 # Kinetic Energy

Kinetic energy is a fundamental concept in physics that quantifies the work performed by an object due to its motion. It plays a crucial role in our understanding of various everyday activities such as walking, jumping, and throwing, as well as more complex phenomena like falling. In this article, we will delve into the concept of kinetic energy, exploring its definition, units of measurement, examples, and transformations. We will also derive the kinetic energy equation and examine its relationship with other forms of energy. Additionally, we will explore the different types of kinetic energy and discuss the distinction between kinetic and potential energy. By the end, you will have a comprehensive understanding of kinetic energy and its significance in the world around us.

## What is Kinetic Energy?

Kinetic energy is a fundamental concept in physics that describes the energy transferred to an object due to its motion. In order to accelerate an object, force must be applied, which involves performing work. When work is done on an object, energy is transferred, resulting in the object moving with a new speed. This transferred energy is known as kinetic energy and is determined by the object’s mass and the speed it achieves.

According to the definition in physics, the kinetic energy of an object is the measure of the work it can do by virtue of its motion. It is important to note that kinetic energy is a scalar quantity, meaning it is solely described by its magnitude and does not have a directional component.

To summarise, kinetic energy is the energy associated with the motion of an object, obtained by performing work on it. It depends on both the mass of the object and the square of its speed. Understanding kinetic energy is crucial in comprehending various physical phenomena.

### Units of Kinetic Energy

The units of kinetic energy are important in understanding and quantifying its measurement. In the International System of Units (SI), the standard unit for kinetic energy is the Joule (J). One Joule is equivalent to 1 kilogram multiplied by metre squared divided by second squared (kg.m²/s²). This unit represents the amount of kinetic energy possessed by an object with a mass of 1 kilogram moving at a velocity of 1 metre per second.

In the CGS system (centimetre-gram-second), the unit of kinetic energy is the erg. One erg is defined as the amount of kinetic energy possessed by an object with a mass of 1 gram moving at a velocity of 1 centimetre per second.

Both the Joule and erg are derived from fundamental units of mass, length, and time, and they provide a standardised way to express and compare kinetic energy across different systems of measurement.

### Kinetic Energy Examples • A river flowing at a certain speed exhibits kinetic energy as the water possesses both velocity and mass.
• An asteroid falling towards Earth exhibits a tremendous amount of kinetic energy due to its large mass and high velocity.
• During flight, an airplane has a higher kinetic energy due to its substantial mass and fast velocity.
• The act of walking involves the movement of a person’s legs and body, resulting in the possession of kinetic energy.
• When a ball is thrown, it acquires kinetic energy while in motion through the air. The amount of kinetic energy it possesses is determined by the speed of the throw.
• Objects falling towards the ground accumulate kinetic energy due to the force of gravity. The impact’s kinetic energy is directly proportional to the object’s mass and velocity.
• A truck traveling down the road possesses greater kinetic energy compared to a car traveling at the same speed. This is because the truck has a significantly higher mass than the car.

## Kinetic Energy Transformation

Kinetic energy can be transferred between objects and transformed into various other forms of energy. Let’s explore more examples of kinetic energy transformation.

### Elastic Collision

When two objects collide, such as two billiard balls, kinetic energy can be transferred between them. As the balls collide and rebound, some of the kinetic energy is transformed into other forms, such as heat and sound energy.

### Pendulum Swing

As the pendulum swings back and forth, it undergoes a continuous transformation between potential and kinetic energy. At the highest point of the swing, the energy is mostly potential energy, and as it descends, the potential energy is converted into kinetic energy.

### Car Braking

When a car applies its brakes, the kinetic energy of the moving vehicle is transformed into other forms of energy, mainly heat energy. The friction between the brake pads and the wheels slows down the car by converting its kinetic energy into thermal energy.

### Wind Turbine

In a wind turbine, the kinetic energy of the moving air is harnessed and transformed into electrical energy. As the wind blows, it causes the blades of the turbine to rotate. The kinetic energy of the wind is converted into mechanical energy, which then drives a generator to produce electricity.

### Waterfall

As water falls from a higher elevation in a waterfall, its potential energy is gradually transformed into kinetic energy. The flowing water gains speed and kinetic energy due to gravity, which can then be used to generate hydroelectric power.

### Yo-Yo Transformation

A yo-yo provides an interesting demonstration of the transformation of kinetic energy. When a person holds a yo-yo in their hand, the yo-yo is at rest, and all the energy is stored in the ball in the form of potential energy. As the person releases the yo-yo, gravity pulls it downward, converting the potential energy into kinetic energy. The yo-yo ball accelerates as it descends, gaining speed and kinetic energy. At the bottom of the yo-yo’s path, the energy is entirely in the form of kinetic energy.

However, the transformation doesn’t end there. As the yo-yo reaches the end of its descent, it starts to climb back up due to the string’s tension. The kinetic energy of the yo-yo ball begins to transform back into potential energy as it gains height. The process repeats as the yo-yo moves up and down, with continuous conversions between potential and kinetic energy.

## Kinetic Energy Formula

The formula of kinetic energy is an essential tool in understanding the energy associated with an object’s motion. The equation for kinetic energy is given by:

$$\begin{array}{l}KE=\frac{1}{2}mv^{2}\end{array}$$

In this equation, KE represents the kinetic energy of the object. The symbol “m” refers to the mass of the object, while “v” represents its velocity.

The equation reveals that kinetic energy is directly proportional to both the mass and the square of the velocity of the object. This means that as either the mass or the velocity increases, the kinetic energy will increase as well. Moreover, the relationship between kinetic energy and velocity is particularly noteworthy because the velocity is squared in the formula. This implies that even small changes in velocity can have a significant impact on the kinetic energy of an object.

To calculate the kinetic energy of an object using this formula, you need to know the mass of the object and its velocity. First, square the velocity of the object, then multiply it by half of the object’s mass. The resulting value will give you the object’s kinetic energy.

### Why is Kinetic Energy a Scalar Quantity?

Kinetic energy is a scalar quantity with no associated direction. It represents the magnitude of energy possessed by an object. While velocity, which is a vector quantity, has both magnitude and direction, the calculation of kinetic energy only considers the magnitude of velocity. The kinetic energy depends on the mass and the square of the magnitude of velocity. The squaring of the velocity term ensures that kinetic energy is always positive, regardless of the direction of motion. Therefore, even though velocity is a vector quantity, the calculation of kinetic energy focuses on magnitude, making it a scalar quantity.

## Deriving Kinetic Energy Formula

The formula for calculating kinetic energy is an essential tool in understanding the energy associated with an object’s motion. The equation for kinetic energy is given by:

$$\begin{array}{l} KE = \frac{1}{2} mv^2 \end{array}$$

To begin, let’s consider a particle of mass “m” moving with a velocity “v.” The kinetic energy of this particle can be defined as the work done to accelerate the particle from rest to its current velocity. According to the work-energy theorem, the work done on an object is equal to the change in its kinetic energy.

The work done on the particle is given by the equation:

$$\begin{array}{l} Work = Force\times Distance \end{array}$$

(i) In the case of constant force, the work done can be calculated as:

$$\begin{array}{l} Work = Force\times Distance \end{array}$$

Now, let’s express the force in terms of mass and acceleration. According to Newton’s second law of motion, force (F) is equal to mass (m) multiplied by acceleration (a). Since the particle is moving with a constant velocity, its acceleration is zero. Therefore, the force acting on the particle is zero.
Substituting the force into the work equation, we have:

$$\begin{array}{l} Work =0\times Displacement \end{array}$$

$$\begin{array}{l}Work= 0 \end{array}$$

This means that no work is done on the particle as it moves with a constant velocity. Consequently, there is no change in its kinetic energy.

(ii) In the case when the particle is initially at rest and then accelerated to a final velocity “v” by an external force.

In this case, work is done on the particle to change its velocity.

The work done on the particle is equal to the change in its kinetic energy. So, we can write:

$$\begin{array}{l}Work= Change\,in\,Kinetic\,Energy\end{array}$$

Using the work formula:

$$\begin{array}{l}Work = Force \times Displacement\end{array}$$

The force acting on the particle is equal to the mass of the particle (m) multiplied by its acceleration (a). Since the initial velocity is zero, the final velocity is “v,” and the displacement is “d,” we have:

$$\begin{array}{l}Work = (m \times a) \times d\end{array}$$

Now, recall that the formula for acceleration (a) is given by:

$$\begin{array}{l}a = \frac{Change\,in\,Velocity}{Time}\end{array}$$

Since the initial velocity is zero, the change in velocity is “v” and the time taken is “t,” we can rewrite the equation as:

$$\begin{array}{l}a =\frac{ v}{ t}\end{array}$$

Substituting this back into the work equation, we have:

$$\begin{array}{l}Work = (m × (v / t)) × d\end{array}$$

$$\begin{array}{l}Work =(m \times v ) \frac{d}{t}\end{array}$$

The quantity (d / t) represents the average velocity (vavg) of the particle. So, we can rewrite the equation as:

$$\begin{array}{l}Work = m \times v \times v_{avg}\end{array}$$

Now, the average velocity (vavg) is equal to half of the final velocity (v), as the particle starts from rest and reaches a final velocity “v.” Therefore, we can further simplify the equation as:

$$\begin{array}{l}Work = m \times v \times (\frac{v}{2})\end{array}$$

$$\begin{array}{l}Work = \frac{1}{2} \times m \times v^2\end{array}$$

Finally, we equate the work done to the change in kinetic energy:

$$\begin{array}{l}(\frac{1}{2}) \times m \times v^2 = Change\, in\,Kinetic\,Energy\end{array}$$

Hence, we have derived the equation for kinetic energy:

$$\begin{array}{l}K.E = (1/2) \times m \times v^2\end{array}$$

This equation relates the kinetic energy (K.E) of an object to its mass (m) and velocity (v). It demonstrates that the kinetic energy is directly proportional to the square of the velocity and the mass of the object.

### Kinetic Energy Calculation

1. Calculate the kinetic energy of a 200 kg object that is moving at a speed of 15 m/s.
Solution:

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

$$\begin{array}{l}KE=\frac{1}{2}mv^2\end{array}$$

Substituting the values in the above equation, we get

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

$$\begin{array}{l}KE=45000\,J\,or\,45\,KJ\end{array}$$

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

$$\begin{array}{l}m=\frac{2KE}{v^2}\end{array}$$

Substituting the values in the above equation, we get,

$$\begin{array}{l}m=\frac{2\times 1500}{40^2}=1.87\,kg\end{array}$$

## Types of Kinetic Energry

There are several types of kinetic energy that can be observed in different contexts. Some of the common types include:

### Translational Kinetic Energy

This refers to the energy associated with the linear motion of an object. It is the most familiar type of kinetic energy and is observed when objects move in a straight line.

### Rotational Kinetic Energy

This type of kinetic energy is exhibited by objects that are rotating or spinning around an axis. Examples include spinning tops, wheels, or rotating machinery.

### Vibrational Kinetic Energy

Vibrational kinetic energy is related to the oscillatory motion of particles within a system. It is observed in vibrating strings, molecules, or atoms that are in a state of vibration.

### Thermal Kinetic Energy

Thermal kinetic energy is associated with the random motion of particles within a substance. It is responsible for the temperature and heat transfer in a material. The faster the particles move, the higher the thermal kinetic energy.

### Electrical Kinetic Energy

Electrical kinetic energy is related to the movement of charged particles, such as electrons, in an electric current. It is utilised in various electrical devices and is a form of energy transfer.

These different types of kinetic energy demonstrate the wide range of phenomena where energy is associated with the motion of objects or particles. Each type of kinetic energy has unique characteristics and plays a role in different areas of physics and everyday life.

 Related Articles

## Overview of Kinetic Energy

 Kinetic Energy Definition Kinetic energy is the measure of the work an object can do by the virtue of its motion. Kinetic Energy Equation $$\begin{array}{l}KE=\frac{1}{2}mv^{2}\end{array}$$ 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 exhibits kinetic energy as the water possesses both velocity and mass. Kinetic Energy Types Translational Kinetic Energy, Rotational Kinetic Energy, Vibrational Kinetic Energy, Thermal Kinetic Energy, Electrical Kinetic Energy

## Difference Between Kinetic Energy and Potential Energy

 Kinetic energy Potential energy Kinetic energy is a form of energy possessed by an object due to its motion. Potential energy is the form of energy possessed by an object due to its position or state. The formula used to calculate kinetic energy is $$\begin{array}{l}KE=\frac{1}{2}mv^{2}\end{array}$$ The formula used to calculate potential energy is $$\begin{array}{l}mgh\end{array}$$ Vibrational energy is an example of kinetic energy Gravitational potential energy is an example of potential energy

### For a more captivating exploration of kinetic energy and potential energy, we encourage you to watch the videos below. These videos delve into the concepts of kinetic energy and potential energy, unravelling their intricacies and showcasing their real-world applications.  ## Frequently Asked Questions – FAQs

Q1

### What is the definition of kinetic energy?

The kinetic energy is the measure of the work an object can do by virtue of its motion.
Q2

### Can the kinetic energy be negative?

The kinetic energy cannot be negative because the square of the speed gives a non-negative number, and the mass cannot be negative.
Q3

### When is the kinetic energy maximum?

The kinetic energy is maximum when the potential energy is minimum, and this occurs when the velocity is maximum and the mass is at the equilibrium position.
Q4

### 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.
Q5

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

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

## Enhance your understanding of Work and Energy in Class 9 Physics with this engaging video episode. Join us as we explore insightful Mind Maps and participate in a stimulating Menti Quiz, designed to enhance your preparation for the final exam. Stay tuned to BYJU’S and Fall in Love with Learning!

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