I noticed that when I threw the rock into the river, it made a circle shape which got bigger
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When I was playing “splash rocks”, I noticed that when I threw the rock into the river, it made a circle shape which got bigger. How does it make the ripple? Why do the circles spread out further and further? Why do they stop? – Rowan, aged six, UK.
Simon Cox, professor of mathematics, Aberystwyth University, UK:
“When water is in its calmest, lowest energy state, it has a flat surface. By throwing the rock into the river, you have given the water some energy.
That causes the water to move around, trying to spread out the energy so it can go back to having a still, flat surface.
This follows a powerful principle of physics, which is that everything seeks to find a state where its energy is as small as possible.
One way energy can move around is by forming waves. For example, the waves you see at the beach are formed by energy from the wind.
Light and sound also move in waves, though we can’t see that directly. And the ripples that you see in the river are small waves carrying away the energy from where you threw the rock.
Water is made of tiny molecules. But during a ripple, the water molecules don’t move away from the rock, as you might expect. They actually move up and down.
When they move up, they drag the other molecules next to them up – then they move down – dragging the molecules next to them down too.
Dragging neighbouring water molecules up and down is hard work, and it slowly uses up energy. So the ripples get smaller as they get further away. Ripples often spread out in circles, but this isn’t the only possibility.
When you throw a stick into the water, the ripples from the middle of the stick eventually catch up with the ripples from the ends, because of the different ways they spread out.
So far away from the stick, the ripples are round … just like they were for your rock.
That’s what creates the peaks and troughs you see on the surface of the water. And that’s how the ripple travels away from your rock – a bit like a human wave around a stadium.”
. To read more of Simon’s answer, go to theconversation.com
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