Certificate Locked
Complete the course to unlock the certificate
0% Completed
Required 100%
Newton’s Third Law states:
If one object exerts a force on a second object, the second object exerts an equal-magnitude and opposite-direction force on the first object.
Mathematically:
\(\vec{F}_{AB}=-\vec{F}_{BA}\)
where:
Forces always occur in pairs.
Every interaction involves:
These forces:
A common misconception is that the forces cancel.
They do not cancel because they act on different objects.
Example:
If a person pushes a wall:
The forces are equal and opposite, but they act on different bodies.
Third-law force pairs always:
Never on the same object.
\(|\vec{F}_{AB}|=\vec{F}_{BA}|\)
\(\vec{F}_{AB}=-\vec{F}_{BA}\)
Examples:
When walking:
The forward force from the ground accelerates the person.
A rocket pushes exhaust gases downward.
The gases push the rocket upward with equal and opposite force.
The book pushes downward on the table.
The table pushes upward on the book.
These are a third-law pair.
Important:
Newton’s Third Law explains conservation of momentum in isolated systems.
Internal interaction forces cancel in pairs:
\(\vec{F}_{12}=-\vec{F}_{21}\)
Thus total momentum remains constant when no external forces act.
When drawing free-body diagrams:
This is essential for avoiding sign and force-counting errors.
Newton’s Third Law explains interactions between objects:
It shows that forces are mutual interactions rather than isolated effects.
Newton’s Third Law:
\(\vec{F}_{AB}=-\vec{F}_{BA}\)
Key ideas:
You have not completed all required lessons and assessments.