1. What Is Kinetic Energy?
Kinetic energy is the energy of a moving object. Anything that moves — a car, a falling stone, a running person, flowing water — has kinetic energy.
The faster an object moves, or the heavier it is, the more kinetic energy it has.
1.1. Why Moving Objects Have Energy
A moving object can do work. For example, a moving hammer can drive a nail into wood. This ability to do work comes from its kinetic energy.
1.2. Real-Life Examples
- A fast cricket ball has high kinetic energy — that’s why it is hard to catch.
- Wind has kinetic energy that rotates windmills.
- A waterfall has kinetic energy that can turn turbines.
2. Formula for Kinetic Energy
The kinetic energy of a moving object is given by:
\( KE = \dfrac{1}{2}mv^2 \)
where:
- \( m \) = mass of the object (kg)
- \( v \) = velocity of the object (m/s)
2.1. Understanding the Formula
Kinetic energy depends on two things:
- Mass — heavier objects have more kinetic energy.
- Velocity — even a small increase in speed greatly increases kinetic energy because of the squared term.
For example, doubling the speed makes kinetic energy four times larger.
3. Deriving the Kinetic Energy Formula (Simple Explanation)
The formula comes from the work–energy idea:
Work done on a body changes its kinetic energy.
3.1. Steps (Easy Version)
Work done to accelerate an object:
\( W = Fd \)
But from Newton’s second law \( F = ma \).
Using the equation of motion:
\( v^2 = u^2 + 2ad \)
If the object starts from rest \( u = 0 \), then:
\( d = \dfrac{v^2}{2a} \)
Substitute into \( W = Fd \):
\( W = ma \cdot \dfrac{v^2}{2a} = \dfrac{1}{2}mv^2 \)
This work becomes the kinetic energy of the object.
4. Kinetic Energy in Everyday Situations
Kinetic energy helps explain why moving objects behave the way they do.
4.1. Vehicles
A fast-moving car has a lot of kinetic energy. That is why brakes need more force to stop a speeding vehicle.
4.2. Sports
A fast tennis ball has high kinetic energy and bounces back strongly after hitting the racket.
4.3. Nature
Flowing rivers have kinetic energy, which is used to generate electricity in hydroelectric power stations.
5. Units of Kinetic Energy
The SI unit of kinetic energy is the Joule (J).
If an object has 1 Joule of kinetic energy, it means it can do 1 Joule of work.
5.1. Large Amounts of Kinetic Energy
Very fast or heavy objects have kinetic energies in thousands or millions of Joules, such as trains, airplanes, or flowing water in dams.