What it does
Two carts roll along a straight track and crash. Choose their masses and velocities and whether they bounce (an elastic collision, like carts with spring buffers) or stick (like carts with Velcro). Press Run and watch them meet. Arrows show each velocity, and the bars under the track show each cart’s momentum before (pale) and after (solid). Whatever you choose, the Total bar stays exactly the same: momentum is conserved.
The Recoil experiment starts with everything at rest, so the total momentum is zero. Fire a toy cannon, jump off a boat or throw a medicine ball from a skateboard, and the two parts move apart with equal and opposite momentum. The heavier part moves more slowly.
It follows the school topics on momentum: momentum, its conservation and recoil (Class 9 Science, how forces affect motion) and collisions, impulse and the coefficient of restitution (Class 11 Physics, laws of motion and work, energy and power). The forces behind a collision are in newton's laws sandbox; for glancing collisions and your own air-track data use collision calculator, and see where the lost kinetic energy fits in energy skate track.
How to use it
- Pick a Level. Class 9 has the bouncy and sticky buttons. Class 11 swaps them for a slider of the coefficient of restitution e (1 is elastic, 0 is sticky) and adds the impulse.
- In Two carts, set the mass and velocity of each cart. Positive velocity is to the right; give B a negative velocity to make a head-on crash. If A cannot catch B, the page says they never meet.
- Press Run. Reset puts the carts back.
- In Recoil, choose an example and set the two masses and the speed of the thrown part, then press Fire.
Worked example
Two 1 kg carts, bouncy: A at 2 m/s, B at rest. After the crash A stops and B moves off at 2 m/s. Momentum 1 × 2 = 2 kg m/s before and after; kinetic energy 2 J before and after.
Make it sticky: they move off together at 2 ÷ 2 = 1 m/s. Momentum is still 2 kg m/s, but the kinetic energy is now ½ × 2 × 1² = 1 J: half went into heat, sound and squashing the Velcro.
Recoil: a 5 kg cannon fires a 0.5 kg ball at 20 m/s. The ball’s momentum is 10 kg m/s forwards, so the cannon gets 10 kg m/s backwards: V = −0.5 × 20 ÷ 5 = −2 m/s.
Tips
- Momentum has a direction. A cart moving left has negative momentum, so a head-on crash can leave both carts stopped when the momenta cancel.
- Momentum is kept in every collision; kinetic energy is kept only in an elastic one.
- In recoil the kinetic energies are not equal: the light, fast part gets most of the energy.
FAQ
Why is momentum always conserved? During the crash each cart pushes the other with an equal and opposite force for the same time (Newton’s third law), so one gains exactly the momentum the other loses.
Are real collisions perfectly elastic? Almost never. Steel balls and spring buffers come close; clay and cars that crumple are nearly sticky.
Is anything stored? No. The questions are made fresh in your browser and nothing is saved or sent.
Where the numbers come from
- Conservation of linear momentum, the velocities after a head-on collision with coefficient of restitution e, v₁ = (m₁u₁ + m₂u₂ + m₂e(u₂ − u₁)) ÷ (m₁ + m₂) and v₂ = (m₁u₁ + m₂u₂ + m₁e(u₁ − u₂)) ÷ (m₁ + m₂), kinetic energy ½mv² and recoil V = −mv ÷ M: standard school physics found in any textbook.
- Checked against an independent implementation (fibub's collision-calculator maths) for random masses, velocities and e, and against hand-worked examples (equal masses swap velocities; 2 kg at 3 m/s sticking to 1 kg gives 2 m/s). Cart and cannon sizes are made-up round values.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.