Physics
Energy
Energy is never lost, it just keeps changing its costume.
What you need first
Your breakfast gets you up the hill, the sun warms your face, and your phone runs on power from the socket. Energy is at work everywhere: in motion, in height, in heat, in electricity, and even in your food. Time to take a closer look at this invisible something.
Energy is never lost
The most astonishing thing about energy: it never disappears and never appears out of nothing. It only transforms from one form into another. This is called conservation of energy. A car turns the energy in fuel into motion and heat, a lamp turns electricity into light and heat. The total amount always stays the same.
The sum always stays 100%. Energy is not lost, it just changes form.
From high up to fast
At the top of the slide you are full of potential energy: energy of height. As you slide down it becomes kinetic energy, and at the bottom you are fastest. A roller coaster works exactly this way: the motor only pulls the car up the first hill, and potential energy pays for the rest of the ride. The higher the start, the faster it gets at the bottom.
Measuring energy: the joule
Energy is measured in joules, J for short. One joule is roughly the energy it takes to lift a bar of chocolate (about 100 g, which weighs around 1 N) by 1 m. So one joule is one newton times one metre. From this follows the rule you need for the calculation tasks: potential energy = weight times height. If you lift a load weighing 40 N by 2 m, you put in 40 · 2 = 80 J of potential energy.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
A ball rolls down the slide. What happens to its energy?
In which unit is energy measured?
A cart has 500 J of potential energy on top of the hill. How many J of kinetic energy does it have at the bottom if there is no friction?
Order how the ball's energy changes on the slide from top to bottom.
- 1At the very bottom: almost only kinetic energy, here it is fastest
- 2At the very top: the ball has only potential energy
- 3At half height: partly potential, partly kinetic energy
A roller coaster starts with 800 J of potential energy. At half height, 400 J of potential energy remain. How many J of kinetic energy does the car have there?
Match each device to the energy change it makes.
Where this leads