CBSE Class 11-science Answered
Please suggest crossing the river problems & holding the umbrella problems in refrence to relative motion.
Asked by saurabh.mahadane | 01 Sep, 2019, 01:42: PM
Expert Answer
Qn.1
Rain is falling vertically with a speed of 30 m s-1. A woman rides a bicycle with a speed of 10 m s-1
in the north to south direction. What is the direction in which she should hold her umbrella ?
Let vw be the velocity of woman riding bicycle at a speed of 10 m/s , from north to South.
Let vR be the rain veocity 30 m/s, falling vertically.
Relative velocity of rain with respect to woman = VR - VW
Vector diagram shown in figure shows the direction of ( VR - VW ).
Angle made by ( VR - VW ) with vertical is given by, tanθ = VW / ( VR - VW ) = 10/30 or θ = tan-1(0.33) ≈ 18°
Hence umbrella should be hold at angle 18° with vertical towards south
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Qn. 2
A man can swim with a speed of 4.0 km/h in still water. How long does he take to cross a river 1.0 km wide
if the river flows steadily at 3.0 km/h and he makes his strokes normal to the river current?
How far down the river does he go when he reaches the other bank ?
Time taken by the swimmer to cross the river = 1 km / 4 km/hr = 0.25 hr = 15 min
distance travelled along the river direction when he reaches other bank = 0.25 hr × 3 km/hr = 0.75 km = 750 m
Answered by Thiyagarajan K | 01 Sep, 2019, 05:06: PM
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