Edexcel GCSE Physics (1PH0) — Motion and forces
Imagine trying to stop a rolling tennis ball compared to a rolling car moving at the exact same speed. The car is much harder to stop because it has significantly more momentum. Momentum is a property possessed by all moving objects, defined precisely as the product of an object's mass and its velocity.
If an object is perfectly stationary, its velocity is zero. This means its momentum is always exactly kg m/s, regardless of how massive the object is. Momentum is directly proportional to both mass and velocity, meaning that doubling either the mass or the velocity of an object will cause its total momentum to double.
Because velocity has a specific direction, momentum is a vector quantity. This means it has both a magnitude (size) and a specific direction.
In Physics, we use positive and negative signs to indicate this direction. Typically, an object moving to the right or upwards is given a positive () momentum, while an object moving to the left or downwards has a negative () momentum.
For example, if two identical kg balls move towards each other at m/s:
To calculate momentum, you must recall and use the following equation:
Where:
Note that momentum can also be expressed in Newton seconds (N s), which is exactly equivalent to kg m/s.
Examiners frequently test your ability to use this equation, sometimes hiding "conversion traps" where mass is given in grams rather than kilograms.
A car of mass 1200 kg travels at a velocity of 25 m/s. Calculate its momentum.
Step 1: Identify the formula and values.
Step 2: Substitute into the equation.
Step 3: Calculate the final answer with units.
An owl has a momentum of 5.04 kg m/s and flies at a velocity of 3.6 m/s. Calculate its mass.
Step 1: Identify the formula and values.
Step 2: Substitute into the equation.
Step 3: Rearrange to find mass.
Step 4: Calculate the final answer with units.
Students often lose the final evaluation mark by writing the unit as 'kg/m/s' or 'kg/s' instead of the strictly required 'kg m/s'.
In Higher Tier collision or 'rebound' questions, you must change the sign of an object's velocity or momentum to negative if it changes direction.
Watch out for the 'conversion trap' — always divide mass in grams (g) by 1000 to convert it to kilograms (kg) before substituting it into the momentum equation.
The formula p = m x v is not provided on the Edexcel formula sheet for the 1PH0 or 1SC0 exams, so you must memorise it.
Momentum
The product of an object's mass and its velocity. It is a measure of how difficult it is to stop a moving object.
Vector quantity
A physical quantity that has both a magnitude (size) and a specific direction.
Mass
The amount of matter in an object, measured in kilograms (kg).
Velocity
The speed of an object in a given direction, measured in metres per second (m/s).
Kilogram metres per second
The standard SI unit for momentum, often written as kg m/s.
Put your knowledge into practice — try past paper questions for Physics
Momentum
The product of an object's mass and its velocity. It is a measure of how difficult it is to stop a moving object.
Vector quantity
A physical quantity that has both a magnitude (size) and a specific direction.
Mass
The amount of matter in an object, measured in kilograms (kg).
Velocity
The speed of an object in a given direction, measured in metres per second (m/s).
Kilogram metres per second
The standard SI unit for momentum, often written as kg m/s.
