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Two carts crash on a track. Where does the push go?
Crashing Carts lab · Last updated: 16 October 2026
Two carts roll towards each other on a track and crash. One may stop dead, both may bounce apart, or they may stick and roll on together. You change the carts and the speeds and track where the push goes.
What you'll discover
- When carts collide, the push one loses is gained by the other.
- The total momentum of both carts is the same before and after the crash.
- Bouncy crashes and sticky crashes share momentum differently, but both keep the total.
The science in plain words
Momentum is how much "oomph" a moving thing has. Scientists calculate it as mass multiplied by velocity, so a heavy cart moving slowly can have as much momentum as a light cart moving fast. Direction matters too: carts moving opposite ways have momentum with opposite signs.
Newton's third law says that when two objects push on each other, the pushes are equal and opposite. During a crash the first cart pushes the second forward exactly as hard as the second pushes the first back, and for exactly the same time. So whatever momentum one cart loses, the other gains.
This means the total momentum of the two carts is the same just before and just after the collision. This is the law of conservation of momentum, and it holds for every collision as long as no outside force, such as friction, interferes much.
Energy is a different story. In a springy, bouncy collision most of the movement energy survives. In a sticky collision, where the carts lock together, some movement energy turns into heat and sound. Yet momentum is still conserved in both. Car safety design, snooker and rocket engines all depend on this idea.
Where it fits
- India, NCERT: Class 9 science, Chapter 8: Force and Laws of Motion
- United States: Grades 6 to 12
- Next Generation Science Standards: MS-PS2-1, HS-PS2-2
Common questions
What is momentum in simple words?
Momentum is how hard it is to stop something that is moving. It is mass times velocity, so a heavy, slow lorry and a light, fast cricket ball can both carry a lot of it.
Is momentum always conserved in a collision?
Yes, the total momentum of the objects colliding stays the same, as long as no large outside force such as friction acts during the crash. Movement energy may be lost as heat and sound, but momentum is not.
What happens when a moving cart hits a still cart of the same mass?
If the crash is perfectly bouncy, the moving cart stops and the still cart rolls away at the first cart's speed. If they stick together, both roll on at half the original speed.
Related labs
- Bouncing Balls: Drop a rubber ball from 30 cm. How high does it come back?
- Friction Drag Test: Pull a block with a newton meter. How hard do you have to pull on ice, wood or sandpaper?
- Slide and Light Gates: Let a toy sled slide down a slide. How fast does it pick up speed?
See all physics labs or every Taz science lab.
Try it yourself
The Crashing Carts lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.