NCERT Class 11 Physics Solutions: Chapter 5 – Laws of Motion Part 1

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Question 5.1:

Given 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 floating on water.

(c) A kite skillfully held stationary in the sky.

(d) A car moving with a constant velocity of on a rough road.

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

Answer:

(a) A drop of rain falling down with a constant speed: Zero net force

Explanation:

The raindrop is falling with a constant speed. Hence, it acceleration is zero. As per Newton’s second law of motion, the net force acting on the raindrop is zero.

(b) A cork of mass 10g floating on water: Zero net force

Explanation:

The weight of the cork is acting downward. It is balanced by the buoyant force exerted by the water in the upward direction. Hence, no net force is acting on the floating cork.

(c) A kite skillfully held stationary in the sky: Zero net force

Explanation:

The kite is stationary in the sky, i.e. it is not moving at all. Hence, as per Newton’s first law of motion, no net force is acting on the kite.

(d) A car moving with a constant velocity of 30 km/h on a rough road: Zero net force

Explanation:

The car is moving on a rough road with a constant velocity. Hence, its acceleration is zero. As per Newton’s second law of motion, no net force is acting on the car.

(e) A high-speed electron in space far from all material objects, and free of electric and magnetic fields: Zero net force

Explanation:

The high-speed electron is free from the influence of all fields. Hence, no net force is acting on the electron.

Question 5.2:

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 answers change if the pebble was

thrown at an angle of 45° with the horizontal direction? Ignore air resistance.

Answer:

0.5 N, in vertically downward direction, in all cases Acceleration due to gravity, irrespective of the direction of motion of an object, always acts downward. The gravitational force is the only force that acts on the pebble in all three cases. Newton’s second law of motion as gives its magnitude:

Where,

The net force on the pebble in all three cases is 0.5 N and this force acts in the downward direction.

If the pebble is thrown at an angle of 45° with the horizontal, it will have both the horizontal and vertical components of velocity. At the highest point, only the vertical component of velocity becomes zero. However, the pebble will have the horizontal component of velocity throughout its motion. This component of velocity produces no effect on the net force acting on the pebble.

Question 5.3:

Given 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 with

(d) Lying on the floor of a train, which is accelerating, with the stone being at rest relative to the

train. Neglect air resistance throughout.

Answer:

(a) Just after it is dropped from the window of a stationary train: 1 N; vertically downward

Explanation:

Mass of the stone,

Acceleration of the stone,

As per Newton’s second law of motion, the net force acting on the stone,

(b) Just after it is dropped from the window of a train running at a constant velocity of 36 km/h: 1 N; vertically downward

Explanation:

The train is moving with a constant velocity. Hence, its acceleration is zero in the direction of its motion, i.e., in the horizontal direction. Hence, no force is acting on the stone in the horizontal direction.

The net force acting on the stone is because of acceleration due to gravity and it always acts vertically downward. The magnitude of this force is 1 N.

(c) Just after it is dropped from the window of a train accelerating with : 1 N; vertically downward

Explanation:

It is given that the train is accelerating at the rate of .

Therefore, the net force acting on the stone, F' = ma = 0.1 × 1 = 0.1 N

This force is acting in the horizontal direction. Now, when the stone is dropped, the horizontal force F,' stops acting on the stone. This is because of the fact that the force acting on a body at an instant depends on the situation at that instant and not on earlier situations.

Therefore, the net force acting on the stone is given only by acceleration due to gravity.

This force acts vertically downward.

(d) Lying on the floor of a train, which is accelerating, with the stone being at rest relative to the train. Neglect air resistance throughout: 0.1 N; in the direction of motion of the train

Explanation:

The weight of the stone is balanced by the normal reaction of the floor. The only acceleration is provided by the horizontal motion of the train.

Acceleration of the train,

The net force acting on the stone will be in the direction of motion of the train. Its magnitude is given by:

Answer:

Explanation:

When a particle connected to a string revolves in a circular path around a centre, the centripetal force is provided by the tension produced in the string. Hence, in the given case, the net force on the particle is the tension T, i.e.,

Where,

F is the net force acting on the particle.