# Edexcel A Level Physics:复习笔记2.13 Momentum

### Defining 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

Which object has the most momentum?

• 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  throughout the question.

Sketching a diagram which includes the signs on positive and negative values will help you avoid mistakes when calculating