Work And Energy — Science STD 9 — Question

Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?

A hockey ball of mass 200g travelling at 10m s^-1 is struck by a hockey stick so as to return it along its original path with a velocity at 5m s^-1. Calculate the magnitude of change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.

Why are plant and animal tissues different?

Does the force of gravitation exist at all the places of the universe?

 What is the cause of buoyant force?

Each beehive consists of thousands of hexagonal thin-walled fragile cells. Name the material which is used to make these cells and mention the significance of the hexagonal shape of the cell.

Why do the passengers in a bus tend to fall backward when it starts suddenly?

A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground. Calculate the work done by him on the luggage.

State the various effects of force.