A force which tries to stop things sliding past one another
Friction only acts on moving objects and it cannot make objects move
Friction
It is useful because it helps objects grip on a surface
It can be a problem as it makes objects wear out and get hot
Parachute
Uses air resistance to work. It is very light in weight and has a very big surface area. It catches a lot of air when it falls down, creating a lot of air resistance or drag
The direction of air resistance is opposite to the direction of motion of an object moving through the air
Air resistance
The force caused by air pushing against moving objects
Friction between a car's tyres and the road makes the tyres wear out
Lubricants
Slippery substances that help things move smoothly and reduce friction
Balanced forces
When two forces acting on an object are equal and opposite
Unbalanced forces
When two or more forces acting in opposite directions on an object are not the same size
Force diagrams
Show all the pushes and pulls acting on an object
Net force
When two opposite forces are not balanced, there is a net force
Work
The amount of energy transferred to an object to make it move
Forces can make things move, speed up moving objects, and change an object's direction of movement
The direction of gravitational force is downward
Forces act in pairs and in different directions
Friction is greater between rough surfaces than between smooth surfaces
Friction is greater between small surfaces than between large surfaces
