Describe examples of momentum in collisions

The idea

Momentum is mass × velocity, so a moving object carries momentum in the direction it travels. In a collision, momentum is conserved when no external resultant force acts: the total momentum before equals the total momentum after. Describe examples by naming the objects, giving their masses and velocities, and comparing total momentum before and after. For instance, a 2 kg trolley moving at 3 m/s has momentum 6 kg m/s; if it collides with a stationary 1 kg trolley and they stick together, the combined 3 kg mass moves at 2 m/s, giving 6 kg m/s again. A rocket expelling gas backwards is a recoil example, not a collision.

What earns the marks

• States that momentum equals mass multiplied by velocity and is a vector quantity with direction.
• Identifies the objects involved in a collision and gives their masses and velocities before impact.
• Calculates total momentum before the collision by summing the individual momenta with correct signs.
• Compares total momentum before and after the collision to show it is conserved when no external resultant force acts.
• Describes a recoil or explosion example, such as a rocket or two separating trolleys, where total momentum stays zero.

Common mistakes and how to put them right

• Adds momenta without regard to direction.

  Put it right: Assign opposite directions opposite signs before summing.

• Treats momentum as a scalar and ignores velocity direction.

  Put it right: State that momentum is a vector.

• Assumes momentum is lost in every collision.

  Put it right: Distinguish elastic and inelastic collisions, where momentum is conserved but kinetic energy may not be.

• Confuses momentum with kinetic energy.

  Put it right: Keep mass × velocity separate from ½mv².

Exam tips

• Write the momentum equation and substitute values with units before choosing an option.
• Check the direction of each velocity and use signs consistently in the total.
• Eliminate options that ignore conservation of momentum when no external force acts.
• Show the before-and-after totals so the reasoning is visible.