Newton's Three Laws and F = ma: Rules and Worked Examples
Newton's three laws in plain language, how to use F = ma, and two worked examples, plus the action-reaction mistake to avoid.
Newton's three laws
- 1st · no net force, no change in motion
- 2nd · F = ma
- 3rd · forces come in equal, opposite pairs
Inertia, acceleration, interaction.
F = ma IN PRACTICE
- F · net force in newtons (N)
- m · mass in kilograms (kg)
- a · acceleration in m/s²
Net force means add up all forces on the object first.
Example 1
A 2 kg cart gets a net force of 10 N. Find its acceleration.
- Use F = ma, so a = F / m.
- a = 10 / 2
a = 5 m/s²
Example 2
A 5 kg box accelerates at 3 m/s². Find the net force.
- Use F = ma.
- F = 5 × 3
F = 15 N
Common mistake
Action and reaction forces cancel each other
They act on different objects, so they never cancel.
Only forces on the same object add up to the net force.
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