SITEMAP   School-college Physics Notes: Forces & motion 6.1 What is momentum

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Forces and Newton's Laws of Motion 6.1 What is the momentum of a moving object and how do we calculate it?

Doc Brown's Physics exam study revision notes

6.1 What is the momentum of an object and how do we calculate it?

All moving objects have what we call momentum.  (sometimes denoted by a lower case p)

The faster an object moves, the bigger its momentum.

The bigger the mass of a moving object, the bigger its moment.

The momentum of an object is directly proportional to its velocity AND its mass.

Momentum (p) is a vector quantity, it has both size and direction

Therefore the equation for momentum is ...

momentum (kilogram metre per second) = mass (kilogram) × velocity (metre per second)

momentum p (kg m/s) = mass m (kg) x velocity v (m/s)

p = m × v

Example of simple calculations using the formula for momentum

Q1  A 120 kg sprinter is travelling at 9.0 m/s, what is the momentum of the runner?

Q2 If a 20 g bullet has a momentum of 8 kg m/s, calculate its velocity.

p = m × v,  v = p/m     (don't forget to change the g to kg!)

Q3 What mass must an object have if it has a momentum of 1.5 x 106 kg m/s and a velocity of 30 m/s?

p = m × v,  m = p/v

Q4 What is the velocity of a 1500 kg car travelling north with 30 000 kg m/s of momentum?

Keywords, phrases and learning objectives for elastic/inelastic collisions and momentum

Know the formula for momentum is p = mv (momentum = mass x velocity)

Know how to calculate the momentum of a moving object.

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Worked out ANSWERS to the simple momentum calculation questions

Example of how to work out simple calculations using the formula for momentum

Q1  A 120 kg sprinter is travelling at 9.0 m/s, what is the momentum of the runner?

momentum p = m × v

p = 120 x 9 = 360 kg m/s

Q2 If a 20 g bullet has a momentum of 8 kg m/s, calculate its velocity.

p = m × v,  v = p/m     (don't forget to change the g to kg!)

velocity = 8/(20/1000) = 400 m/s

Q3 What mass must an object have if it has a momentum of 1.5 x 106 kg m/s and a velocity of 30 m/s?

p = m × v,  m = p/v

mass = 1.5 x 106/30 = 5.0 x 104 kg

Q4 What is the velocity of a 1500 kg car travelling north with 30 000 kg m/s of momentum?

p = m x v, v = p / m

v = 30 000 / 1500 = 20 m/s in a northerly direction

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