A moving cart can keep going without another forward push. Friction and air resistance can slow real carts. In this model, turning resistance on makes the same cart slow down and stop. Compare the same starting speed with and without a resisting force. Inertia is not another forward push.
Forces, motion and energy · Discover & try
Why does a moving object not always need another push?
Reading level 2 of 5 · DevelopingChange
Word help
Inertia
The tendency to preserve velocity unless a net force acts.
Resistance
An interaction that can oppose relative motion.
Reference frame
The viewpoint relative to which motion is described.
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See how it works
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Ideal free motion compared with a selected opposing force; no reverse motion after the model stops.
What to look for
- The speed stays at 3 m/s; equal-time position marks stay equally spaced.
- The speed falls to zero and the cart stays stopped.
Explain what you noticed
A separate probe coasts through space with no net external force. Its engine is switched off. What does the ideal first-law model predict?
Keep this idea: Inertia preserves motion when net force is zero; it is not an extra forward force.
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