Force and Laws of Motion — Science STD 9 — Question

Why does an object float or sink when placed on the surface of water?

A man standing 825 metres away from a cliff (steep rock) fires a gun. After how long will he hear its echo? Speed of sound in air is 33m/s.

Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

What will be the residue left on the watch glass on heating blue ink in it?

Explain why, a solution of salt in water is considered a mixture and not a compound.

brief state what happens when: Golgi apparatus is removed from the cell?

A stone is thrown in a vertically upward direction with a velocity of 5ms^-1. If the acceleration of the stone during its motion is 10ms^–2 in the downward direction, what will be the height attained by the stone and how much time will it take to reach there?

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

• Suma is swimming in a pond.
• A donkey is carrying a load on its back.
• A wind mill is lifting water from a well.
• A green plant is carrying out photosynthesis.
• An engine is pulling a train.
• Food grains are getting dried in the sun.
• A sailboat is moving due to wind energy.