Momentum and Conservation of Momentum
Momentum and Collisions
This GCSE physics lesson explores Momentum and Conservation of Momentum. Students calculate momentum, apply conservation of momentum to collisions and explosions and connect changes in momentum to force and stopping time. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Lesson duration is: 55 minItems in this lesson
Momentum and Collisions
This GCSE physics lesson explores Momentum and Conservation of Momentum. Students calculate momentum, apply conservation of momentum to collisions and explosions and connect changes in momentum to force and stopping time. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Momentum, KS4 Physics, GCSE Physics, Conservation of Momentum, Momentum Formula, $p = mv$, Impulse, Collisions, Explosions, Safety Features, Force and Momentum, Change in Momentum, MCQs, Open Questions, Polls, Emoji Polls, Exit Tickets
Learning goal
I can explain momentum as mass × velocity, calculate momentum in simple situations, and apply conservation of momentum to one-dimensional collisions and explosions.
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What words come to mind when you hear momentum?
Use this to surface prior ideas such as motion, mass, speed, force, collisions or direction. Pick up misconceptions like 'force' and revisit them later.
Momentum as mass × velocity
Momentum is the quantity of motion of an object
It is given by the equation p = mv
p = momentum, m = mass, v = velocity
A larger mass gives a larger momentum
A larger velocity gives a larger momentum
Keep the wording consistent: momentum, mass, velocity. Avoid switching to speed at this point, because the later vector idea depends on direction.
Momentum depends on two things
Heavy objects can have large momentum even when moving slowly
Light objects can have small momentum even when moving quickly
To compare moving objects, consider mass and velocity together
A useful contrast is a lorry at modest speed versus a thrown ball at high speed. Students should see that speed alone does not tell the whole story.
Momentum is a vector
Momentum has size and direction
Its direction is the same as the object's velocity
In one dimension, one direction is often taken as positive
The opposite direction is then negative
This is the key idea students need before conservation calculations. Link signs directly to direction on a line, such as right positive and left negative.
Units of momentum
From p = mv
mass is measured in kg
velocity is measured in m/s
so momentum is measured in kg m/s
You may note that kg m/s is the standard GCSE unit. Keep unit handling explicit in every example.
Comparing momenta
To compare momentum, calculate p = mv for each object and include direction when needed.
Example: a 2 kg object moving at 3 m/s has momentum 6 kg m/s. A 4 kg object moving at 2 m/s has momentum 8 kg m/s, so it has the larger momentum.
If direction is added, remind students that 6 kg m/s to the left would be written as -6 kg m/s if right is chosen as positive.
Which change would double an object's momentum?
Double its mass
Halve its mass
Halve its velocity
Reverse direction
Tests p = mv. Reversing direction changes the sign, not the size, of the momentum.
Complete the summary
Momentum is given by p = mv. It depends on both mass and velocity. Momentum is a vector, so direction matters. Its unit is kg m/s.
All blanks come from the first subtopic: formula, factors, vector idea and unit.
Why can a lorry have more momentum than a faster ball?
Expected elements: momentum depends on both mass and velocity; a heavier object can have more momentum even if it moves more slowly; a valid comparison should mention calculating p = mv for both objects.
Conservation of momentum
In a closed system, total momentum stays constant
Momentum may move from one object to another
The total before an interaction equals the total after
This applies to collisions, explosions and recoil
Define closed system simply as one with no external forces changing the total momentum during the interaction.
The conservation equation
total momentum before = total momentum after
In one dimension, add momenta with signs:
positive for one direction
negative for the opposite direction
Students often lose marks by ignoring direction. Stress that momentum is added algebraically, not by using only positive numbers.
Collision in one dimension
Two objects move along the same straight line
Calculate total momentum before collision
Calculate total momentum after collision
Set the two totals equal
Keep examples one-dimensional throughout: objects moving in the same or opposite directions along a line.
Worked collision example
A 3 kg trolley moves at 4 m/s and hits a 1 kg trolley at rest. They stick together.
Before collision:
total momentum = (3 × 4) + (1 × 0) = 12 kg m/s
After collision:
combined mass = 4 kg
velocity = 12 ÷ 4 = 3 m/s
This example also sets up inelastic collisions, because the trolleys stick together after impact.
Explosion and recoil
In an explosion, one object separates into parts
If it starts at rest, total momentum before is zero
After the explosion, momenta in opposite directions add to zero
Recoil is the backward motion when something is pushed forward
Emphasise that zero total momentum before does not mean zero momentum for each part afterwards. It means the positive and negative momenta balance.
Worked recoil example
A 0.05 kg bullet moves forwards at 200 m/s from a 2.0 kg rifle initially at rest.
bullet momentum = 0.05 × 200 = 10 kg m/s
rifle momentum = -10 kg m/s
rifle velocity = -10 ÷ 2.0 = -5 m/s
Use this to reinforce sign convention. The negative sign shows the rifle moves in the opposite direction to the bullet.
In a closed system, what is always conserved?
Total momentum
Each velocity
Total speed
Each mass
Students may confuse conservation of total momentum with each object's velocity staying the same.
Complete the conservation idea
In a closed system, total momentum before an interaction equals total momentum after. In one dimension, momentum in one direction can be taken as positive and the opposite direction as negative. In an explosion starting from rest, the total momentum before is zero.
Reinforces the key language needed for collision and recoil calculations.
Elastic and inelastic collisions
In all collisions in a closed system, momentum is conserved
In an elastic collision, kinetic energy is also conserved
In an inelastic collision, kinetic energy changes
Objects that stick together are inelastic
Keep the distinction qualitative: the lesson focus is what is always conserved and what may change.
What changes in collisions?
Momentum stays the same overall in a closed system
Velocity of each object may change
Kinetic energy may stay the same or may decrease
Sound, heating and deformation can occur in inelastic collisions
Students often think conservation means every object's momentum stays the same. Clarify that only the total momentum of the system is conserved.
GCSE-style calculation method
Choose a positive direction
Calculate momentum of each object before
Add to find total momentum before
Set this equal to total momentum after
Solve for the unknown velocity or momentum
Check the sign and unit
This method works for both collisions and explosions. Encourage students to write the momentum equation before substituting numbers.
A 2 kg object moves right at 5 m/s. What is its momentum?
10 kg m/s
7 kg m/s
3 kg m/s
25 kg m/s
Simple application of p = mv before the class moves to more involved conservation questions.
Which statement about an inelastic collision is correct?
Momentum is conserved
Velocity is conserved
Mass is destroyed
Direction is unchanged
Checks the qualitative distinction: momentum is always conserved in a closed system, while kinetic energy may change.
What is one thing you will remember about momentum
Use the responses to summarise key takeaways before the glossary: p = mv, direction, kg m/s, total before = total after, recoil, elastic/inelastic.
How confident do you feel about explaining what momentum means?
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Glossary
momentum - the quantity of motion of an object, given by mass multiplied by velocity
mass - the amount of matter in an object, measured in kilograms
velocity - speed in a stated direction
vector - a quantity with both size and direction
kg m/s - the unit of momentum
closed system - a system where no external forces change the total momentum
conservation of momentum - the rule that total momentum before equals total momentum after
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Glossary
collision - an interaction in which moving objects hit each other
elastic collision - a collision where momentum and kinetic energy are both conserved
inelastic collision - a collision where momentum is conserved but kinetic energy changes
recoil - backward motion caused when something else moves forwards
explosion - an event where parts of an object move apart and share momentum in opposite directions
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