Linear Momentum
- When an object with mass is in motion and therefore has a velocity, the object also has momentum
- Linear momentum is the momentum of an object that is moving in only one dimension
- The linear momentum of an object remains constant unless the system is acted upon by an external resultant force
- Momentum is defined as the product of mass and velocity
- Where:
- = momentum, measured in kg m s−1
- - mass, measured in kg
- = velocity, measured in m s−1
- Momentum is a vector quantity with both magnitude and direction
- The initial direction of motion is usually assigned the positive direction
When the ball is travelling in the opposite direction, its velocity is negative. Since momentum = mass × velocity, its momentum is also negative
Worked example
A tennis ball of mass 60 g travels to the right with a speed of 75 m s−1.
A brick of mass 3 kg is thrown to the right at a speed of 1.5 m s−1.
Determine which object has the greatest 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)
Examiner Tip
Since the SI units for momentum are kg m s−1:
- If the mass is given in grams, you need to convert to kg by dividing the value by 1000
- If the velocity is given in km s−1, you need 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