earth-gravityHave you ever tried to jump?

Probably, yes! And you know that when we jump from the ground, we just fall back after few seconds.children-jumping

Why? And How?

These are the questions that, almost every human had thought in his childhood. So, the simple answer to all these questions is “gravity”.

So, let’s learn something about gravity and the interesting history about it.

What is Gravity?

Gravity is the force that attracts us and other objects towards each other; much like a magnet, but not as a magnet. Then, why things in your room are not attracted to each other? It is because gravity depends on the mass; Massive objects have more gravity.

Gravitational force is so weak that the object must be very massive to have sufficiently strong gravity to attract other objects. Consider our earth, it so massive that its gravity pulls us towards the ground. When we jump, earth’s gravity pulls us and we fall back on the ground.

Importance of Gravity

What would happen, if gravity suddenly disappears? Take some time and think about it.

galaxyYou will be amazed to find out that, the current appearance of our earth, planets, and stars depends on the gravity. Without gravity, there will be no earth’s pull to attract you back to the ground when jumping. Even the planets and stars would not exist without gravity. Because planets and stars have been formed from the accumulation of cosmic dust and gas that was only possible due to gravity.

Here are some of the effects of gravity on our everyday life

  • We can stand, jump, walk, and run on the ground due to gravity. If the earth doesn’t pull you back while jumping, then you would never come back to the ground, and you may reach space in a single jump! Wow!
  • On the earth, gravity keeps the air around us as a blanket, and it prevents air from escaping into space.
  • Tides in the oceans are caused by the gravitational pull of moon and sun.

Universal Law of Gravitation

law-of-gravitationDon’t be afraid!

It is just a simple law that was proposed by a famous scientist Sir Isaac Newton. This law gives us the relation between the gravitational force, the mass of objects, and the distance between objects.

Here are two simple points that explain the law:

  • The gravitational force between massive objects is more as compared to light objects.
  • The gravitational force between two objects decreases exponentially as they go away from themselves.

Here is the mathematical expression of the universal law of gravitation:

F = G*M1*M2/r^2

G = Gravitational constant, whose value is equal to
M1 = mass of the first object
M2 = mass of the second object
r = distance between two objects

By knowing gravity, you can calculate when a ball will touch the ground after being released into the air. You can measure and graph the path of a projectile. You can calculate how much propellant is required to push a rocket into space, and many more.

Do Heavy and Light Objects Fall at the Same Time?

galileo-galilei-gravity-experimentThe simple answer is, yes. But let’s understand, why it happens.

Gravity at the surface of the earth is 9.8 m/s, which means an object falling towards earth gains 9.8 m/s speed in every second. If a ball is dropped from a building it will have 9.8 m/s speed at the end of the 1st second, 19.6 m/s at end of the 2nd second, and 29.4 at the end of the 3rd second and so on. Every object is accelerated at the same rate towards earth, whether it is a paper clip or a stone. If two objects are dropped from a height, they both will hit the ground at the same time.

But, wait a minute!

You may be thinking, why a paper falls slower than a paper clip when both are dropped from the same height? It is because of the air resistance. The paper clip has lower surface area than the paper, which results in higher air resistance for the paper. But, if you drop paper and paper clip into a vacuum chamber both will hit the bottom at the same time. It is because vacuum chamber does not have air, so there will be no air friction to slow any object.

Difference between Mass and Weight

mass and weightIn everyday life, you may have used the term mass and weight interchangeably. But in physics, mass and weight are two different quantities.

The mass of an object is the quantity of matter in it; usually measured in kilograms. But, weight is the force of an object that it is putting on a platform due to gravity.

For example, if you hold a brick in your hand you will feel a downward force. If you somehow take that brick to the moon, you will notice much less downward force. Because the moon has less gravitation than the earth. In both cases, the amount of the matter in the brick will be same on the earth and the moon.

Difference between Gravity and Gravitation

There is a difference between gravity and gravitation. Gravitation is the force of attraction between objects that have mass. But, gravity is the gravitational force between earth and other objects. It means that gravity is the special case of gravitation.

Gravitational force exists since the formation of our universe. It is one of the four fundamental forces of nature. It is the force that attracts atoms together to form stars and planets. It keeps the moon in orbit around the earth and the earth around the sun.

Fun Facts

  • Gravity keeps all the planets in the orbit of the sun, and it keeps our moon in orbit of the earth.
  • If any object goes upward with a velocity of 11 km/s, it will escape the gravitational pull of the earth. This velocity is called the escape velocity, and it is different for different planets and moon.
  • The gravity of our sun is about 274 m/s^2. Anything weighing 1 kg on the earth will weigh about 28 kg (If it was possible!).

A Brief History

Let’s learn something about the history of gravity, and find out about the people who contributed to explaining the gravity.

Who first pointed to the presence of gravity? And when? It is still unknown. But you may consider that early human beings had probably thought about it. Because, it is the force whose effects we can clearly see, like falling back to the ground after jumping.

  • The first person who gave the hypothesis about gravity was, Aristotle. He hypothesized that “heavier objects are accelerated more towards ground than lighter objects”. He was actually wrong because he didn’t know about air resistance which slows down the objects. But, he at least provided the ground for the great scientists to think and research about gravity.

  • In late 16th century, a famous scientist Galileo Galilei performed an experiment. In this experiment, he dropped two balls from a height and proved that light objects and heavy objects fall at the same rate.

    Galileo Galilei
  • In 1687, Sir Isaac Newton developed the universal law of gravitation. Which explained that lighter objects have less gravitational force of attraction than massive objects. Also, the gravitational force decreases exponentially as the distance between two objects increase. This is also known as the inverse square law.

    Isaac Newton
  • The modern understanding of gravity was described by Albert Einstein. He described the gravity as a curvature in space and time. This curvature is caused by the objects with mass; the massive objects cause more curvature than the lighter objects.

    Albert Einstein


What is Power?

In physics, power is the rate of doing work. Because work does not deal with time, it only tells us that how much energy is consumed in performing the work.

The work can be done at different rates. For example, you and your friend are pushing the boxes having the same weight. Now, your friend reaches the target in 60 seconds and you reach the same target in 100 seconds. Both of you have expanded the same amount of energy to push the box. But, your friend did the work faster than you, so its power was more than you.

How is Power Measured?

Power is measured in watts in the international system of units. One watt of power is equal to one joule energy expanded in one second. The mathematical formula for calculating the power is:

Power = Work/time
P = W/t

Example: Two persons are carrying boxes towards their home. The first person is reaching the home in 100 seconds, whereas the second person in 40 seconds. Calculate the power of both persons, if the work performed by both is 400 joules.


Power of the first person is:

P1 = work / time = 400 joules / 100 seconds = 4 watts

Power of the second person:

P2 = work / time = 400 joules / 40 seconds = 10 watts

Clearly, the second person is doing the work with more power than the first person.

The power is also measured in another unit, which is called horsepower. Horsepower is bigger unit than a watt and is mostly used for heavy duty machines. One horsepower is equal to 745.7 watts.

Electrical Power

Light-BulbOur rate of electricity consumption is also measured in watts. To find out how much power we are using, just multiply the current flowing in the circuit with the voltage of the circuit.


P = Voltage*Current


  • Chemicals that release energy very fast result into the explosions.
  • Your electricity bill is calculated in kilowatt-hours. This is the unit of energy, not the power.
  • In real practice, machines with high power consume more energy than the machines with low power to perform the same Because increasing power can decrease the efficiency.

worker-draging-loadYou might think about work as, reading books and doing homework. You may have also heard the sentence “hard work leads to success”. But, in physics work has its own meaning and it is different from the previously described example.

What is Work?

boy-kicking-a-ballIn physics, work is done when a force is applied to an object and it travels some distance. For example, you kicked a ball with a certain amount of force and ball traveled few meters, then you had done a certain amount of work on the ball.

How is Work measured?

Work is the scalar quantity, means it doesn’t need direction to define it. Work is measured in joule or newton-meter. This is the same unit in which energy is measured. The work done on an object is found by multiplying the force exerted on an object with the distance that it traveled.

Work = Force*distance

But, there are also some complications in calculating the work which may sometimes confuse you. Here are the few things to remember when calculating the work.

  1. If you are applying a force on an object but it is not moving, then work will be zero. Because object didn’t travel any distance. For example, you are pushing a wall with all of your force, but the wall is not moving. So, your work done will be zero.
  2. If you are applying a force on an object and it is moving in a path that brings it back from where it started. Then, work done on that object is also zero. Because the net distance traveled by the object is zero.
    For example, a race car is starting from a starting point and traveling in a circular path and it comes back again at the same starting point. Then, work done by the car is zero because it didn’t travel any net distance.

Example: A man is pushing a box with a force 50 newtons. The box has traveled the distance of 100 meters. Calculate the work done by the man.


Applied Force = 50 N

Distance covered = 100 meters

Work = Force*distance = 50*100 = 5000 joules or N-m


  • If a force is applied in opposite direction to the covered distance, then the work is said to be negative work.
  • An object that falls from a height also performs work due to gravitational force and height.
  • In space when an object is accelerated to a certain speed then the force is removed from it. But, the object doesn’t lose its speed because there is no any air friction in space. So, work done by objects flying in space is also zero.


What is Energy?

The ability to do work is known as energy. Anything that is doing something uses energy. Your body needs the energy to walk, think and lift your school bag. We get this energy from our food. All the mechanical objects such as cars, bikes, buses, and airplanes need some kind of energy source like gasoline to do their work.

Energy is measured in joules in the international system of units. But, there are also other units of energy like kilowatt-hour. This unit is mostly used is your home’s electricity meters. Your electricity bills are calculated according to the usage of energy in kilowatt-hour.

Types of Energy

There are many forms of energy including chemical, electrical, heat, light, kinetic, potential and nuclear energy. These different energy forms can be converted into each other. For example, we use electric heaters in winter to warm up our homes. These heaters use electrical energy and convert them into the heat energy. Let’s dig into each type of energy.

  • Chemical Energy: It is stored in the bonds of molecules. When chemical reaction happens, they release chemical energy. We use chemical energy in our car in form of gasoline to drive. Batteries also use chemical energy to produce electricity.
  • Electrical Energy: It is the widely used form of energy. Today it is very difficult to think about life without electricity. We are almost doing every work with electricity. Electrical energy is obtained from the movement of electrons. It is mostly produced from fossil fuels like coal and gasoline.
  • Heat Energy: We use this energy to cook our food and heat our homes during winter. Heat energy is obtained from molecules at high temperature. For example, electrical or gas stoves are used to heat up the pots for cooking food.
  • Light Energy: Light energy is the form energy that we are able to see. Our Earth receives a lot of light from the Sun. Light is also produced by electricity at homes. We can also generate electricity from light by using Photovoltaic cells.
  • Kinetic Energy: Kinetic energy is also known as the energy of motion. If an object is moving, it has kinetic energy. Cars moving on roads have kinetic energy due to their motion.
  • Potential Energy: Potential energy is also called stored energy. Energy stored in springs is potential energy. Also, objects at heights have also potential energy due to the gravity. This energy is released when objects fall.
  • Nuclear Energy: This energy is released during a nuclear reaction; fission and fusion. Nuclear reactions release a huge amount of energy. Many power plants across the globe utilize fission reaction to generate electricity.

Renewable and Non-renewable Energy

Energy is classified into two types, one in renewable energy and the other is non-renewable energy. Renewable energy is the types of energy whose resources are not consumed up and can be recycled.

Examples of renewable energy include solar energy, wind energy, and geothermal energy etc. Non-renewable energy sources are consumed in the process and can’t be recycled. Examples of this types of energy include coal and gasoline.

Goto: Renewable Energy section


  • People have been using wind energy for more than a thousand year.
  • One lightning bolt can power a 60 watts light bulb for six months.
  • Geothermal power plants are used to generate electricity by using the Earth’s internal heat.

Man lifting a boxIn everyday life, we walk, drive cars, and lift objects. All these tasks need a force to be performed. So, it is necessary to understand this force scientifically. This force was described scientifically by Isaac Newton. He described the force and its relationships in his laws that are known today as the Newton’s Laws of Motion.

First Law of Motion

velocity of carThe first law states that an object in motion will continue to move with same speed in the same direction until another force acts on it. Furthermore, an object which is at rest (not moving) will not move until some external force acts on it.

You may be thinking, how this is possible. Because as you kick a ball, it will immediately start to slow down and will come to rest at a distance. This happens because of air friction. As you kick a ball, frictional forces immediately start to oppose the motion of the ball until it comes to rest.

Second Law of Motion

boy-kicking-a-ballThe Second Law states that force is equal to mass times the acceleration. Let’s say it in other words, the more force is applied to an object the more acceleration will be produced in it. As a result, the object will speed up faster. For example, if you hit a ball harder, it will go faster and farther.

The Second law of motion can also be defined in the mathematical form. It is defined as, F = mass * acceleration.

F = m*a; m is the mass and “a” is the acceleration

Third Law of Motion (Every action has reaction)

You may be familiar with the statement of this law which states that “every action has equal and opposite reaction”. In other words, each applied force also has the opposing force, which has the same magnitude of applied force but in opposite direction. For example, when you kick a ball with 10 N force, your feet will also experience the same 10 N force from the ball.


  • First law of motion also tells us that, mass has a resistance to change in motion. This resistance is called inertia.
  • Sir Isaac Newton is also the founder of Calculus; a branch of mathematics which deals with the rate of change.
  • An object in motion experiences friction from the ground and air. Friction also helps our cars to slow down, when the brakes are pressed.