# 3.1.4 Linear Momentum

### Linear Momentum

• Linear momentum (p) is defined as the product of mass and velocity

Momentum is the product of mass and velocity

• Momentum is a vector quantity – it has both a magnitude and a direction
• This means it can have a negative or positive value
• If an object travelling to the right has positive momentum, an object travelling to the left (in the opposite direction) has a negative momentum
• The SI unit for momentum is kg m s−1

When the ball is travelling in the opposite direction, its velocity is negative. Since momentum = mass × velocity, its momentum is also negative

#### Worked Example

• 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)

#### Exam Tip

Since momentum is in kg m s−1:

• If the mass is given in grams, make sure to convert to kg by dividing the value by 1000.
• If the velocity is given in km s−1, make sure to convert to m s−1 by multiplying the value by 1000
• The direction you consider positive is your choice, as long the signs of the numbers (positive or negative) are consistent with this throughout the question

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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