Momentum (Edexcel GCSE Physics: Combined Science)

Revision Note

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Katie M

Last updated

24 July 2023

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Momentum

Higher Tier Only

A moving object has momentum which is defined by the equation:

p = mv

Where:
- p = momentum in kilogram metre per second (kg m/s)
- m = mass in kilograms (kg)
- v = velocity in metres per second (m/s)

This means that an object at rest (i.e v = 0) has no momentum
Momentum keeps an object moving in the same direction, making it difficult to change the direction of an object with a large momentum

Since velocity is a vector this means that the momentum of an object also depends on its direction of travel
This means that momentum can be either positive or negative
- If an object travelling to the right has positive momentum, an object travelling in the opposite direction (to the left) will have negative momentum

Negative momentum, downloadable AS & A Level Physics revision notes

The tennis ball's momentum is negative when it moves in the opposite direction to which it initially was travelling in

Therefore, the momentum of an object will change if:
- The object accelerates (speeds up) or decelerates (slows down)
- Changes direction
- Its mass changes

Worked example

Which object has the most momentum? WE - Momentum comparison question image, downloadable AS & A Level Physics revision notes

WE - Momentum comparison answer image, downloadable AS & A Level Physics revision notes

Both the tennis ball and the brick have the same momentum
Even though the brick is much heavier than the ball, the ball is travelling much faster than the brick
This means that on impact, they would both exert a similar force (depending on the time it takes for each to come to rest)

Examiner Tip

Remember the units of momentum as kg m/s which is the product of the units of mass (kg) and velocity (m/s).Which direction is taken as positive is completely up to you in the exam. In general, the right and upwards are taken as positive, and down or to the left as negative.

Examples of Momentum

Higher Tier Only

Examples of momentum in an event are collisions
Objects will either:
- Collide and move in opposite directions - this is an elastic colision
- Collide and move in the same direction together - this is an inelastic collision
When the objects move in opposite directions:
- Each object will have a different velocity depending on its mass and initial momentum of the system
When the objects move in the same direction together:
- They will have a combined mass and velocity
Momentum is always conserved in a collision

Elastic & Inelastic Collisions, downloadable AS & A Level Physics revision notes

The difference between elastic and inelastic collisions

Examiner Tip

If an exam question asks you to analyse a collision, follow these tips for full marks:

Always consider the motion before and after the collision and state:
- The velocities of each object
- The direction each object moves
State whether the collision was elastic or inelastic and explain your reasoning
- In a perfectly elastic collision, the kinetic energy is the same before and after
- In a perfectly inelastic collision, the two objects stick together after colliding
Describe any energy transfers that occur if kinetic energy is not conserved
- For example, it may be converted into heat, sound, elastic potential energy etc