IGCSE Physics / Momentum
Momentum revision notes
Edexcel International GCSE Physics (4PH1), chapter 4: Momentum, specification point 1.25P, 1.2P. Free lesson.
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Momentum combines an object's mass and velocity. It matters because both affect how difficult the object is to stop, and momentum has a direction as well as a size.
How to calculate momentum
Use momentum = mass × velocity. The equation is on the equation list, but you need to know what each symbol and unit means. The mass must be in kilograms and the velocity in metres per second, so the answer is in kg m/s. p is momentum, measured in kg m/s. m is mass, measured in kg. v is velocity, measured in m/s. Velocity includes a direction. Momentum is not kinetic energy. It uses a different equation, has a different unit, and has a direction.
How direction affects momentum
Momentum is a vector, which means it has both a size and a direction. Choose one direction as positive. The opposite direction is negative, so objects moving at the same speed in opposite directions have equal-sized momenta with opposite signs. For two 2 kg trolleys moving at 3 m/s in opposite directions, choose right as positive. Trolley A has velocity +3 m/s and momentum +6 kg m/s. Trolley B has velocity −3 m/s and momentum −6 kg m/s. Their total momentum is zero because +6 + (−6) = 0.
Convert units before substituting
Questions may give mass in grams or speed in kilometres per hour. Convert these first, because the momentum equation needs mass in kilograms and velocity in metres per second. Convert grams to kilograms by dividing by 1000. Convert km/h to m/s by dividing by 3.6.
Find a missing mass or velocity
You can rearrange p = m × v when a question gives momentum and asks for another quantity. To find velocity, divide momentum by mass. To find velocity: v = p ÷ m. To find mass: m = p ÷ v.