speed from rest? Assume the rocket accelerates at a rate of 750 m/s2

May 18th, 2015

19.

Speed of boat = 20 km/h & speed of current = 1.5 km/h....

Since the boat traveled (30 minutes) against direction of current therefore  effective speed of boat was

20-1.5 =18.5 km/h

distance traveled in 30 minutes ( 0.5 hrs) against the current =18.5 x 0.5 = 9.25 km

NOw crew has to return 9.25 km along the direction of current hence speed will be 20+1.5 = 21.5 km/h

therefore time required to reach starting point = distance/speed = 9.25/21.5 = 0.4302 hrs    or     25.8 minutes


20.   v = u+at

3x 10^8 = 0+ 750 * t

hence,   t = 3x10^8/750 =400000 sec    =111.111 hrs


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May 18th, 2015

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