Laws of Motion Questions and Answers Part-9

1. A plumb line is suspended from a ceiling of a car moving with horizontal acceleration of a. What will be the angle of inclination with vertical
a) \[\tan^{-1}\left(a\diagup g\right)\]
b) \[\tan^{-1}\left(g\diagup a\right)\]
c) \[\cos^{-1}\left(a\diagup g\right)\]
d) \[\cos^{-1}\left(g\diagup a\right)\]

Answer: a

2. Mass of a person sitting in a lift is 50 kg. If lift is coming down with a constant acceleration of \[10 m\diagup sec^{2}\]  .Then the reading of spring balance will be \[\left(g=10 m\diagup sec^{2}\right)\]
a) 0
b) 1000N
c) 100 N
d) 10 N

Answer: a
Explanation: R = m(g − a) = 0

3. A body of mass 2 kg has an initial velocity of 3 meters per second along OE and it is subjected to a force of 4 N in a direction perpendicular to OE. The distance of the body from O after 4 seconds will be
a) 12 m
b) 20 m
c) 8 m
d) 48 m

Answer: b

4. A block of mass m is placed on a smooth wedge of inclination \[\theta\] . The whole system is accelerated horizontally so that the block does not slip on the wedge. The force exerted by the wedge on the block (g is acceleration due to gravity) will be
a) \[mg\cos\theta\]
b) \[mg\sin\theta\]
c) mg
d) \[mg\diagup\cos\theta\]

Answer: d

5. A machine gun fires a bullet of mass 40 g with a velocity \[1200 m s^{-1}\]  . The man holding it can exert a maximum force of 144 N on the gun. How many bullets can he fire per second at the most
a) One
b) Four
c) Two
d) Three

Answer: d

6. An automobile travelling with a speed of 60 km/h, can brake to stop within a distance of 20 m. If the car is going twice as fast, i.e. 120 km/h, the stopping distance will be
a) 20 m
b) 40 m
c) 60 m
d) 80 m

Answer: d

7. A man of weight 75 kg is standing in an elevator which is moving with an acceleration of \[5 m s^{-2}\]  in upward direction the apparent weight of the man will be \[\left(g=10 m\diagup sec^{2}\right)\]
a) 1425 N
b) 1375 N
c) 1250 N
d) 1125 N

Answer: d
Explanation: The apparent weight, R = m(g + a) = 75(10 + 5) = 1125 N

8. The linear momentum p of a body moving in one dimension varies with time according to the equation \[p=a+bt^{2}\]  where a and b are positive constants. The net force acting on the body is
a) A constant
b) Proportional to \[t^{2}\]
c) Inversely proportional to t
d) Proportional to t

Answer: d

9. The spring balance inside a lift suspends an object. As the lift begins to ascent, the reading indicated by the spring balance will
a) Increase
b) Decrease
c) Remain unchanged
d) Depend on the speed of ascend

Answer: a
Explanation: When the lift moves upwards, the apparent weight, = m(g + a) . Hence reading of spring balance increases.

10. There is a simple pendulum hanging from the ceiling of a lift. When the lift is stand still, the time period of the pendulum is T. If the resultant acceleration becomes , g / 4 then the new time period of the pendulum is
a) 0.8 T
b) 0.25 T
c) 2 T
d) 4 T

Answer: c