Newton’s Laws of Motion
Identify the object, list its forces, and reason from the net force.
Newton’s Laws
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Motion changes only when forces are unbalanced
First law: zero net force means no acceleration. Second law: Fnet=ma. Third law: interacting objects exert equal and opposite forces on each other.
One object at a time
Only combine forces acting on the same object.
Third-law pair
The two forces act on different objects, so they do not cancel on one free-body diagram.
Object → forces → net force → acceleration
Problem: A 3-kg cart has 12 N net force east.
Recognize: Net force and mass are given.
Do it fast: a=F/m=12/3=4 m/s² east.
Why it works
Separating the object from its interactions prevents you from mixing action-reaction forces or unrelated forces.
Five forms you should recognize
A passenger continues forward when a bus stops.
12 N on 3 kg gives 4 m/s².
A swimmer pushes water back; water pushes the swimmer forward.
Weight and normal force balance for a resting book.
Kinetic friction opposes sliding.
Check before you commit
- Saying zero net force means zero velocity
- Putting a third-law pair on one object
- Using mass as weight
- Adding opposite forces without direction
- Calling inertia a force
- Assuming the heavier object always falls faster
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Newton’s Laws of Motion FAQ
Do balanced forces mean an object is stopped?
No. It may also move with constant velocity.
Why do action-reaction forces not cancel?
They act on different objects.
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