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# What is friction?

Friction occurs when two things are in contact and at least one of them is moving.

Friction can occur between solid objects, such as a toy car and the floor, or between a solid and a liquid, such as a person swimming in a pool. It can also occur between a solid and gas, when an aeroplane if flying for example. The last two examples of friction are more commonly known as water resistance and air resistance.

The amount of friction of a surface can be looked at as the amount by which it resists something moving over it, or how ‘grippy’ it is. The more resistance, the grippier a surface and the higher the friction. The less resistance to movement, or more slippy a surface is, then the lower the friction of the surface.

Friction slows the movement of an object, such as a toy car on a table. A push force will start it moving, but the friction between the car wheels and the surface of the table will work against this forward push to slow the car and bring it to a stop. Some children may think that a smooth surface like ice has no friction, but it does, as it will oppose movement across its surface. However, It has much lower friction than a rougher surface like a pavement.

## Describe

Imagine a toy car sitting on a shelf. The shelf breaks and the toy rolls along the shelf slowly at first, then at a steady speed and it finally falls off the shelf and drops to the floor. Can you explain when friction is acting on the car?