# 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.

**Answer:**

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.

**Answer:**

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 .

**Answer:**

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,