Forces and Newton's Laws
Why Forces Matter
Newton's first law
An object stays at rest or moves at constant velocity unless acted on by a net external force. This is the idea of inertia.
Newton's second law
The net force on an object is related to its acceleration by . More generally, force is the cause of changing motion.
Newton's third law
For every action force, there is an equal and opposite reaction force. Forces come in pairs acting on different objects.
Common mistake
Action-reaction forces do not cancel each other because they act on different bodies.
According to Newton's second law, what happens when net force increases and mass stays the same?
From , if is fixed and increases, then increases.
Correct answer: Acceleration increases
What does inertia mean?
Inertia is the resistance to changes in velocity.
Correct answer: The tendency of an object to resist changes in its motion.
Newton's laws at a glance
First law
- Describes motion without net force
- Introduces inertia
Second law
- Connects force, mass, and acceleration
- Written as
Why do action-reaction forces not cancel?
Each force in the pair acts on a different body.
Correct answer: They act on different objects