Mass measures how strongly an object resists a change in velocity. Inertia is a property, not a hidden force pushing forward. The drawing can show either ideal motion with zero horizontal net force or a constant opposing force until the cart stops. It does not reverse a stopped cart by continuing friction backward.
Forces, motion and energy · Discover & try
Why does a moving object not always need another push?
Reading level 3 of 5 · IndependentChange
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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
- Follow motion with no opposing force.
- Find the interaction responsible for slowing.
- Check that the stopped model does not reverse.
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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Source material checked 2026-10-28. Explanations and activities by Nepedia. Source links do not imply endorsement.
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