# NCERT Class 9 Science Solutions: Chapter 9 – Force and Laws of Motion Part 3

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Question 6:

Q. A stone of 1 kg is thrown with a velocity of 20 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. what is the force of friction between the stone and the ice?

Initial velocity of the stone,

Final velocity of the stone, (Finally the stone comes to rest)

Distance covered by the stone, m

According to the third equation of motion:

Where,

Acceleration, a

The negative sign indicates that acceleration is acting against the motion of the stone.

Mass of the stone, m=1 kg

From Newton’s second law of motion:

Force,

Hence, the force of friction between the stone and the ice is

Question 7

Q. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate:

(a) The net accelerating force;

35000 N

Force exerted by the engine

Frictional force offered by the track

Net accelerating force,

Hence, the net accelerating force is 35000 N.

(b) the acceleration of the train; and

1.944

Acceleration of the train

The engine exerts a force of 40000 N on all the five wagons.

Net accelerating force on the wagons,

Mass of the wagons,

Mass of a

Number of wagons

Mass of the engine,

Total mass,

From Newton’s second law of motion:

Hence, the acceleration of the wagons and the train is 1.944 m/s2.

(c) The force of wagon 1 on wagon 2.

28000N

Mass of all the wagons except wagon 1 is

Acceleration of the wagons

Thus, force exerted on all the wagons except wagon 1

Therefore, the force exerted by wagon 1 on the remaining four wagons is 28000 N.

Hence, the force exerted by wagon 1 on wagon 2 is 28000 N.

Question 8:

Q. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7

Mass of the automobile vehicle,

Final velocity (Finally the automobile stops)

Acceleration of the automobile,

From Newton’s second law of motion:

Force

Hence, the force between the automobile and the road is in the direction opposite to the motion of the automobile.