<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Dismiss
Skip Navigation
Our Terms of Use (click here to view) and Privacy Policy (click here to view) have changed. By continuing to use this site, you are agreeing to our new Terms of Use and Privacy Policy.

5.2: Kinetic Energy

Difficulty Level: At Grade Created by: CK-12
Atoms Practice
Estimated2 minsto complete
%
Progress
Practice Kinetic Energy
Practice
Progress
Estimated2 minsto complete
%
Practice Now

What could these four photos possibly have in common? Can you guess what it is? All of them show things that have kinetic energy.  

Defining Kinetic Energy

Kinetic energy is the energy of moving matter. Anything that is moving has kinetic energy—from atoms in matter to stars in outer space. Things with kinetic energy can do work. For example, the spinning saw blade in the photo above is doing the work of cutting through a piece of metal. You can see a cartoon introduction to kinetic energy and how it is related to work at this URL: http://www.youtube.com/watch?v=zhX01toLjZs

Calculating Kinetic Energy

The amount of kinetic energy in a moving object depends directly on its mass and velocity. An object with greater mass or greater velocity has more kinetic energy. You can calculate the kinetic energy of a moving object with this equation:

\begin{align*}\mathrm{Kinetic\; Energy\; (KE)=\frac{1}{2} mass*velocity^2}\end{align*}

This equation shows that an increase in velocity increases kinetic energy more than an increase in mass. If mass doubles, kinetic energy doubles as well, but if velocity doubles, kinetic energy increases by a factor of four. That’s because velocity is squared in the equation.

Let’s consider an example. The Figure below shows Juan running on the beach with his dad. Juan has a mass of 40 kg and is running at a velocity of 1 m/s. How much kinetic energy does he have? Substitute these values for mass and velocity into the equation for kinetic energy:

\begin{align*}KE=\frac{1}{2}*40kg*(1\frac{m}{s})^2=20kg*\frac{m^2}{s^2}=20N*m,\end{align*} or \begin{align*}20 J\end{align*}

Notice that the answer is given in joules (J), or N ∙ m, which is the SI unit for energy. One joule is the amount of energy needed to apply a force of 1 Newton over a distance of 1 meter.

What about Juan’s dad? His mass 80 kg, and he’s running at the same velocity as Juan (1 m/s). Because his mass is twice as great as Juan’s, his kinetic energy is twice as great:

\begin{align*}KE=\frac{1}{2}*80kg*(1\frac{m}{s})^2=40kg*\frac{m^2}{s^2}=40N*m,\end{align*} or \begin{align*}40 J\end{align*}

Q: What is Juan’s kinetic energy if he speeds up to 2 m/s from 1 m/s?

A: By doubling his velocity, Juan increases his kinetic energy by a factor of four:

\begin{align*}KE=\frac{1}{2}*40kg*(2\frac{m}{s})^2=80kg*\frac{m^2}{s^2}=80N*m,\end{align*} or \begin{align*}80 J\end{align*}

Summary

  • Kinetic energy (KE) is the energy of moving matter. Anything that is moving has kinetic energy.
  • The amount of kinetic energy in a moving object depends directly on its mass and velocity. It can be calculated with the equation: \begin{align*}KE=\frac{1}{2}mass*velocity^2\end{align*}.

Vocabulary

  • kinetic energy: Energy of moving matter.

Practice

At the following URL, review kinetic energy and how to calculate it. Then take the quiz at the bottom of the Web page. Be sure to check your answer. http://www.physicsclassroom.com/class/energy/u5l1c.cfm

Review

  1. What is kinetic energy?
  2. The kinetic energy of a moving object depends on its mass and its
    1. volume.
    2. velocity.
    3. distance.
    4. acceleration.
  3. The bowling ball in the Figure below is whizzing down the bowling lane at 4 m/s. If the mass of the bowling ball is 7 kg, what is its kinetic energy?

Vocabulary

kinetic energy

kinetic energy

Energy of moving matter.

Image Attributions

Show Hide Details
Description
Difficulty Level:
At Grade
Grades:
7 , 8
Date Created:
Oct 31, 2012
Last Modified:
Jun 22, 2016
Files can only be attached to the latest version of Modality
Reviews
72 % of people thought this content was helpful.
12
Loading reviews...
Load More (9)
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original
 
SCI.PSC.223.3.L.1