1. Is it useful or a nuisance?
FRICTION
What is it?
Is it useful or a nuisance?

2. What is friction?
Friction is a force that opposes (tries to stop) movement
It can makes things that rub together
get hot , slow down and wear out
So it can be a nuisance!
But without it we couldn’t walk across the floor and cars couldn’t move on the road
So it is useful too!

3. Friction is a force that resists the movement of an object
Friction is a force that resists the movement of an object. It acts in the opposite direction to the object’s motion.
pull
friction
The force you need to just start the object moving is equal to the STATIC FRICTION between the object and the ground.
Once the object is moving you need a smaller force to keep it moving
- this is called the DYNAMIC (or KINETIC) FRICTION between the object and the ground.

4. Is friction useful or a nuisance in these cases?
skiing downhill
striking a match
useful
tying knots in a rope
useful
walking along the ground
useful
starting a car engine
nuisance
putting on the brakes on a bike
useful