Page 108 - Understanding NCERT Science 09

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• The SI unit of force is kg m s . This is also known as newton
and represented by the symbol N. A force of one newton
produces an acceleration of 1 m s –2 on an object of
mass 1 kg.
• The momentum of an object is the product of its mass and
velocity and has the same direction as that of the velocity.
Its SI unit is kg m s .
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• Third law of motion: To every action, there is an equal and
opposite reaction and they act on two different bodies.

Exercises

1. An object experiences a net zero external unbalanced force.
Is it possible for the object to be travelling with a non-zero
velocity? If yes, state the conditions that must be placed on
2. When a carpet is beaten with a stick, dust comes out of it,
Explain.
3. Why is it advised to tie any luggage kept on the roof of a bus
with a rope?
4. A batsman hits a cricket ball which then rolls on a level
ground. After covering a short distance, the ball comes to
rest. The ball slows to a stop because
(a) the batsman did not hit the ball hard enough.
(b) velocity is proportional to the force exerted on the ball.
(c) there is a force on the ball opposing the motion.
(d) there is no unbalanced force on the ball, so the ball
would want to come to rest.
5. A truck starts from rest and rolls down a hill with a constant
acceleration. It travels a distance of 400 m in 20 s. Find its
acceleration. Find the force acting on it if its mass is
7 tonnes (Hint: 1 tonne = 1000 kg.)
6. A stone of 1 kg is thrown with a velocity of 20 m s –1 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?
7. 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 and
(b) the acceleration of the train.
8. 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

FORCE AND LAWS OF MOTION 97

Rationalised 2023-24