This type of friction occurs when dry surfaces rub together.

The frictional force depends only on:-

1. the type of surfaces
2. how hard the surfaces are pressed together.

In this diagram, the weight of the block provides the force pressing the surfaces together. Watch the animation carefully:

If this picture isn't moving, click on "Refresh"/"Reload"

If we push the block harder and harder, the frictional force will increase, until it reaches a maximum (in this case, 2.5N).
If we push harder still, (say, 2.6N), the block will start to move, because we're now pushing harder than the frictional force.

Advanced information:

We can calculate the maximum frictional force using

F=mN

Where
- F is the frictional force,
- m is the "coefficient of friction", a number we use for those particular surfaces,
- N is the "Normal reaction", which is the force pressing the surfaces together.

This type of friction is what happens with liquids and gases
(In Physics, liquids and gases are both called "fluids". They behave in similar ways.)

Fluid friction is also known as "drag". On aircraft it's also called "air resistance".
It depends on:-

1. how thick the fluid is
        (its "viscosity")
2. the shape of the object
3. the speed of the object

A thin, runny liquid has a low viscosity.
A "viscous" liquid is thick and gooey.

Aircraft and car designers want to reduce drag, so that the vehicle can go fast without having to waste too much fuel.

To reduce drag, we need a shape that the fluid can flow past easily and smoothly, without any swirls (called "eddies"). This tends to mean using long, pointed, "streamlined" shapes.

When an object falls, it accelerates. As its speed increases, the air resistance increases.
Eventually, the force from the air resistance will equal the force from the weight of the object.
At that point, the speed will remain constant: the object has reached its "terminal velocity" and can't fall any faster.

Terminal velocity depends on the drag, so a streamlined shape will fall quickly, whilst a parachute will fall slowly.

For a free-fall parachutist, before the parachute opens, terminal velocity is around 120 miles per hour. When the parachute opens, the terminal velocity decreases to only a few miles per hour - a much better way to hit the ground!

Click on the graph to find out more.

Friction can be a nuisance, because it changes kinetic energy into heat which is usually wasted. Friction also tends to wear away at the surfaces, causing damage.

We can reduce friction by oiling ("lubricating") the surfaces. This means that the surfaces no longer rub directly on each other, but slide past on a layer of oil. It's now much easier to move them.

Hovercraft ride on a cushion of air, which reduces the drag dramatically compared to the drag on the hull of a ship.
Thus hovercraft can easily achieve much higher speeds than ships.

"Air Hockey" tables in amusement arcades use the same principle. The puck goes a long way when you hit it, because there's almost no friction to slow it down.

In winter sports, we need friction to be as low as possible so that we can achieve high speeds.
Ice skaters actually move on a layer of water, and don't skate on ice at all. When ice is subjected to high pressure it melts.
The narrow blades of the skates create a very high pressure and thus the skaters glide along on a layer of water they've just melted. The water refreezes as soon as they've moved on.
This is called "regelation" (sounds like something that happens to a football team, but it's spelt differently!)

Other methods of reducing friction include:-

• using "ball bearings" or "roller bearings", where balls or rollers allow the surface to move easily without actually touching each other
  
  
• using special materials, for example, Teflon, which have a very low coefficient of friction and thus slide easily (Teflon is used in "non-stick" frying pans for this reason)

We use friction to help us grip. This means that our shoes grip the floor, so we don't fall over.
Right now you're using a mouse, which works because of friction between the ball and the mouse mat.

If it wasn't for friction between the tyres and the road, driving a car would be like trying to drive on an ice rink. This would make cornering and stopping very difficult!

Friction provides the force to accelerate, stop or change the direction of the car. Ice and water on the road reduce this friction, and make is easier to skid.

Friction Questions: (Click on the questions to reveal the answers)