Force and Newton's First Law
An object stays at rest or in constant motion unless a net force acts.
About this topic
A force is a push or pull, measured in newtons. Newton's first law, the law of inertia, says an object at rest stays at rest and an object in motion continues in a straight line at constant speed unless a net force acts on it.
Inertia is the tendency to resist changes in motion, and mass measures it: the more mass, the harder it is to start, stop or turn. Forces are vectors, so several forces must be added to find the net force. If the net force is zero, the velocity does not change, whether the object is at rest or moving.
Example: a hockey puck on smooth ice glides far because friction is tiny, and passengers lurch forward when a car brakes because their bodies tend to keep moving. A tablecloth can be yanked from under dishes for the same reason. Seat belts and headrests exist because of inertia.
Misconception: many people think a force is needed to keep an object moving. Aristotle thought so, but Galileo and Newton showed that force is needed only to change motion, such as to overcome friction. A moving object with balanced forces is perfectly natural.
Key ideas
- Inertia is resistance to changes in motion
- Net force zero means no acceleration
- Seat belts exploit inertia
Quick check
Use cases: where Force and Newton's First Law helps
- Homework: review the key idea behind Force and Newton's First Law before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Force and Newton's First Law without notes.
- Visual learning: play with the interactive lab to see how Force and Newton's First Law changes when you adjust its inputs.
- Teaching a friend: explain Force and Newton's First Law in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Force and Newton's First Law?
An object stays at rest or in constant motion unless a net force acts.
What will I learn in Force and Newton's First Law?
Inertia is resistance to changes in motion. Net force zero means no acceleration. Seat belts exploit inertia.
Is Force and Newton's First Law part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Force and Newton's First Law lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Force and Newton's First Law?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.