Newton's laws of motion
Three laws govern how forces affect motion.
First law — inertia
An object remains at rest, or in uniform motion in a straight line, unless acted on by a resultant force.
Implications:
- No force needed to keep something moving (in space, satellites coast forever).
- What we call "needing a push" on Earth is to overcome friction.
- Mass measures inertia — resistance to changes in motion.
Second law
Resultant force equals mass times acceleration.
$F = ma$
- $F$ in newtons (N).
- $m$ in kilograms (kg).
- $a$ in m/s² (in the direction of $F$).
A larger force gives more acceleration; a heavier object accelerates less for the same force.
Third law — pairs
When object A exerts a force on object B, object B exerts an equal and opposite force on A.
Pairs are always:
- Same magnitude.
- Opposite direction.
- On different objects.
Example — walking:
- You push back on the ground (force from your foot).
- Ground pushes forward on you (reaction).
- The reaction propels you forward.
✦Worked example
A 1500 kg car accelerates at 3.0 m/s². Find the resultant force.
- $F = ma = 1500 \times 3.0 = 4500$ N.
Inertia and inertial mass
The inertial mass of an object is its resistance to acceleration. From $a = F/m$, larger $m$ means smaller $a$ for the same $F$. So a 5 kg object has more inertia than a 0.5 kg one.
⚠Common mistakes
- Saying the third-law pair is on the same object (it's two forces on different objects).
- Confusing equilibrium ($F = 0$) with motion ($v = 0$). An object can be moving with $F = 0$.
- Forgetting that mass and weight are different (mass is in $F = ma$, not weight).
- Saying "objects naturally come to rest" — they only do so because of friction.
AI-generated · claude-opus-4-7 · v3-deep-physics