NCERT Solutions Curiosity Chapter 5 .6 Floating and Sinking (Fig. 5.15) — Activity 5.13: Let us investigate
Book page 765 Updated on2026-09-05
Q1.
Push the bottle in the water (Fig. 5.15). Do you feel an upward push?
Answer
Yes. As you press the closed empty bottle down into the bucket, your hand feels the water pushing the bottle upwards, against you — and the deeper you push it, the stronger that push becomes.
This upward push is the upthrust, or buoyant force. It is applied by the water on the bottle, and it acts in the direction opposite to the way you are pressing.
Q2.
Release the bottle. Does it bounce up?
Answer
Yes. The moment you let go, the bottle shoots back up and bobs on the surface of the water.
Why it happens: once your hand is removed, only two forces are left on the bottle — the Earth's pull downwards and the water's upthrust upwards. Held under, the upthrust on the closed, air-filled bottle is far greater than its weight, so the bottle is driven upwards until enough of it is out of the water for the two forces to balance. It then floats. In fact, all liquids apply a similar force on objects placed in them.
Try this: repeat with the same bottle filled to the brim with water and capped. Now it barely rises at all. The bottle occupies the same space either way — what has changed is the weight of the bottle, and so which of the two forces wins.