## Laws of Motion Questions and Answers Part-13

1. A man is standing on a balance and his weight is measured. If he takes a step in the left side, then weight
a) Will decrease
b) Will increase
c) Remains same
d) First decreases then increases

Explanation: Remains same

2. A man is standing at a spring platform. Reading of spring balance is 60 kg wt. If man jumps outside platform, then reading of spring balance
a) First increases then decreases to zero
b) Decreases
c) Increases
d) Remains same

Explanation: For jumping he presses the spring platform, so the reading of spring balance increases first and finally it becomes zero.

3. A cold soft drink is kept on the balance. When the cap is open, then the weight
a) Increases
b) Decreases
c) First increases then decreases
d) Remains same

Explanation: Gas will come out with sufficient speed in forward direction, so reaction of this forward force will change the reading of the spring balance.

4. Action and reaction forces act on
a) The same body
b) The different bodies
c) The horizontal surface
d) Nothing can be said

Explanation: The different bodies

5. A bird is sitting in a large closed cage which is placed on a spring balance. It records a weight of 25 N. The bird (mass m = 0.5 kg) flies upward in the cage with an acceleration of $2 m s^{-2}$  . The spring balance will now record a weight of
a) 24 N
b) 25 N
c) 26 N
d) 27 N

Explanation: Since the cage is closed and we can treat bird, cage and the air as a closed (isolated) system. In this condition the force applied by the bird on cage is an internal force, due to this the reading of spring balance will not change.

6. A light spring balance hangs from the hook of the other light spring balance and a block of mass M kg hangs from the former one. Then the true statement about the scale reading is
a) Both the scales read M/2 kg each
b) Both the scales read M kg each
c) The scale of the lower one reads M kg and of the upper one zero
d) The reading of the two scales can be anything but the sum of the reading will be M kg

Explanation: As the spring balance are massless therefore both the scales read M kg each

7. A machine gun fires 20 bullets per second into a target. Each bullet weighs 150 gms and has a speed of 800 m/sec. Find the force necessary to hold the gun in position
a) 800 N
b) 1000 N
c) 1200 N
d) 2400 N

Explanation: F = mnv = 150 * 10-3 * 20 * 800 = 2400 N

8. The tension in the spring is a) Zero
b) 2.5 N
c) 5 N
d) 10 N

Explanation: 5N force will not produce any tension in spring without support of other 5N force. So here the tension in the spring will be 5N only.

9. A book is lying on the table. What is the angle between the action of the book on the table and the reaction of the table on the book
a) $0^{\circ}$
b) $30^{\circ}$
c) $45^{\circ}$
d) $180^{\circ}$