Test: Relative Motion

10 Questions MCQ Test Physics Class 11 | Test: Relative Motion

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A boat sails across a river with a velocity of 10 km/hr. If resultant boat velocity is 14 km/hr, then what is the velocity of river water? 


As the boat moves at 10 kph across the river it is being swept downstream by the river current at a 90 degree angle. resulting i a motion of 14 kph. the downstream velocity of the river can be calculated by the Pythagoras theorem:




x=9.798 approximately


The angular speed of the earth about its own axis is:


Hour hand i.e- 60x60=3600sec
Time period=2π/angular speed.
T=2π/ ω
to complete one rotation, time taken=12 hours.
For 12 hours=12x60x60 sec
=43200 sec
Angular speed of hour hand=2π/T
(i.e., a complete round) =2π/43200
= π/21600 rad/s


Rain is falling vertically with the speed of 30 m/s. A man rides a bicycle with the speed of 10 m/s in east to west direction. What is the direction in which he should hold the umbrella?


When the man rides, he should put an umbrella in the opposite direction to which the drops are falling with respect to him.
Hence wrt man, the speed of rain drop is
at an angle of tan-1
Hence he would put umbrella to the direction of tan-1


The angular speed of a wheel of radius 0.50 m moving with the speed of 20 m/s is:


Radius r = 0.5 m
Speed, v = 20 m/s
We know,
v = r ω
ω = angular velocity
20 = 0.5 x ω
ω = 20/0.5 = 40 rad/sec


In 5 seconds a body covers a distance of 50 m and in 10 seconds it covers 80 m with uniform acceleration. What is the distance traveled in 15 seconds?


Correct Answer :- d

Explanation : Let the initial velocity of the body be u m/s; and the constant acceleration be a m/s². Then displacement (distance covered) of the body after t second is given by

s = u t + ½ a t²       —(1)

The body covers 50 m in 5s and 80 m in 10s.

50 = u×5 + ½ a 5²; => 50 = 5u + 25/2a

10u + 25a = 100     —(2)

80 = u×10 + ½ a 10²; => 80 = 10u + 50a; this gives,

u + 5a = 8  —(3)

Solving (2) and (3) for u and a we get

u = 12 m/s and a = -4/5 m/s²  — (4)

To obtain the distance covered in the next 15s we need to find the velocity at the beginning of this interval using,

v = u + a t => v = 12 m/s + (-4/5) m/s² × 15s = 0m/s

s = u t + ½ a t²

Substituting various values we get

s = 12×15 +½ ×(-4/5)×15² = 180 - 90

= 90 m


The angular speed of a flywheel making 210 revolutions per minute is:


As the flywheel makes 210 revolution in a minute, and during each revolution the angular displacement of the flywheel  is 2π thus total angular distance covered is
 210 × 22/7 × 2 = 1320 rad
Therefore angular speed=1320 rad/60 s = 22 rad/s


 A boat is moving with a velocity 14486_111 with respect to the ground. The water in the river is moving with a velocity 14486_0000000000000000000 with respect to the ground. What is the relative velocity of boat with respect to river?


Velocity of boat wrt river = velocity of boat wrt ground - velocity of river wrt ground
I.e. v = (6i + 8j) - (-6i - 4j)
= 12i + 12j


The angular speed of the second hand of a watch is :


The second hand goes through 2π radians in 1 min, or 2π radian/60 seconds, so ω = π/30 rad.s-1 = 0.03 rad.s-1


A monkey is running on the ground, next to a train, in the direction of the train. The speed of the monkey is 5km/h and the speed of the train is 100km/h. What is the velocity of the monkey with respect to the train?


Assuming the direction of velocity of train to be positive, wrt train the speed of monkey will be
-100 + 5 = -95 kmph
I.. 95 kmph in direction opposite to the train.


The angular speed of a wheel of radius 0.50 m moving with the speed of 20 m/s is:


Radius r = 0.5 m

Speed, v = 20 m/s

We know,

v = r ω


ω = angular velocity

20 = 0.5 x ω

ω = 20/0.5 = 40 rad/sec

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