Revision Notes

1.6.1 Momentum

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Momentum Defined

  • Momentum is defined by the equation:
momentum  =  mass × velocity


p  =  m × v

(where p stands for momentum)

Momentum mass velocity, IGCSE & GCSE Physics revision notes

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


Momentum comparison, IGCSE & GCSE Physics revision notes


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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

conservation-of-momentum, IGCSE & GCSE Physics revision notes

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


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  • 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)


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