How to Calculate Joules

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In science, the joule is the unit of energy or work. It is a compound unit defined as 1 newton of force over a distance of 1 meter, or as the kinetic energy of a 1-kilogram mass moving at one meter per second. Joules can also be converted from calories, as calories are another unit of energy. There are 4.19 joules in every calorie. You can calculate joules by calculating the kinetic energy, or energy of motion, of an object. You can also calculate the joules by calculating the amount of work accomplished by a person or machine. Lastly, you can calculate joules by converting directly from a measurement in calories.

Calculating Joules of Kinetic Energy

    Write the equation for kinetic energy:

    \text{kinetic energy} =\frac{1}{2}\times \text{mass}\times \text{velocity}^2

    where kinetic energy is in joules, mass is in kilograms, and velocity is in meters per second.

    Consider the moving object for which you would like to calculate the kinetic energy in joules. For example, suppose you want to calculate the energy of a 700 kilogram (kg) car moving at 20 meters per second (m/s).

    Plug the values into the kinetic energy equation to calculate the joules, as shown below:

    \text{kinetic energy} =\frac{1}{2}\times 700\text{ kg}\times (20\text{ m/s})^2=140,000\text{ J}

Calculating Joules of Work

    Write the equation for work:


    where work is in joules, force is in newtons, and displacement is in meters.

    Consider the situation for which you would like to calculate the joules of work being done. For example, suppose you want to calculate the work done by a man who uses 250 newtons (n) of force to push a box 8 meters (m) across the floor.

    Plug the values into the work equation to calculate the Joules, like so:

    \text{work}=250\text{ N}\times 8\text{ m}=2000\text{ J}

Converting Calories to Joules

    Construct a conversion factor for converting calories to joules. You know that there are 4.19 joules in a calorie, which can be written mathematically as 4.19 joules/calorie.

    Consider the calorie value you want to convert to joules. For instance, suppose you want to know how many joules of energy are in 300 calories.

    Multiply the calorie value by the conversion factor, as the following line shows:

    300\text{ calories}\times\frac{4.19\text{ J}}{\text{calorie}}=1257\text{ J}



About the Author

Timothy Banas has a master's degree in biophysics and was a high school science teacher in Chicago for seven years. He has since been working as a trading systems analyst, standardized test item developer, and freelance writer. As a freelancer, he has written articles on everything from personal finances to computer technology.