Grade 10 · Newton's Laws and Momentum · Newton's Laws
Momentum and Collisions
Why crumple zones save lives.
Learning objectives
- Calculate momentum from mass and velocity.
- Apply conservation of momentum to a collision.
- Explain safety features using change in momentum over time.
NGSS alignment
HS-PS2-3
Apply science and engineering ideas to design, evaluate, and refine a device that minimizes the force on a macroscopic object during a collision.
HS-PS2-2
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.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion C — Processing and Evaluating
- Criterion D — Reflecting on the Impacts of Science
ENGAGE
A car designed to crumple
Modern cars are built to crumple in a crash. A rigid car body would survive better, yet its passengers would fare far worse.
Think about it
- What does crumpling change about the collision?
- Which quantity has to change to zero either way?
EXPLAIN
Momentum and safety
Key equation
momentum p = mass x velocity, in kg m/s. A 1200 kg car at 15 m/s has 18000 kg m/s of momentum.
In any collision with no external forces, total momentum before equals total momentum after. This lets you calculate an unknown velocity after two objects collide.
| Before | After | Result |
|---|---|---|
| 2 kg at 3 m/s hits 1 kg at rest | They stick together | Combined 3 kg moves at 2 m/s |
| Equal masses, equal opposite velocities | They stick together | Both stop; total momentum was zero |
Why crumple zones work
- Force equals change in momentum divided by time
- The change in momentum in a crash is fixed by the mass and speed
- Extending the collision time reduces the force on the passengers
- Crumple zones, airbags, seat belts and helmets all increase the stopping time
Careful
Safety features do not reduce the change in momentum. They spread the same change over a longer time, which lowers the force.
Watch
Collisions: Crash Course Physics
CrashCourse · 10 min
Conservation of momentum applied to real collisions.
Before you watch: What stays constant in a collision?
- Calculate the momentum of a 2 kg ball at 3 m/s.
- Explain a crumple zone using force and time.
Interactive simulation · PhET
Projectile Motion
Fire objects of different mass at the same speed and compare how far each pushes the target.
While you explore
- Which object carried more momentum?
- How does mass affect the impact for the same speed?
Key vocabulary
- Crumple zone
- A part of a car designed to fold in a crash.
- Momentum
- Mass in motion: how hard something is to stop.
Practice questions
0/2 correct
Level 1 · Criterion A
Calculate the momentum of a 4 kg object moving at 5 m/s.
Level 2 · Criterion A
A 2 kg trolley at 3 m/s collides and sticks to a 1 kg trolley at rest. Their combined speed is:
MYP criterion tasks
Level 3 · Criterion D
Evaluate the claim that a stiffer, stronger car would be safer for its passengers in a crash.
Reflect & track
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