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NCERT Class 9 Science Solutions: Chapter 9 – Force and Laws of Motion Part 3
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, u = 20 m/s
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 -4 N.
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:
28000 N
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.