Look at the ball and the pins above. Why is it when I bowl, sometimes I strike (all pins down), but other times only 2/3 pins were knocked down? 

When a car ramped into a tree, the tree collapsed. But, when a lorry which is traveling at the same speed as the car, knocked the tree, the tree does not collapsed.

Why?

This is because of Momentum. What is that?

Momentum is a mass in motion. 

All moving objects have momentum.

The faster it moves, the more momentum it has. The bigger its mass, the more its momentum.

For example, comparing momentum of a car and a lorry traveling at same velocity.

Or comparing momentum of a car speeding from 10 ms^-1 to 20 ms^-1.

Is momentum a scale quantity or vector quantity?

Of course, it is a vector quantity because it has direction. Its direction is the same as the direction of its velocity.

What is the unit of momentum?     

Since momentum = mv, therefore                                         (m = mass, v = velocity)

unit of momentum = (unit of mass) x (unit of velocity)

                            = (kg) x (ms^-1 )

                            = kgms^-1

^{What is the momentum of trolley P and Q?}