# 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?

**Answer:**

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;

**Answer:**

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

**Answer:**

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.

**Answer:**

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

**Answer:**

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.