Center of Mass
The point where an object's mass acts as if concentrated.
About this topic
The centre of mass is the point where an object's mass can be treated as concentrated, and it moves as if all outside forces act on that point. For uniform symmetric shapes it is at the geometric centre; it can lie outside the object, like in a ring or boomerang.
An object is stable if the vertical line from it falls within its base of support. Lower centres of mass and wider bases give more stability, which is why sports cars sit low and a tall load makes a truck prone to tipping. To find it for a few masses, use x = (m1x1 + m2x2 + ...)/(m1 + m2 + ...).
Example: two masses of 1 kg and 3 kg, 4 m apart, have their centre of mass 3 m from the lighter one, closer to the heavier one. A high jumper arches the body over the bar so the centre of mass may pass below it. A thrown spanner spins around its centre of mass while that point follows a smooth parabola.
Tip: you can find the centre of a flat shape by hanging it from two points and drawing plumb lines. Do not confuse it with the geometric centre when density varies.
Key ideas
- Balances the object
- A symmetric object has it at the center
- Low center of mass gives stability
Quick check
Use cases: where Center of Mass helps
- Homework: review the key idea behind Center of Mass before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Center of Mass without notes.
- Visual learning: play with the interactive lab to see how Center of Mass changes when you adjust its inputs.
- Teaching a friend: explain Center of Mass 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 Center of Mass?
The point where an object's mass acts as if concentrated.
What will I learn in Center of Mass?
Balances the object. A symmetric object has it at the center. Low center of mass gives stability.
Is Center of Mass 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 Center of Mass 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 Center of Mass?
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.