Center of mass
What makes an object stay balanced? Look around you. Most of the objects in the room are probably balanced—and not on the verge of tipping over. If someone hands you an object and asks you to put it down, you probably intuitively know how to place it so it won’t fall over. But what’s the science behind how an object balances? Why do certain objects only balance on some sides and not others? Try this project to find out!
In general we use the word “balanced” to refer to an object that is upright and not falling over. The technical term for an object that won’t tip over—even if it is pushed—is stable. An object that can be knocked over by a light push or a gentle puff of wind is unstable. A chair sitting on the floor on all four legs, for example, is stable—it’s hard to knock it over. If you try to balance the chair on one leg, however, it’s unstable. The moment you let go of the chair it will probably fall.
What determines if an object is stable or unstable? It depends on two things: the location of the object's center of mass and where the object is in contact with the ground. For a perfectly symmetrical object (such as a ball), the center of mass is directly in the middle. For an irregularly shaped object (such as a chair), the location of the center depends on how the object's mass is distributed. Imagine looking down on a chair from directly above and drawing imaginary lines connecting its four legs, forming a square on the ground. The chair’s center of mass will be inside this square. Gravity pulls down on the chair acting like a single force concentrated at the center of mass. Because this force acts inside the chair's contact area with the ground, it does not cause the chair to tip over.
When you try to balance the chair on one leg, conversely, it has a very small contact area with the ground. It’s almost impossible to get the chair’s center of mass to line up inside this contact area—and even if you do, the slightest motion will move it back outside. This will cause the chair to tip over because the force exerts a torque on the chair.
In this activity you’ll try this simple test with a variety of objects of different shapes. Do you think you can balance them all?
- An assortment of objects from around the house (that you have permission to use) to try to balance. Get a variety of different-shaped objects, such as: rectangular objects (blocks, boxes, etcetera); long, skinny objects (for example, pencils, rulers); and irregularly shaped objects (coffee mugs, hair brushes, etcetera).
- Helper (optional)
- Gather your objects on a flat, firm surface on which they can be balanced.
- Take one of your rectangular objects. Predict which sides you think it will balance on then try balancing it on each side.
- Now try balancing it on the edges and corners. Can you still get it to balance?
- Now take one of your long, skinny objects. Predict which sides you think it will balance on. First try to balance it on its longest side.
- Now try balancing it on one of the skinny sides or points. Can you still get it to balance?
- Try one of your irregularly shaped objects. Again first predict how you think it will balance then try it out. You can probably get a coffee mug, for example, to balance easily on its bottom, flat side. Can you get it to balance on its rounded side or handle?
- Extra: After you get an object to balance test whether it is stable or unstable. Gently blow on it or nudge it with your finger. Does it fall over or remain standing?
- Extra: Get a helper and try this test with larger objects, such as a chair or broomstick. Be careful not to drop anything heavy!
- Extra: Try balancing objects on your fingertip instead of on a flat surface. How do your results change?
Observations and results
You probably found it was easier to balance objects on larger, flatter sides than on smaller, pointier or rounder sides. It’s also easier to balance an object that has three or more points of contact with the ground (like the legs of a chair). This is because when an object has a large, flat surface in contact with the ground (or multiple points forming a polygon) most of the time its center of mass lies inside this area, and the object is stable. Thin edges, points and round surfaces form very small contact areas with the ground. It’s difficult to get a center of mass to stay inside this size area, making the object unstable. You might be able to temporarily balance an unstable object—for example, balancing a pencil vertically on its eraser—but it only takes a gentle push to knock the object over.
More to explore
Circus-Trick Science: How to Balance Anything, from Science Buddies
“X” Marks the Spot: Finding the Center of Mass, from Scientific American
Seesaw Science: the Hammer–Ruler Trick, from Scientific American
What Is Center of Mass?, from Khan Academy
STEM Activities for Kids, from Science Buddies
This activity brought to you in partnership with Science Buddies