Energy

YOU ARE LEARNING:

Kinetic Energy

Anything that is moving possesses kinetic energy, and we will look at an equation to calculate it using mass and speed.

Kinetic energy is movement! In fact, any moving object or being has kinetic energy and the amount of kinetic energy depends on the size (mass) in $kg$ and the speed $(v)$ in $m/s$ at which it moves.

A car weighs 1000 kg and drives at speed 10 m/s. We can calculate its kinetic energy (KE) like this: $KE=\frac12\times1000 kg \times10 m/s^2=50000 J$

A cat weighs $3 kg$ and moves at $5 m/s$ . How much kinetic energy does the cat have?

Which equation do you think is the one we use for calculating kinetic energy?

Kinetic energy can be worked out from the equation: $KE=\frac{1}{2}\times mass \times speed^{2}$ which we write as $KE=\frac{1}{2} mv ^{2}$ . Remember that all energy is measured in Joules (J).

Using the KE equation, what do you think the KE of a 2kg object travelling at 1 m/s could be?

Imagine 2 cars of the same mass, travelling at different speeds. Car 1 is travelling at 45mph and car 2 is travelling at 50 mph. Pick all the options below that you think are true.

You can select multiple answers

Have a think about which one of the following statements is true.

Imagine a 5000 kg elephant and a 10 kg tortoise, both travelling at 2m/s. Which one of the following statements is correct?

Using the equation $KE=\frac{1}{2} mv ^{2}$ , we can work out the speed (v) if we know the amount of KE and the mass (m). We can also work out the mass (m) if we know the speed (v) and amount of KE. To do this, you would have to rearrange the equation.

Lets assume a moving body has a KE of $300 J$ and a mass of $6 kg$ . At what speed do you think the body is travelling?

Let's recap! We are given the KE and the mass. Putting these numbers into the KE equation gives us this:

$\frac{1}{2} \times 6kg \times v^{2} = 300 J$

We can change that into this:

$3kg \times v^{2} = 300J$

Now, we can divide by 3kg on both sides and get:

$v^{2} = \frac{300J}{3kg}$

Finally, taking the square root gives the answer:

$v = \sqrt{\frac{300J}{3kg}} = \sqrt{100} = 10m/s$