Try standing on one foot. Why is it harder than standing on two feet? What are you actually "doing" when you're moving around and trying to balance?

The last thing we'll cover in this section is center of mass. The center of mass of an object (or a system of objects) is the mass-weighted average position of all of the parts of that object (or all of the objects in that system).

Put another way, the center of mass of a group of objects is the sum of each object's position multiplied by its mass, all divided by the total mass. The same is true for a single object, but instead of a group of "individual" masses we are looking at how the mass is distributed (or spread out) across that object. Don't worry if that seems confusing, it'll make more sense after a few examples.

You may also hear the term center of gravity. If an object or a system is in a uniform gravitational field (which we assume is true on earth), then the center of mass and the center of gravity will be the same point.

One way to think about center of mass is the point at which you can balance an object from below (if you placed it on a fulcrum, like a seesaw) or from above (if you tied it to a string, like a hanging mobile). However, we'll learn that the center of mass may not always be directly on the physical object itself.

Also, remember projectile motion? Although we talked about an entire object following a trajectory, we were really describing the motion of the object's center of mass.

Multiple-Choice Questions

Answers

• Flipping Physics - Center of Mass Introduction
• Michel van Biezen - When Will A Car Tip Over?
• Khan Academy - Center of mass equation
• Matt Anderson - Center of Mass

• Organic Chemistry Tutor - Center of Mass Problems
• Michel van Biezen - Earth-Moon System
• Michel van Biezen - Four Objects
• Michel van Biezen - Odd-Shaped Object

• TED Ed - An athlete uses physics to shatter world records

• Center of Mass

Other Resources

Try standing on one foot. Why is it harder than standing on two feet? What are you actually "doing" when you're moving around and trying to balance?

The last thing we'll cover in this section is center of mass. The center of mass of an object (or a system of objects) is the mass-weighted average position of all of the parts of that object (or all of the objects in that system).

Put another way, the center of mass of a group of objects is the sum of each object's position multiplied by its mass, all divided by the total mass. The same is true for a single object, but instead of a group of "individual" masses we are looking at how the mass is distributed (or spread out) across that object. Don't worry if that seems confusing, it'll make more sense after a few examples.

You may also hear the term center of gravity. If an object or a system is in a uniform gravitational field (which we assume is true on earth), then the center of mass and the center of gravity will be the same point.

One way to think about center of mass is the point at which you can balance an object from below (if you placed it on a fulcrum, like a seesaw) or from above (if you tied it to a string, like a hanging mobile). However, we'll learn that the center of mass may not always be directly on the physical object itself.

Also, remember projectile motion? Although we talked about an entire object following a trajectory, we were really describing the motion of the object's center of mass.