Forces can be classified into two groups; CONTACT and NON-CONTACT FORSES.

• A CONTACT FORCE acts between two bodies which are touching. Examples of contact forces include; frictional forces, up thrust, normal reaction forces, tensions and forces in collisions.
• Non contact force or an action at a distance force acts between two bodies irrespective of whether they are touching each other or not. Examples of non contact force include; gravitational force, electrostatic force and magnetic force.

Note;

• A vector quantity is a quantity which has both magnitude and direction e.g. force, weight, velocity etc.
• A scalar quantity is a quantity which has a magnitude with no specific direction e.g. mass, speed etc.

1. Weight

Like any other planet in the solar system, the earth pulls every object from above it. This is called gravity pull.

The weight of an object is the gravitational pull of the object by the earth or any other massive bodies like moon and other planets. The weight of a body, W, is given by

                                         W=mg

Where m= mass of the body, g= acceleration due to gravity.

Acceleration due to gravity varies from planet depending on the mass of the planet.

On earth, the value of g= 10m/s². Meanwhile on the moon the value of g= 1.6m/s².

N.B: the mass of an object is constant everywhere meaning the mass of a bag of cement in the moon is the same as the mass of cement on earth.

Deep in space where acceleration due to gravity is ZERO, all objects become weightless. Objects can also feel weightless on earth. For example; if a boy is in a lift falling freely, his apparent weigh is zero because he moves towards the floor of the lift at the same rate as the lift moves away. If he was standing on a scale balance, the reading of the scale balance will be zero.

Differences between mass and weight

UPTHRUST

This is the upward force exerted on a body by fluid molecule when the body is partially or wholly immersed in the fluid as shown in the figure below; par Claude Foumtum