Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Does energy make things happen?

You often read that “Energy is what makes things happen”. But you need to be careful to think a bit before you say it. Take an example: 

“Energy is needed to get a car moving”. Sounds good. 

“Energy is needed to stop a car moving” Sounds less good. But both are “something happening” and both do involve energy. 
 
What can we say that is true of both? (After all, stopping your car safely is no less important than getting it going!) Perhaps: 

“To get a car moving, energy has to be put into it from somewhere. It could be from someone pushing, but is usually burning fuel in the engine. There has to be a store of energy (fuel + air) and a pathway for it (the engine)” ... plus, of course, links to the wheels. 

“To stop a car moving energy has to be taken out of it somehow. It could be done by pushing against a brick wall, but it’s better to use the brakes. Energy is stored in the moving car, and must go somewhere else. This needs a pathway (the brakes) and a place for the energy to go (the brake discs and the air that cools them, warming the atmosphere). 
 
Take another example: 

“Energy is needed to warm up your house.” Sounds good. 

“Energy is needed for your house to get cold at night.” Sounds less good. But both are “something happening" and both do involve energy. 
 
What can we say that is true of both? Perhaps: 

“To warm up a house, energy is needed from somewhere. This is often a store of fuel. And the energy has to be put into the house. This is often via a boiler burning fuel and hot water circulating in radiators. So there’s a suitable source and a suitable pathway.” 

“For a house to get cold at night, the energy stored in it must get out. This is not difficult: there is somewhere for it to go – the cold outside air which warms up – and a way for it to go – through the walls and roof.” 
 
It becomes clear that the phrase “Energy is what is needed to make things happen” puts the focus on changes when something has to gain energy from something else. Certainly if the energy isn’t there or there is no pathway for it, the change won’t happen. The reverse kind of change, when something has to give energy to something else, is just as important. And these changes go by the same rules: there has to be a pathway and a place for the energy to go to. 
 
In summary, in lots of kinds of changes, energy has to go from one thing to another spontaneously. The change can only happen if it is possible, that is, if there is a pathway for the energy, connecting suitable stores.