Scivra
Pro šŸ”’~14 min

Newton's Three Laws of Motion

Inertia, F=ma, and action-reaction pairs

How it works

Newton's First Law (Inertia): An object at rest stays at rest; an object in motion stays in motion at constant velocity, unless acted upon by a net external force. Inertia is the resistance to change in motion. Newton's Second Law (F=ma): The net force on an object equals its mass times its acceleration. A larger force or smaller mass means greater acceleration. Newton's Third Law (Action-Reaction): When object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. Note: these forces act on DIFFERENT objects ā€” they never cancel. Rocket engines work by Newton's Third Law: exhaust gas pushed backward ā†’ rocket pushed forward.

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Step-by-step

  1. Select which law to demonstrate (Law selector, Pro).
  2. In Law 2 mode, adjust the Force and Mass sliders ā€” watch acceleration (a = F/m) change.
  3. Add friction to see it oppose motion.
  4. In Law 3 mode, watch the collision force pairs ā€” equal magnitude, opposite direction.
  5. Free body diagrams update in real time.

Key formulas

  • 1st:Ā āˆ‘F=0ā‡’a=0(inertia)\text{1st: } \sum F=0 \Rightarrow a=0 \quad \text{(inertia)}Law 1: An object at rest/motion stays that way unless acted on by net force
  • 2nd:Ā āˆ‘F=maa=Fnetm\text{2nd: } \sum F = ma \quad a = \frac{F_{net}}{m}Law 2: Net force equals mass times acceleration
  • 3rd:Ā FAB=āˆ’FBA(action-reaction)\text{3rd: } F_{AB} = -F_{BA} \quad \text{(action-reaction)}Law 3: Every action has an equal and opposite reaction

Frequently asked questions

A 5 kg box has a 30 N push force and 10 N friction. What is the acceleration?
Net F = 30 - 10 = 20 N. a = F/m = 20/5 = 4 m/sĀ².
Why doesn't a book sitting on a table fall? What forces act on it?
Two forces: gravity pulls it down (W=mg), the table pushes it up (normal force N). These are balanced (N=W), so net force = 0, acceleration = 0 ā€” the book stays at rest (Law 1).
A rocket expels 100 N of force backward. What force pushes the rocket forward?
Newton's 3rd Law: equal and opposite. The rocket pushes exhaust backward with 100 N, so the exhaust pushes the rocket forward with exactly 100 N.
Why does a car accelerate more slowly when fully loaded vs empty?
F=ma ā†’ a=F/m. With more mass (loaded car), the same engine force (F) produces smaller acceleration. To get the same acceleration with more mass, you need more force.