When two carts push apart, each pushes the other. The pushes act on different carts. With equal masses, the carts move away equally quickly. A heavier cart gains less speed from the same brief push. The two interaction forces are equal in size and opposite in direction, even when the masses differ. Newton’s third law holds during an interaction even when the masses and accelerations differ. Our isolated two-cart system keeps total momentum at zero as they separate. The selected impulse determines each velocity change through Δv = J ÷ m, with opposite signs assigned to the two carts.
Forces, motion and energy · Discover & try
Why can equal pushes produce different motions?
Reading level 4 of 5 · FluentChange
Word help
Interaction pair
Equal and opposite forces acting on two different objects.
Impulse
The accumulated effect of force over time, changing momentum.
Momentum
Mass multiplied by velocity.
Watch · Try · Explain
See how it works
The interactive activity is loading.
Ideal two-cart recoil after a selected impulse, with no external horizontal force.
What to look for
- They coast away at equal speeds in opposite directions.
- The heavier right cart moves more slowly after the same brief push.
Explain what you noticed
Two different skaters push apart, one with twice the other’s mass. Which statement fits the ideal interaction?
Keep this idea: Interaction forces are equal and opposite on different objects; different masses can accelerate differently.
Follow the sources
Source material checked 2026-10-29. Explanations and activities by Nepedia. Source links do not imply endorsement.
You’ve reached the end of this journey
Choose another journey →