HS-PS2-2: Conservation of Momentum in Collisions
Also called NGSS HS-PS2-2 · Topic: Forces and Interactions · Physical Science
The Official Wording
Use mathematical representations to support the claim that the total momentum of a system of objects is conserved when there is no net force on the system.
Clarification statement: Emphasis is on the quantitative conservation of momentum in interactions and the qualitative meaning of this principle.
Assessment boundary: Assessment is limited to systems of two macroscopic bodies moving in one dimension.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-PS2-2 Means in Plain English
High school students calculate momentum, mass times velocity, for two objects before and after they collide or push apart. They use the numbers to argue that the total stays the same when nothing outside the pair exerts a net force, and explain what that means.
Problems involve just two everyday objects moving along one line, not angled or three-body collisions. The work follows Newton's second law (HS-PS2-1) and supports designing collision-safety devices in HS-PS2-3.
HS-PS2-2 I Can Statements
- I can calculate momentum as mass times velocity, with direction.
- I can show that total momentum is the same before and after a collision.
- I can predict a final velocity using conservation of momentum.
HS-PS2-2 Worked Example
A collision problem worked from the kind of numbers students measure on an air track with Velcro bumpers.
On a level air track, a 0.20 kg glider moving at 0.60 m/s hits a 0.30 kg glider at rest. Velcro makes them stick. How fast do they move together?
- Find the momentum of each glider before the collision.0.20 × 0.60 = 0.12 kg·m/s and 0.30 × 0 = 0
- Add them for the total momentum before.Total before = 0.12 kg·m/s
- Write the momentum after, when they move as one mass.(0.20 + 0.30) × v = 0.50v
- Set the totals equal and solve for v.0.50v = 0.12, so v = 0.24 m/s
Check it: Momentum after is 0.50 kg × 0.24 m/s = 0.12 kg·m/s, matching the total before, and the speed dropped because the mass grew.
Common Mistakes with HS-PS2-2
Ignoring direction
Adding two momenta of carts moving toward each other as both positive
Try this: Momentum has direction. Choose one direction as positive and give objects moving the other way a negative velocity before adding.
Thinking speed is conserved
“The gliders should still go 0.60 m/s together.”
Try this: Momentum is conserved, not speed. When mass increases after sticking, the velocity must drop to keep the total momentum the same.
Applying conservation with an outside force
Using conservation for a cart dragged by friction over a long distance
Try this: Conservation holds only when the net outside force is close to zero, such as during a brief collision on a low-friction track.
Where HS-PS2-2 Fits
Practice HS-PS2-2
Quick Check
- What is the momentum of a 1,200 kg car moving at 15 m/s?
- Two skaters at rest push apart. What is their total momentum afterward?
- Why must a heavier object move slower to have the same momentum as a lighter one?