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 3 laws of motion.

Momentum

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, wheras 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 $p=mv$. 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:

Within a closed system, the total momentum in any specified direction remains constant.

Momentum is a vector quantity and has the units $kgm{s}^{-1}$ or $Ns$ (Newton-seconds) in the SI system.

Collisions

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.

Before After
mv1 + mv2 = mv1 + mv2

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:

Before After
mv1 + mv2 = mv1 + mv2
3x3 + 2x1 = 3v + 1x4
11 = 3v + 4
11-4 = 3v
7 = 3v
7/3 = v

So the velocity at which ball 1 is moving after the collision is 2.3m/s (7/3)m/s

Newton's laws of motion

Newton's first law of motion

An object will remain at rest or in a state of uniform motion unless it is acted on by an external resultant force.

Newton's second law of motion

Originally, you learnt this to be:

For an object with constant mass, its acceleration is proportional to the force producing the acceleration, and is in the direction of the force.

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:

The rate of change in momentum of an object is proportional to the force that produces it, and takes place in the direction of the force.

Newton's third law of motion

When two bodies interact, the forces they exert on each other are equal and opposite.