# NCERT Class 11 Physics Solutions: Chapter 5 – Laws of Motion Part 3

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Question 5.8:

The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg.

Initial speed of the three-wheeler,

Final speed of the three-wheeler,

Time,

Mass of the three-wheelers,

Mass of the driver,

Total mass of the system,

Using the first law of motion, the acceleration (a) of the three-wheeler can be calculated as:

v = u + at

The negative sign indicates that the velocity of the three-wheeler is decreasing with time.

Using Newton’s second law of motion, the net force acting on the three-wheeler can be calculated as:

The negative sign indicates that the force is acting against the direction of motion of the three-wheeler.

Question 5.9:

A rocket with a lift-off mass 20,000 kg is blasted upwards with an initial acceleration of Calculate the initial thrust (force) of the blast.

Mass of the rocket,

Initial acceleration,

Acceleration due to gravity,

Using Newton’s second law of motion, the net force (thrust) acting on the rocket is given by the relation:

Question 5.10:

A body of mass 0.40 kg moving initially with a constant speed of to the north is subject to a constant force of 8.0 N directed towards the south for 30 s. Take the instant the force is applied to be t = 0, the position of the body at that time to be, and predict its position at .

Mass of the body,

Initial speed of the body,

Force acting on the body,

Acceleration produced in the body,

( I ) At t = –5 s

Acceleration, and

( ii ) At,

Acceleration, and

For

As per the first equation of motion, for ,final velocity is given as:

Velocity of the body after

For motion between 30 s to 100 s,

i.e., in 70 s:

∴ Total distance,
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