Momentum
p = m v
Higher tier: in a collision, total momentum before = total momentum after. Track the whole system.
Know the equation
| Symbol | Quantity | Unit |
|---|---|---|
| p | momentum | kg m/s |
| m | mass | kg |
| v | velocity | m/s |
Rearrangements
- m = p / v
- v = p / m
Apply it — mark your own working
Work each one out on paper first, then reveal the mark scheme and tick the marks you actually earned. That is exactly how you should mark past papers.
Trolley A of mass 2.0 kg moves at 3.0 m/s and collides with a stationary trolley B of mass 4.0 kg. The trolleys stick together after the collision. Calculate their common velocity after the collision.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Forgetting the stationary object still counts (its momentum before is 0, but its mass joins the total after they stick).
- Using only one trolley's mass for the combined object after they stick together.
- Dropping the direction: momentum is a vector, so opposite directions need opposite signs.
Exam tip: Write 'total before = total after' as your first line every time. In a stick-together collision the masses add, so the moving object slows down — sanity-check that your answer is smaller than the starting speed.
Still losing marks on the calculations?
I'll go through your working line by line and show you exactly where the marks are — your first lesson is free.