 # Kinetic Energy

## Kinetic Energy Explanation

To accelerate an object we have to apply force. To apply force, we need to do work. When work is done on the object, energy is transferred and the object now moves with a new constant speed. The energy that is transferred is known as kinetic energy and it depends on the mass and speed achieved.

The definition of kinetic energy in Physics

“Kinetic Energy is the energy possessed by the body by virtue of its motion”

Kinetic energy is a scalar quantity. Kinetic energy is completely described by magnitude alone.

### Kinetic Energy Transformation

Kinetic energy is transferred between objects and can be transformed into other forms of energy. Yo-Yo is a great example to describe the transformation of kinetic energy. While beginning to play with it, one starts by letting it rest in the hand, at this point, all the energy is stored in the ball in the form of potential energy. Once the person drops the yo-yo the stored energy is transformed into kinetic energy, which is the energy of movement. Once the ball reaches the very bottom of the yo-yo all the energy is converted to kinetic energy. As it moves back to the hand, all the energy is once again converted to potential energy when it reaches the hand.

### Units of Kinetic Energy

• The SI unit of kinetic energy is Joule which is equal to 1 kg.m2.s-2.

• The CGS unit of kinetic energy is erg.

## Measuring Kinetic Energy

The kinetic energy equation is given as follows:

$KE=\frac{1}{2}mv^{2}$

where, KE is the kinetic energy, m is the mass of the body and v is the velocity of the body.

### Deriving Kinetic Energy Equation

Kinetic energy equation can be obtained by the basic process of computing the work (W) that is done by a force (F). If the body of mass m was pushed for a distance of d on a surface by applying a force that’s parallel to it, then the work done would be:

$W=F.d=m.a.d$

The acceleration in this equation can be substituted by the initial (vi) and final (vf) velocity and the distance. This we get from the kinematic equations of motion.

$W=m.a.d\\ \\ =m.d.\frac{v_{f}^{2}-v_{i}^{2}}{2d}\\ \\ =m.\frac{v_{f}^{2}-v_{i}^{2}}{2d}\\ \\ =\frac{1}{2}.m.v_{f}^{2}-\frac{1}{2}.m.v_{i}^{2}$

Simplifying the equation further, we get

$K.E=\frac{1}{2}mv^{2}$

Alternately, one can say that the total work that is done on a system is equivalent to the change in kinetic energy. This statement is equated as follows:

$W_{net}=\Delta K$

This equation is known as the work-energy theorem and has large applications even if the forces applied to vary in magnitude and direction.

Read More: Derivation Of Kinetic Energy

## Examples of Kinetic Energy

• A semi-truck travelling down the road has more kinetic energy than a car travelling at the same speed because the truck’s mass is much more than the car’s.
• A river flowing at a certain speed comprises kinetic energy as water has certain velocity and mass.
• The kinetic energy of an asteroid falling towards earth is very large.
• The kinetic energy of the aeroplane is more during the flight due to large mass and speedy velocity.

## Types of Kinetic Energy

There are five types of kinetic energy: radiant, thermal, sound, electrical and mechanical. Let us look at some of the kinetic energy examples and learn more about the different types of kinetic energy.

Radiant energy is a type of kinetic energy as it is always in motion travelling through medium or space. Examples of radiant energy are:

• Ultraviolet light
• Gamma rays

### Thermal energy

Thermal energy is also known as heat energy which is generated due to quick motion of atoms when they collide with each other. Examples of thermal energy are:

• Hot springs
• Heated swimming pool

### Sound energy

Sound energy is produced by the vibration of an object. Sound energy travels through the medium but cannot travel in vacuum as there are no particles to act as a medium. Examples of sound energy are:

• Tuning fork
• Beating drums

### Electrical energy

Electrical energy is obtained from the free electrons that are of positive and negative charge. Examples of electrical energy are:

• Lightning
• Batteries when in use

### Mechanical energy

The sum of kinetic energy and potential energy is known as mechanical energy which can neither be created nor destroyed but can be converted from one form to other. Examples of mechanical energy are:

• Orbiting of satellites around the earth
• A moving car

## Overview of Kinetic Energy

 Kinetic Energy Definition The energy that a body possesses by virtue of being in motion Kinetic Energy Equation $KE=\frac{1}{2}mv^{2}$ Kinetic Energy Units The SI unit of kinetic energy is Joules which is equal to kg-m2s-2 Kinetic Energy Examples A river flowing at a certain speed Kinetic Energy Types Radiant energy, Thermal energy, Sound energy, Electrical energy and Mechanical energy.

## Difference Between Kinetic Energy and Potential Energy

 Kinetic energy Potential energy Kinetic energy is defined as the energy present in an object from the state of rest to motion Potential energy is defined as the energy contained in an object by the virtue object’s position Formula used is $KE=\frac{1}{2}mv^{2}$ The formula used is mgh Vibrational energy is an example of kinetic energy Gravitational energy is an example of potential energy

Watch the video below to understand how kinetic energy is different from potential energy. ## Kinetic Energy Numericals

#### 1. Calculate the kinetic energy of 200 kg object that is moving with a speed of 15 m/s?

Solution:

The kinetic energy of the body can be calculated using the following equation:

$KE=\frac{1}{2}mv^2$

Substituting the values in the above equation, we get

$KE=\frac{1}{2}(200\,kg)(15\,m/s)^2$

$KE=45000\,J\,or\,45\,KJ$

#### 2. Calculate the mass of the object moving at a speed of 40 m/s and having a kinetic energy of 1500 J.

Solution:

Rearranging the kinetic energy equation, we get

$m=\frac{2KE}{v^2}$

Substituting the values in the above equation, we get,

$m=\frac{2\times 1500}{40^2}=1.87\,kg$

## Frequently Asked Questions – FAQs

### What is the definition of kinetic energy?

The kinetic energy of the body is defined as the energy possessed by the body by the virtue of its motion.

### Can the kinetic energy be negative?

The kinetic energy of a body cannot be negative because the square of the speed gives a non-negative number and the mass cannot be negative.

### When is the kinetic energy maximum?

The kinetic energy is maximum when the potential energy is maximum. This occurs when the velocity is maximum and the mass is at the equilibrium position.

### What happens to the kinetic energy when the speed decreases?

When the speed decreases the kinetic energy will be transformed into potential energy, thermal energy or other forms of energy.

### How is the kinetic energy different from the potential energy?

The energy possessed by the body by the virtue of its position is known as potential energy. The energy possessed by the body by the virtue of its motion is known as kinetic energy.