Forces and Motion

Friction.

Friction is an opposing force.
It acts in the opposite direction to a force which is applied to an object.

Friction occurs where two solid objects rub against each other,
or where a liquid or gas is pushed out of the way of a moving object.

For two solid objects,
the amount of friction depends on how well the surfaces grip each other.
Tyres on a road have very high friction (they don't slip),
skates on ice have very low friction (they slip easily).

For a solid object moving through a liquid or gas,
the amount of friction depends on the shape and surface area of the object
and the viscosity (thickness or thinness) of the liquid.

For an object moving through air, friction is called "air resistance" or "drag".
A parachute will have a very high air resistance,
a missile will have a very low air resistance.
An object moving through a high viscosity liquid (syrup) will have a high friction,
an object moving through a low viscosity liquid (water) will have a low friction.

Is friction good or bad?

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