Where slipperiness occurs
Think of a park that you know well. There should be a large area of grass where children can play games. Perhaps there are grassy banks or the whole park is on a hillside. There may be a road running close to the park. In one corner there may be a play area with swings and slides. Imagine what the park is like in summer and winter. As you do, think about how friction or slipperiness affects the people using the park.

Begin your thoughts with the park in summer. It is a warm, dry day. Some children are playing football in the open space. As their boots touch the ground the surfaces of the soles press into the ground and grip it. This grip is due to microscopic ridges and grooves in both surfaces. The ridges in one surface push into the grooves of the other. The surfaces lock together while the boot and ground are in contact. This provides a firm support for standing on one leg and kicking a ball.

Some more children are on a grassy bank. They are trying to slide down it but are not having much success. Friction between their bodies and the ground is stopping them moving. Eventually they decide to roly-poly down the bank. By making themselves into human rollers they can reduce the effect of friction and move quite fast.

In the children's playground friction is holding children to the seats of swings and letting feet grip climbing frames. However, when someone sits at the top of a slide and pushes themselves off, something different happens. The surface of the slide is very smooth. There are few ridges and grooves. The ridges are small and the grooves are shallow. This means that there are few opportunities for the surfaces of clothes to grip the slide. The force of friction between someone's clothes and the surface of the slide is too small to hold the person on the slide so they shoot quickly down the slide.

Later in the day there is a shower of rain but it soon passes. Some children come out to play football again. As they run around they sometimes slip. When the ball is kicked up the grassy bank a boy chases after it to bring it back. He kicks the ball from the bank then runs quickly down the bank. One of his feet begins to slide down the bank. He puts out his arms to help him balance but it is too late: he crashes to the ground. Fortunately he is not hurt so he picks himself up and joins in the game again. What has happened to make the game more difficult?

The rainwater has filled the grooves on the surface of the ground. This means that the grooves cannot take in the ridges from the surfaces of the children's shoes. The friction between the shoes and the ground is reduced so there is a greater chance of slipping and sliding - especially when standing on one leg to make a powerful kick at goal.

Now imagine the park in winter when it is covered in snow and the weather is freezing. As people trudge through the snow they flatten it and make a smooth surface. In some places it is as smooth as the slide in the children's playground. The paths are dangerous places for people to walk but on the grassy bank the children make a long strip of flat snow. They run at the top of the bank, jump on the end of the strip and slide all the way down the bank. They have made a slide of their own.

While the flattened snow is providing fun for some in the park it is proving a nuisance for others in the road. During the summer cars could go up and down the park road without a problem. Even in the rain, the treads of the tyres channelled the water out of the way so the tyres could get a good grip. Now in this winter weather the flattened snow has turned to ice and car wheels are spinning. The wheels spin because they cannot get a grip on the ice. Even when a car is moving there is still danger. If the brakes are put on quickly the tyres lose their grip of the road and the car skids.

Next time you are in your local park look for places where friction is at work and where a lack of friction makes a place slippy.

Does friction act on any surface?
Yes, it does. There are many different kinds of surfaces and you may not think that friction can work on them all. For example, the grassy surface of a park seems different from the hard surface of a road. However the surfaces of leaves have grooves and ridges as does the surface of the road. There is even friction on the surface of a slide because there are some grooves and ridges there, but it is too weak to stop most objects sliding.

Why do they put grit on the roads in winter?
They put more than grit on the roads. Mixed with the grit is salt. On the surface of the ice is a thin film of water. The salt dissolves in this. When salt dissolves in water a mixture is made which is called a salt solution. A salt solution does not freeze in cold conditions but stays as a liquid. The salt can change all the ice on the road to water and make the road less slippy. The grit makes the surface of the road rougher. This also helps to increase grip.

Some people use metal trays as sledges. How do they work?
The metal tray has a very smooth surface, like the surface of a slide. The flattened snow has a very smooth surface too. When a person sits on a metal tray on flattened snow they can slide easily because there are few ridges and grooves on the two surfaces.

Why can't a car move when its wheels are spinning?
The power to turn the wheel comes from the car's engine. When the road is dry the engine makes the wheels turn and at the place where tyre and road touch, the wheel pushes backwards on the road. This is matched by a force of friction, which points the other way. The two forces provide a firm grip for the wheel to push against and it rolls forwards. When the road is covered with ice the wheel turns and pushes back on the ice but there is a very weak force of friction pointing the other way. This means that the forces are not matched and the pushing force makes the wheel turn round and round while the weak force of friction fails to hold the tyre's surface. There is no firm grip to help the wheel push forwards so it, and the car, stays stuck on the ice.

How does a car skid?
A car may go into a skid when the driver puts on the brakes. When a car is moving along, at the place where the tyre and road meet, the wheel pushes backwards on the road and the friction pushes forwards to match it. The two forces provide a firm grip for the wheel to push itself along. When the driver puts on the brakes the pushing force of the tyre becomes less and the force of friction falls to match it. This means that the wheel still pushes itself along but more slowly. If the driver puts on the brakes quickly while riding over ice the sudden change of the pushing force is not matched by the force of friction. The tyre loses its grip and begins to slide. We call this slide a skid.

How does an ice skater move on ice?
There is a narrow strip of metal on an ice skate. It is called a runner. When an ice skater moves on the ice all their weight presses down on the ice through the runner. The pressure of the weight squashes the ice so hard under the runner that some of it turns to water. The water forms a thin film or layer on the surface of the ice. This acts as a lubricant making the force of friction between the skater and the ice very low. The low friction allows the skater to move at speed across the ice. Ice skaters do not leave a trail of water behind them on the ice because as soon as the ice is no longer under pressure the water on its surface freezes again.