Friction

Friction happens when things rub against one another.

Rub your hands together. Does it feel warm? This is because of friction.

Your hand may look smooth to you, but zoom in very very close, you will see that it is actually rough. The rough edges of one hand catch the rough edges of another, but each hand tries to ‘get away’ and continue to slide. This is friction.

Friction is a good thing to have though. Cars stop moving because of friction between the brake and the tyre.

Let’s try something out. Take your toy car. Give it a push and see how far it goes on a smooth floor. Try the same on a carpet, and on your driveway. Where does the car move the most?

Gritting roads

It does not allow water to freeze, and makes ice melt faster.

First of all, what is gritting?

A gritting truck at work (Stephen Craven)

A gritting truck at work (Stephen Craven)

Those of you in cold countries where there is snow and ice must have seen it. Huge ‘gritting trucks’ go out in large numbers at night, to add something on the road. You will probably see the ‘grit’ on the roads the next day.

Although it is called gritting, there is actually no grit involved. Grit is small pieces of stone, pebbles and sand. But, gritting roads uses rock salt. Rock salt is similar to cooking salt, but it’s too rough and has too many other things in it, so we don’t use it for cooking.

How does it work?

For water to freeze, enough of its ‘molecules’ have to meet together and hold hands. (A molecule is too small for you to see, but it’s there is everything – it is the building block of things, same as bricks are used to make a building.)

When rock salt is added, water molecules keep meeting up with salt molecules instead of other water molecules. They cannot hold hands anymore, so water is unable to freeze and form ice.

Rock salt also interferes with ice. Ice is just water molecules, packed tightly together. Rock salt manages to get inbetween the molecules in ice, and breaks them apart, making ice melt.

So rock salt does two things: it prevents water from forming ice, and makes ice melt and form water again. Now you can walk safely on the roads, and people with cars can drive without skidding off the roads.

Why do we slip on ice?

Because the uppermost layer of ice has a thin layer of water on it. 

An icy road

An icy road (Erin McKittrick)

Ice is like an extremely thick stack of newspaper. The top most layer of the ice we move on is ‘loose’ compared to the lower layers. Lower layers have each other to stick to. The upper layer is in contact with the air above. It is warmer than the lower layers, and so tries to melt and become water.

When we move on ice, we are actually helping it become water. I’ll tell you how. Our shoes (and skates) rub on ice, causing friction. Friction warms the ice further, making it more water than ice. This makes ice slippery.

So it’s actually the water on the ice that makes us fall – not the ice itself.

How does gritting help? Go here to find out.