Momentum Defined
- Momentum is defined by the equation:
momentum = mass × velocity
p = m × v
(where p stands for momentum)
Momentum is defined as the product of mass and velocity
- The units of momentum are kg m/s (the units of mass multiplied by the units of velocity)
- Momentum is a vector quantity – it has direction as well as magnitude
- This means that momentum can be negative as well as positive:
- If an object travelling to the right has positive momentum, an object travelling in the opposite direction (to the left) will have negative momentum
https://player.vimeo.com/video/367557274?title=0&byline=0&portrait=0Extended Only
The Conservation of Momentum
- In the absence of external forces (such as friction), the total momentum of a system remains the same
- This means that in a collision, the sum of the momentums before the collision will be the same as the sum of momentums after the collision
Diagram showing the total momentum of a system before and after a collision
- In the above diagram the total momentum before and the total momentum after must be equal:
m × u = M × V – m × v
- Note that because the red ball is travelling to the left after the collision, its momentum will be negative – hence the minus sign in the above equation
https://player.vimeo.com/video/367557540?title=0&byline=0&portrait=0Extended Only
Impulse
- When a resultant (unbalanced) force acts on a mass, the momentum of that mass will change
- The impulse of a force is equal to that force multiplied by the time for which it acts:
impulse = F × t
- The change in momentum of a mass is equal to the impulse provided by the force:
impulse = change in momentum
F × t = mv – mu
(Where u is the initial velocity of the mass and v is the final velocity of the mass)