High School Physics

# What is weight in physics class 9

The earth attracts every object towards its center with a force that depends on the mass of the body and the acceleration due to gravity (g) at that place. The weight of a body is the force with which it is attracted toward the center of the earth.

Definition of weight | What is weight in physics

The force of the earth’s gravity acting on a body is known as the weight of that body.

Formula for weight in physics

We know that, Force = mass × acceleration
The acceleration produced by the force of attraction of the earth is known as acceleration due to gravity and is written as ‘g’.

Thus, the downward force F acting on a body of mass ‘m’ is given by:
F = mass × acceleration due to gravity
or F = m × g
But, by definition, the force of attraction of the earth on a body is known as weight W of the body, so by
writing weight W in place of force in the above equation, we get :

Weight, W = m × g
where m = mass of the body
and g = acceleration due to gravity

Unit of weight | SI Unit of weight

Weight is measured in the same units as force. We know that the SI unit of force is Newton (N).

So, the SI unit of weight is also Newton which is denoted by the letter N.

Weight of 1 kg mass

Let us calculate the weight of the 1-kilogram mass.
We know that : Weight, W = m × g = 1 kg × 9.8 m/s2 = 9.8 × 1 kg × 1 m/s2
Now, by definition 1 kg × 1 m/s2 is equal to 1 Newton, so :
Weight, W = 9.8 Newtons (or 9.8 N)
Thus, the weight of a 1-kilogram mass is 9.8 Newton.

This means that the gravity force acting on a mass of 1 kilogram at the surface of the earth is 9.8 Newton.

Weight is a Vector Quantity

Weight is a vector quantity having magnitude as well as direction. The weight of a body acts in a vertically downward direction.

Weight of a body is not constant

The weight of a body is usually denoted by W. The weight of a body is given by W = m × g.
Since the value of g (the acceleration due to gravity) changes from place to place, therefore, the weight of a body also changes from place to place. Thus, the weight of a body is not constant.

Weight can be zero

In the interplanetary space, where g = 0, the weight of a body becomes zero and we feel true weightlessness. Thus, the weight of a body can be zero. Read more about weightlessness.

Weight and mass

Weight and mass are different. Read more on the difference between mass and weight here.

Weight of a body at the center of the earth

The value of acceleration due to gravity, g, decreases as we go down inside the earth and becomes zero at the center of the earth. So, irrespective of the weight of a body on the surface of the earth, its weight becomes zero when it is taken to the center of the earth (because the value of g is zero at the center of the earth).

The weight of a body becomes zero when it is taken to the center of the earth.

How is the weight of a body measured?

The weight of an object is measured with a spring balance.

The spring balance gives us the weight of the object because the extension of the spring depends on the force with which it is pulled downwards by the earth. Thus, it is the gravitational force acting on an object which operates a spring balance, and it is not its mass.

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