Chapter 5 Laws of Motion:

Question

1. Give the magnitude and direction of the net force acting on

(a) a drop of rain falling down with a constant speed

(b) a cork of mass 10 g floating on water

(c) a kite skillfully held stationary in the sky

(d) a car moving with a constant velocity of 30 km/h on a rough road

(e) a high-speed electron in space far from all material objects, and free of electric and magnetic fields.

2. A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble,

(a) during its upward motion

(b) during its downward motion

(c) at the highest point where it is momentarily at rest. Do your Solutions change if the pebble was thrown at an angle of 45° with the horizontal direction? Ignore air resistance

3. Give the magnitude and direction of the net force acting on a stone of mass 0.1 kg,

(a) just after it is dropped from the window of a stationary train

(b) just after it is dropped from the window of a train running at a constant velocity of 36 km/h

(c ) just after it is dropped from the window of a train accelerating with1 m s-2

(d) lying on the floor of a train which is accelerating with 1 m s-2, the stone being at rest relative to the train. Neglect air resistance throughout.

Answer 1

Q1

(a) Zero,

(b) Zero,

(c) Zero,

(d) Zero,

(e) Zero

Q2.

(a) F = mg = 10 × 5/100 = 0.5N (Vertically upwards)

(b)  F = mg = 10 × 5/100 = 0.5N (Vertically downwards)

(c) Even in this case, the net force is F = Mg = 0.49 N (vertically downwards)

If the pebble was thrown at an angle of 45° with horizontal, the pebble will have components of velocity along horizontal and vertical. These components of velocity along horizontal or along vertical do not lead to any force on the pebble. Therefore, if the pebble were thrown at some angle with the horizontal, the answer in case (a), (b) and (c) will not change.

Q3.

(a) F = mg = 0.1 × 10 = 1N(Vertically downwards)

(b) Same as in (a) (Vertically downwards)

(c) Same as in (a) because the force at an instant depends on the situation at that instant, not on the history.

(d) F = ma = 0.1 × 1 = 0.1N (the force of gravity is balanced by the normal reaction of the floor)