A-level Physics/Forces, Fields and Energy/Further dynamics
From last year, you should remember kinematics and dynamics, the branch of physics that relates to the motion of objects. We will now expand on this and have a look at what happens when two objects collide, the concept of momentum, and we will take a closer look at Newton's three laws of motion.
If you have seen collisions involving two objects, you may have noticed that the velocity of one object seems to be passed to the other object. You may also have noticed that heavier objects seem to pass more velocity on to smaller objects, whereas smaller objects seem to pass less velocity to more massive ones.
What is in fact happening is that momentum is being conserved. Momentum is the product of an objects mass and velocity, or . This means that, after a collision, an object that is heavier will have a lower velocity than a lighter object in its place, and vice versa. Momentum is conserved for all collisions. The principle of the conservation of momentum states that:
Momentum is a vector quantity and has the units or (Newton-seconds) in the SI system.
Since momentum is conserved, the momentum before a collision is equal to the momentum after a collision. You can use this fact to solve problems involving collisions.
For instance, a ball is moving at 3m/s with mass 3kg. It hits another ball with mass 1kg moving at 2m/s; the two balls collide and the second ball rebounds at 4m/s. Find the velocity at which ball 1 is moving:
So the velocity at which ball 1 is moving after the collision is 2.3m/s (7/3)m/s 1
Newton's laws of motion 
Newton's first law of motion 
Newton's second law of motion 
Originally, you learnt this to be:
However, since you now know that a force changes the rate of change of momentum of an object, we can use a more accurate interpretation of Newton's second law: