# Mass and Weight

Mass of a body is the quantity of matter contained in the body. Mass of an object is constant and does not change from place to place. It remains the same whether the object is on earth, on moon or anywhere in outer space.

The mass of an object is measured with the help of a pan balance.

Mass of an object is the measure of its inertia. It means that greater the mass, the greater is the inertia of the object.

The weight of an object is the force with which it is attracted towards the earth.

Force = Mass × Acceleration

Therefore, F = mg

If weight of an object is denoted by W, then

W = mg

As weight is a force, therefore, its SI unit is the same as that of the force - newton (N). Force of weight acts vertically downwards. It has both magnitude and direction. The weight of an object is generally measured by a spring balance.

Weight of an object depends on its mass and value of g. As the value of g is constant at a given place, therefore, the weight of the object at a given place is directly proportional to its mass. However, the weight of an object will be different on different parts of the earth as the value of g is different on different parts of the earth.

### Weightlessness

You notice increase in weight while in moving in Lift or Elevator upward and decrease in weight when moving downward. Similar case you can experience in merry-go-round. Also, an astronaut experiences weightlessness in space.

According to the third law of motion the machine exerts an upward reaction 'R' on the boy which is equal to W. The weighing machine measures the reaction R, which is your weight. Now imagine that the floor below the weighing machine is suddenly removed. What would happen?

You would fall towards the earth with the same acceleration. In this case the you cannot exert a force on the weighing machine. The weighing machine in this case would show a zero weight. Thus, a body falling freely under gravity is weightless.

Why an astronaut experiences weightlessness in a spaceship? The spaceship with the astronaut falls freely towards the earth. Therefore, the astronaut appears to be floating weightlessly.