Particles rub together because they are uneven and catch on each other
Friction is a force caused by two surfaces moving over each other. Rough surfaces produce more friction than smooth ones.
If it is a downwards force it is often weight
Distance / time = speed
Metres per second (m/s)
Speed * Time = Distance
Moving at a constant speed
Arrows equal in size and in opposite directions
Length of arrow = size of force
Where it points shows the direction.
Measured in Newtons (N)
Measured with newton meter
1N=1kg on earth