Page 117 - Understanding NCERT Science 09

of the net force in a particular direction (thrust)
Example 9.4 Mass of an object is 10 kg.
What is its weight on the earth? and the force per unit area (pressure) acting
on the object concerned.
Solution: Let us try to understand the meanings of
thrust and pressure by considering the
Mass, m = 10 kg
Acceleration due to gravity, g = 9.8 m s –2 following situations:
Situation 1: You wish to fix a poster on a
W = m × g
-2
W = 10 kg × 9.8 m s = 98 N bulletin board, as shown in Fig 9.3. To do this
Thus, the weight of the object is 98 N. task you will have to press drawing pins with
your thumb. You apply a force on the surface
area of the head of the pin. This force is directed
Example 9.5 An object weighs 10 N when perpendicular to the surface area of the board.
measured on the surface of the earth. This force acts on a smaller area at the tip of
What would be its weight when the pin.
measured on the surface of the moon?

Solution:

We know,
Weight of object on the moon
= (1/6) × its weight on the earth.
That is,
W 10
W = e = N.
m
6 6
= 1.67 N.
Thus, the weight of object on the
surface of the moon would be 1.67 N.

1 Q
uestions
1. What are the differences between
the mass of an object and its
weight?
2. Why is the weight of an object on

the moon th its weight on the
6
earth?

9.5 Thrust and Pressure
Fig. 9.3: To fix a poster, drawing pins are pressed
with the thumb perpendicular to the board.
Have you ever wondered why a camel can run
in a desert easily? Why an army tank weighing
more than a thousand tonne rests upon a Situation 2: You stand on loose sand. Your
continuous chain? Why a truck or a motorbus feet go deep into the sand. Now, lie down on
has much wider tyres? Why cutting tools have the sand. You will find that your body will not
sharp edges? In order to address these go that deep in the sand. In both cases the
questions and understand the phenomena force exerted on the sand is the weight of your
involved, it helps to introduce the concepts body.

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