Momentum
What is momentum?
Momentum is a measure of how easily an object may be stopped; objects with a larger momentum will need more force to stop them:
- Larger object (more mass) - harder to stop
- Faster object (greater velocity) - harder to stop
Using this we can work out that
Momentum = mass X velocity
Momentum is a Vector quantity so it will always act in a given direction. The units for momentum are kg m/s.
(Note: the symbol for momentum is p)
Newton's 2nd Law shows that the rate of change of momentum is proportional to how much force is being acting on the object.
This leads to the equation:
Force = change in momentum/time taken
We can rearrange this equation to give us:
Force = mass X acceleration
Conservation of Momentum
The momentum of bodies colliding is always conserved. This means that the momentum of the bodies before the collision is always equal to the momentum of the bodies after the collision.
Remember, that because momentum is a vector, the direction of movement must be taken into account.
The equation sometimes associated with this is:
(where m1 = starting mass, m2 = final mass and V1 = starting velocity and V2 = final velocity)
Example Calculation
Example
Collisions:
Elastic collision - no energy is lost as a result of the collision, (eg gas particles)
Partially elastic collision - some energy is lost, usually as heat and sound, (eg snooker balls)
Inelastic collison - The objects do not rebound, but stick together
Exam question usually refer to inelastic collisions because they are easier to work with.
Resources
You should know:
- the momentum of an object can be calculated by multiplying its mass by its velocity
- that momentum is a vector quantity
- when bodies collide, they causes a momentum change for each body which is equal but in opposite directions
- to use the equation force = change in momentum/time taken
- how to calculate the momentum of bodies before and after collisions
- the differences between elastic, partially elastic and inelastic collisions and give examples of each