What makes an object 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 know, without thinking about it too much, 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!
An assortment of random objects from around the house will work for this project. Try to get a variety of objects, such as:
- Rectangular objects (blocks, boxes)
- Long, skinny objects (pencils, rulers)
- Irregularly shaped objects (coffee mugs, hair brushes)
- 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 like a ruler or pencil. Predict which sides you think it will balance on. First try to balance it on the longest side.
- Now try balancing it on one of the skinny sides (like the edge of a ruler) or points (like the tip of a pencil). 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. For example, you can probably get a coffee mug to balance easily on the bottom, flat side. Can you get it to balance on the rounded side, or on the handle?
You probably found that 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 a very small contact area with the ground. It's difficult to get the center of mass to stay inside this 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. See the Digging Deeper section to learn more.
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. For example, a chair sitting on the floor on all four legs 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. Imagine looking down on a chair from directly above, and drawing imaginary lines connecting the chair's 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. Since this force acts inside the chair's contact area with the ground, it does not cause the chair to tip over.
On the other hand, when you try to balance the chair on one leg, 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 experiment with a variety of objects of different shapes. Do you think you can balance them all?
*For a perfectly symmetric object (like a ball), the center of mass is directly in the middle. For an irregularly shaped object like a chair, the location of the center of mass depends on how the object's mass is distributed.
Ask an Expert
For Further Exploration
- 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 stay standing?
- Get a helper and try this experiment with larger objects, like a chair or broomstick. Be careful not to drop anything heavy!
- Try balancing objects on your fingertip instead of on the ground/table. How do your results change?