Solution to Activity 9.4 of NCERT Class 9 Science Chapter 9 Gravitation

4. Complete Activity 9.4 (Page 108).

• Take an empty plastic bottle. Close the mouth of the bottle with an airtight stopper. Put it in a bucket filled with water. You see that the bottle floats.

• Push the bottle into the water. You feel an upward push. Try to push it further down. You will find it difficult to push deeper and deeper. This indicates that water exerts a force on the bottle in the upward direction. The upward force exerted by the water goes on increasing as the bottle is pushed deeper till it is completely immersed.

• Now, release the bottle. It bounces back to the surface.

• Does the force due to the gravitational attraction of the earth act on this bottle? If so, why doesn’t the bottle stay immersed in water after it is released? How can you immerse the bottle in water?

Answer:

Aim: To observe what happens when you push the bottle into the water and then release it and conclude based on that.

Materials Required: Empty plastic bottle, bucket with water.

Procedure:

(i) Place the empty plastic bottle with mouth closed securely in the bucket of water. Observe what happens.

(ii) Push the bottle deep into the water and observe what happens.

(iii) Next release the bottle and observe what happens.

Observations:

Initially the bottle is observed to float on the surface of the water.

Bottle floating on water in a bucket. — **Bottle Floats:** The bottle initially floats on the water. Let us find out why.

When pushed down into the water, an upward push is felt. This upward push becomes larger and larger as the bottle is pushed deeper till it is completely immersed.
When released the bottle bounces back to the surface.

Hand pushing bottle into a bucket of water and the bottle rising to the surface. — **Push the Bottle:** When you push the bottle into the water, it bounces back to the surface. Let us find out why.

Conclusions:

The force due to the gravitational attraction of the earth acts on this bottle.
The bottle initially floats because buoyant force acting upwards on the bottle is greater than its weight acting downwards.
Even when the bottle is immersed it bounces back up because buoyant force acting upwards on the bottle is greater than its weight acting downwards.
You can immerse the bottle in water by applying a force downwards which is at least equal to the difference between the upward force and the weight of the bottle.

“• Take an empty plastic bottle. Close the mouth of the bottle with an airtight stopper. Put it in a bucket filled with water. You see that the bottle floats.

• Now, release the bottle. It bounces back to the surface.