Page 65 - Innovator's Science-8

done with a spring balance or by comparing it with another object of
known mass using a beam balance. Since an object's weight is almost

the same everywhere on Earth, we can use its weight to find its mass in
most situations.
Floating and Sinking

While taking out water from a bucket filled with water using a mug, do
you notice that the mug feels lighter when it is inside water? Let us try to

understand this.
Take an empty plastic bottle with its lid closed and push it into a bucket

filled with water. You will feel an upward push from the water. When you
let go, the bottle quickly comes back to the surface. This shows that
water pushes the bottle upward with a force.

In fact, all liquids apply a similar force. The force applied by a liquid on
an object in the upward direction is known as up thrust or buoyant force.
When an object is placed in a liquid, the gravitational force due to the
Earth acts on it downwards. But a buoyant force is applied on it by the

liquid in the upward direction. If the gravitational force is more than the
buoyant force, the object sinks, but if the two forces are equal, the
object floats. One of the factors on which the buoyant force depends
upon, is the density of the liquid.

FRICTION
We see cycle slowing down after some time when we do not pedal, we

see rolling ball stopping after some time. These are few causes of
friction. But it plays important role in our life. When we stop pedaling,
the bicycle slows down and gradually comes to a stop. Why does this
happen? This happens because of a force called friction .

Not only vehicles, any object, moving over the surface of another object
slows down when no external force is applied on it. Finally it stops. Have
you seen a moving ball on the ground stopping after some time? Why do

we slip when we step on a banana peel? Why is it difficult to walk on a
smooth and wet floor.

We will find the answers to such questions in this chapter.
The force of friction always opposes the motion of an object.

• It tends to stop a moving object.

• It also tends to prevent a stationary object from moving.

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