4PH1

Momentum

Forces & Motion · 1 question type

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

stable

Medium

Stable10%

Conservation of momentum and impulse calculations appear consistently across both papers.

Momentum is a property carried by every moving object, telling you how hard it would be to stop or to turn
A heavier object moving at the same speed has more momentum than a lighter one; the same object moving faster has more momentum than when it was moving slowly
A stationary object has zero , so at v = 0, p = 0

p = m × v

where:
- p = momentum (kg m/s)
- m = mass (kg)
- v = velocity (m/s)
The SI unit follows from the equation: kilograms × metres per second → kg m/s

Velocity carries a direction, and momentum is mass × velocity, so momentum is a vector quantity that points the same way the object is moving
When working with momentum on a straight line:
- Pick one direction as positive (rightwards and upwards are the usual conventions)
- The opposite direction is then negative
- A momentum that comes out negative simply means the object is moving the opposite way to the direction you chose as positive

An object's momentum changes whenever any of the following happen:
- its velocity increases or decreases (any acceleration along the line of motion)
- its direction of travel changes (which is also an acceleration, even at constant speed)
- its mass changes (such as a rocket burning off fuel)

Worked example

Calculating momentum: p = mv

A 3.0 kg ball rolls along a track at 8.0 m/s.

Solution: