Momentum is usually measured in kg m/s.
WORKED EXAMPLE
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m = mass in kg v = velocity in m/s
momentum = mass × velocity momentum = mv
Calculate the momentum of a racing car of mass 600 kg travelling at 75m/s. Write down the formula: Substitute the values for m and v: Work out the answer and write down the units:
REMEMBER
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International GCSE: physics
MOMENTUM Momentum is a quantity of motion. It is a measure of how hard it is to change the motion of an object. Momentum is useful in analysing the motion of objects. It can be calculated (and is defined) using the formula:
momentum = mass × velocity momentum = 600 kg × 75 m/s momentum = 45 000 kg m/s
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The unit of momentum is quite a tricky one – usually kg m/s. It will often be given to you in the question, but make sure you learn it; if it isn’t given in the answer space there will be a mark for it. In other words, if a question does not give a unit beside the space for the answer, then you must insert the unit (if the quantity has one) or you will lose a mark!
QUESTIONS
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1. What is the momentum of a 58 g tennis ball travelling at 40 m/s? 2. What is the momentum of a 2000 kg car travelling at 25 m/s? 3. What is the mass of a car that is travelling at 20 m/s and has a momentum of 20 000 kg m/s?
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4. What is the velocity of a car that has a mass of 1500 kg and a momentum of 37 500 kg m/s?
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CHANGING MOMENTUM In practical applications, such as safety in cars (see page 00) we are interested in the change in momentum. Since velocity is a vector quantity (it has a direction), momentum is also a vector quantity and has a direction. When working out problems, one direction is called the positive direction, for example, going to the right, with the opposite direction taken as the negative direction, for example, going to the left.
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