Define momentum, recall and use the equation: momentum (kilogram metre per…

The idea

Momentum is a vector quantity defined as the product of a body's mass and its velocity, measured in kilogram metre per second (kg m/s). The equation is p = m × v, where p is momentum in kg m/s, m is mass in kg and v is velocity in m/s. Because velocity is a vector, momentum has direction: a body moving in the opposite direction has momentum of the opposite sign. For example, a 2 kg trolley moving at 3 m/s has momentum 2 kg × 3 m/s = 6 kg m/s in its direction of travel. Convert grams to kilograms and other units before substituting, and keep the direction when combining momenta.

What earns the marks

• Defines momentum as mass multiplied by velocity and identifies it as a vector quantity.
• States the unit of momentum as kilogram metre per second (kg m/s).
• Uses the equation momentum = mass × velocity with mass in kg and velocity in m/s.
• Calculates momentum correctly after converting mass to kilograms where necessary.
• Assigns the correct sign or direction to momentum when velocity reverses.

Common mistakes and how to put them right

• Uses speed in place of velocity and ignores direction.

  Put it right: Treat momentum as a vector and include its direction.

• Substitutes mass in grams without converting.

  Put it right: Convert grams to kilograms before multiplying.

• Confuses the unit of momentum with the unit of force.

  Put it right: Write momentum in kg m/s, not newtons.

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

  Put it right: Give opposite directions opposite signs before combining.

Exam tips

• Write the equation p = m × v, substitute values with units, then state the answer with its unit and direction.
• Convert all masses to kilograms and all velocities to metres per second before calculating.
• Check the sign of each momentum when a question involves bodies moving towards each other.