Frames of Reference
Choosing a frame simplifies a physics problem.
About this topic
A frame of reference is the viewpoint from which motion is measured. Choosing a convenient frame can turn a difficult problem into a simple one, such as viewing a collision from the centre-of-mass frame or a train as at rest.
In inertial frames, which move at constant velocity, Newton's laws hold. In accelerating frames, fictitious forces such as the push you feel when a bus brakes seem to appear, although they are really inertia. Earth itself is a good approximation of an inertial frame for most everyday problems.
Example: a ball dropped inside a smoothly moving train falls straight down for a passenger, but to someone on the platform it follows a curved path, because it also moves forward with the train. Both descriptions are correct in their own frame. Astronomers use the Sun as a frame to describe planets, and spacecraft navigation depends on carefully chosen frames.
Tip: always state your frame, even if it is just the ground. Saying an object is moving means nothing unless you say relative to what. Einstein later extended this idea in relativity.
Key ideas
- Inertial frames move at constant velocity
- Newton's laws hold in inertial frames
- Ground is approximately inertial
Quick check
Use cases: where Frames of Reference helps
- Homework: review the key idea behind Frames of Reference before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Frames of Reference without notes.
- Visual learning: play with the interactive lab to see how Frames of Reference changes when you adjust its inputs.
- Teaching a friend: explain Frames of Reference 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 Frames of Reference?
Choosing a frame simplifies a physics problem.
What will I learn in Frames of Reference?
Inertial frames move at constant velocity. Newton's laws hold in inertial frames. Ground is approximately inertial.
Is Frames of Reference 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 Frames of Reference 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 Frames of Reference?
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.