Define momentum and describe examples of momentum in collisions (to…

The idea

Momentum is a vector quantity defined as mass multiplied by velocity, p = m × v, measured in kilogram metres per second (kg m/s). Because velocity has direction, momentum has direction too: an object moving left has momentum in the opposite sense to one moving right. In a collision, the total momentum of the interacting objects before the collision equals the total momentum after it, provided no external resultant force acts on the system. This is the law of conservation of momentum. For example, two trolleys that collide and stick together share a combined momentum equal to the sum of their individual momenta before impact, allowing the combined velocity to be calculated. When objects move in opposite directions, treat one direction as positive and the other as negative before adding.

What earns the marks

• Defines momentum as mass multiplied by velocity, p = m × v, with units of kg m/s.
• States that momentum is a vector, so direction must be considered.
• Applies the law of conservation of momentum: total momentum before a collision equals total momentum after, with no external resultant force.
• Calculates total momentum before a collision by summing individual momenta, treating opposing directions with opposite signs.
• Uses conservation of momentum to find an unknown velocity or mass after a collision, such as objects sticking together.

Common mistakes and how to put them right

• Uses speed instead of velocity when defining momentum.

  Put it right: State that momentum depends on velocity and therefore has direction.

• Adds momenta of objects moving in opposite directions without signs.

  Put it right: Assign one direction positive and the other negative.

• Applies conservation of momentum when an external resultant force acts.

  Put it right: State that the law holds only when no external resultant force acts on the system.

• Omits units or uses the wrong units.

  Put it right: Give momentum in kg m/s.

Exam tips

• Write down p = m × v and list each object's mass and velocity before substituting values.
• Choose a positive direction and mark opposing velocities as negative before summing momenta.
• Check that the total momentum after the collision equals the total before as a final verification.