What is the weight of an object?

Every object has a quantity known as the mass. The mass is simply the quantity of matter that it contains.

When according to the law of gravitation, a mass that is situated inside the gravitational field of the earth would experience an attractive force, a pull, as shown in fig 1 below.

Fig 1

Now this pull on the object is called the weight.

Hence the weight can be defined as the gravitational pull that the earth exerts on an object. 

Since it is gravitational pull it is a  thus a force and its unit is the Newton (N).

The weight is calculated according to the equation below:

weight = mass * acceleration due to gravity

w  = mg 

Example

A stone has a mass of 5.0 kg. Calculate it weight if the acceleration due to gravity is 9.81 m s^-2 .

Weight w = mg

               = 5.0 * 9.81

                = 49.05 N

                = 49 N

Gravitational potential energy

It is simply the energy that a body possesses due to its position above the surface of the earth. There is a second form of potential energy called elastic potential energy that we are going to see later.

The equation to calculate the gravitational energy is

Gravitational potential energy Ep = mgh

Where m is the mass of the object
g is the acceleration due to gravity
h is the height of the object above the ground


Let us have a look at example where the gravitational potential energy of an object.

Example 1

A high jumper has a mass of 62 kg. He jumps to a maximum height of 2.3 m. If the acceleration due to gravity is 9.81 ms^-2, calculate the gravitational potential energy of the object.

Gravitational potential energy Ep = mgh
= 62*9.81*2.3
= 1398 kg
= 1400 kg


You should also know how to obtain the mass or the height of an object if the gravitational potential energy is known.

Let us see an example.

Example 2

A person has a potential energy of 11000 J. If the mass of the person is 85 kg and the acceleration due to gravity is 9.81ms^-2, determine his height above the ground.

Gravitational potential energy Ep = mgh
11000 = 85*9.81*h
h = 11000/ (85*9.81)
` = 13.2 m
= 13 m

I hope you have understood. If you have any questions, leave them in the comment section below. See you later my students.

If you want to test your knowledge of gravitational potential energy, do the following questions leave the answer in the comment section?

Question

A bird of mass 0.12 kg is flying at a height of 12 m above the ground. If the acceleration due to gravity is 9.81 m s^-2, what would be the gravitational potential energy of the bird?

If the bird has a gravitational potential energy of 110 J, at what height would it be flying?