2.3.2Motion and Forces
Conservation of Momentum — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 18 of 23
Section 1 of 6Momentum
Definition
- Definition Momentum is the product of an object's mass and its velocity: p = m × v.
- It is measured in kilogram metres per second (kg m/s).
A vector quantity
- A vector quantity Momentum has a direction, the same as the velocity.
- Choose one direction as positive and give motion the other way a negative sign.
Depends on both factors
- Depends on both factors A slow lorry can have more momentum than a fast cyclist, because its mass is so much greater.
Section 2 of 6The principle
Closed system, no external force
Statement
- Statement In a closed system the total momentum before an event equals the total momentum after the event.
Closed system
- Closed system No resultant external force acts, such as friction from outside, so no momentum enters or leaves.
Momentum is shared
- Momentum is shared One object can gain momentum only if another loses the same amount.
Section 3 of 6Solving a collision problem
1
Choose a positive direction, usually the direction of the first object's motion.
2
Calculate total momentum before: m1u1 + m2u2.
3
Write the total momentum after: m1v1 + m2v2, or (m1 + m2) × v if they stick together.
4
Set the two totals equal and solve for the unknown velocity.
5
Check the sign. A negative answer means that object moves in the opposite direction.
Section 4 of 6Worked examples
| Situation | Working | Result |
|---|---|---|
| A 2 kg trolley at 3 m/s hits and sticks to a stationary 1 kg trolley | 2 × 3 = 6 kg m/s before; (2 + 1) × v = 6 | v = 2 m/s forwards |
| A 60 kg skater pushes off a 40 kg skater, both at rest; the lighter moves at 3 m/s | 0 = 40 × 3 + 60 × v | v = −2 m/s (opposite direction) |
| Gun of mass 2 kg fires a 0.02 kg bullet at 300 m/s | 0 = 0.02 × 300 + 2 × v | v = −3 m/s, recoil |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Momentum | The product of mass and velocity, p = m × v, measured in kg m/s. |
| Closed system | A set of objects with no external forces acting on it. |
| Conservation | A quantity stays constant; the total before equals the total after. |
| Explosion | An event where objects initially together fly apart, so the total momentum stays zero. |
| Recoil | Backward movement of a gun or similar object when it fires something forwards. |
Section 6 of 6Exam technique
1
Ignoring direction and adding momenta as if all were positive. Use signs.
2
Saying that kinetic energy is always conserved in collisions. Only momentum is conserved in every collision.
3
Using weight instead of mass in p = m × v.
4
Writing the answer in m/s when asked for momentum, which is in kg m/s.
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