Page 102 - Understanding NCERT Science 09

Activity ______________ 8.2 five-rupees coin if we use a one-rupee coin, we
find that a lesser force is required to perform
• Set a five-rupee coin on a stiff card the activity. A force that is just enough to cause
covering an empty glass tumbler a small cart to pick up a large velocity will
standing on a table as shown in produce a negligible change in the motion of a
Fig. 8.7. train. This is because, in comparison to the
• Give the card a sharp horizontal flick cart the train has a much lesser tendency to
with a finger. If we do it fast then the change its state of motion. Accordingly, we say
card shoots away, allowing the coin to
fall vertically into the glass tumbler due that the train has more inertia than the cart.
to its inertia. Clearly, heavier or more massive objects offer
• The inertia of the coin tries to larger inertia. Quantitatively, the inertia of an
maintain its state of rest even when object is measured by its mass. We may thus
the card flows off. relate inertia and mass as follows:
Inertia is the natural tendency of an object to
resist a change in its state of motion or of
rest. The mass of an object is a measure of
its inertia.

Q inertia: (a) a rubber ball and a

Fig. 8.7: When the card is flicked with the uestions
1. Which of the following has more
finger the coin placed over it falls in the
tumbler. stone of the same size? (b) a
Activity ______________ 8.3 bicycle and a train? (c) a five-
rupees coin and a one-rupee coin?
• Place a water-filled tumbler on a tray. 2. In the following example, try to
• Hold the tray and turn around as fast identify the number of times the
as you can. velocity of the ball changes:
• We observe that the water spills. Why? “A football player kicks a football
to another player of his team who
Observe that a groove is provided in a kicks the football towards the
saucer for placing the tea cup. It prevents goal. The goalkeeper of the
the cup from toppling over in case of opposite team collects the football
sudden jerks. and kicks it towards a player of
his own team”.
8.3 Inertia and Mass Also identify the agent supplying
the force in each case.
All the examples and activities given so far 3. Explain why some of the leaves
illustrate that there is a resistance offered by may get detached from a tree if
an object to change its state of motion. If it is we vigorously shake its branch.
at rest it tends to remain at rest; if it is moving 4. Why do you fall in the forward
it tends to keep moving. This property of an direction when a moving bus
object is called its inertia. Do all bodies have brakes to a stop and fall
the same inertia? We know that it is easier to backwards when it accelerates
push an empty box than a box full of books. from rest?
Similarly, if we kick a football it flies away.
But if we kick a stone of the same size with 8.4 Second Law of Motion
equal force, it hardly moves. We may, in fact,
get an injury in our foot while doing so! The first law of motion indicates that when an
Similarly, in activity 8.2, instead of a unbalanced external force acts on an object,

FORCE AND LAWS OF MOTION 91

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