Forces: Momentum

Momentum

Momentum is a vector quantity, which means it has both magnitude (size) and direction.
It is calculated by multiplying an object’s mass by its velocity. The equation for this is momentum (kg m/s) = mass (kg) x velocity (m/s).
The unit for momentum is kilograms metres per second (kg m/s).
When no external forces act, the total momentum of a system remains constant - this is known as the principle of conservation of momentum.

If an external force is applied to an object, its momentum will change.
In a collision or an explosion between two objects in isolation (with no external forces), the total momentum of the objects before the event is equal to the total momentum after it. This is because momentum is conserved.
To solve problems involving conservation of momentum, you must remember to take the direction of movement into account. Objects moving in opposite directions will have opposite signs for their momentum.

An applied force can cause a change in an object’s momentum. This change in momentum is known as impulse.
Impulse can be calculated using the formula: Impulse (Ns) = Force (N) x Time (s).
Impulse is also equal to the change in momentum. Therefore, Force x Time = change in momentum.
A larger force or a longer application of the force will result in a greater change in momentum.
Since collisions involve changes in momentum, they also involve impulse. Measures like car crumple zones or airbags increase the time taken for a collision to occur, reducing the force experienced by the occupants.

Safety features in vehicles, such as airbags, seat belts and crumple zones, work by increasing the time taken for the wearer to stop. This decreases the force experienced by the wearer, as the change in momentum is spread out over a longer time.
Helmets and padded surfaces also work by increasing collision time, reducing impact forces and therefore lessening the risk of injury.