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

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Question 5.13:

A man of mass 70 kg stands on a weighing scale in a lift which is moving

(a) Upwards with a uniform speed of

(b) Downwards with a uniform acceleration of

(c) Upwards with a uniform acceleration of . What would be the readings on the scale in each case?

(d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?

(a) Upwards with a uniform speed of :

Mass of the man,

Acceleration,

Using Newton’s second law of motion, we can write the equation of motion as:

Where,

is the net force acting on the man.

As the lift is moving at a uniform speed, acceleration;

∴ Reading on the weighing scale

(b) Downwards with a uniform acceleration of :

Mass of the man,

Acceleration, downward

Using Newton’s second law of motion, we can write the equation of motion as:

R = m(g – a)

= 350 N

(c) upwards with a uniform acceleration of :

Mass of the man,

Acceleration, upward

Using Newton’s second law of motion, we can write the equation of motion as:

R = m (g + a)

(d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity:

When the lift moves freely under gravity,

R = m(g – a)

The man will be in a state of weightlessness.

Question 5.13:

Figure 5.16 shows the position-time graph of a particle of mass 4 kg. What is the

(a) Force on the particle for?

(b) Impulse at t = 0 and t = 4 s? (Consider one-dimensional motion only).

(a) Force on the particle for:

For,

It can be observed from the given graph that the position of the particle is coincident with the time axis. It indicates that the displacement of the particle in this time interval is zero. Hence, the force acting on the particle is zero.

For,

It can be observed from the given graph that the position of the particle is parallel to the time axis. It indicates that the particle is at rest at a distance of 3 m from the origin. Hence, no force is acting on the particle.

For

It can be observed that the given position-time graph has a constant slope. Hence, the acceleration produced in the particle is zero. Therefore, the force acting on the particle is zero.

(b) Impulse at t = 0 and t = 4 s:

Impulse

Mass of the particle,

Initial velocity of the particle,

Final velocity of the particle,

∴Impulse

At,

Initial velocity of the particle,

Final velocity of the particle,

∴ Impulse