Center of mass
Did you ever come across a challenge that looked almost too easy to try—but turned out to be surprisingly difficult, if not impossible? This activity challenges you in a fun way. Something as simple as picking up a piece of candy can be way harder than it looks. Find out why some movements are harder than you’d expect, and then trick your friends into trying them. Astonish them with your stunning knowledge of the laws of physics—but most of all, have fun!
Gravity is a force that acts between any two masses, but you only notice it if at least one of the masses is huge, such as the mass of Earth (about 5.97 x 1024 kilograms). We encounter the effects of Earth's gravity daily. Gravity prevents you from floating off into space when you jump, makes objects fall to the ground when you release them and makes it difficult to hold your balance on one foot without falling over. When balancing or jumping, gravity acts on us as if all our body's mass is squished into one point and gravity pulls on that point. The point is called the center of mass, and how the body's mass is distributed determines its location. If you are a child, your center of mass is usually below the belly button when you are standing. Wearing a heavy backpack will shift your center of mass up because more mass is now located on the upper part of your body. Leaning to the left moves part of your mass to the left so your center of mass shifts to the left as well. Similarly leaning forward moves it forward. As you grow, your mass distribution changes and the center of mass shifts accordingly. It tends to stay low for adult females whereas in most adult males their center of mass is above the belly button when standing.
Why is this important? If our center of mass is directly above whatever supports us (our feet, a chair, etcetera) we feel balanced. If it is not, we feel unbalanced and need to do something or else we will topple over and fall. The challenges in this science activity allow you to experience this. Go ahead, feel it for yourself!
- Wall to lean against
- Candy in a wrapper
- Ruler (optional)
- Chair without armrests and a straight back (optional)
- Gather your materials and bring your partner to the location where you will perform the test.
- Stand with your back against a wall with your heels touching the wall and your feet together.
- Ask your partner to put a piece of candy about 30 centimeters (about a foot) in front of your feet. Do you think you could pick up the candy without bending your knees or moving your feet?
- Try it. Did you succeed? Was your prediction correct?
- Stand away from the wall, with your feet together and a piece of candy again about 30 centimeters in front of your feet. Do you think you can pick up the candy now? Remember, you are not allowed to bend your knees or move your feet.
- Try it. Did you succeed? Was the latter challenge easier or harder than the first one? Why would this be?
- For the next challenge stand tall on two feet and away from the wall. You are allowed to bend your knees but keep your arms next to your body. Can you lift your left foot without moving your right foot? Do you think you could do this with you right side leaning against the wall?
- Stand with your right side against the wall so your foot, hip, arm and shoulder touch the wall. You are allowed to bend your knees. Can you lift your left foot without moving the right foot? Why would standing against a wall make such a big difference?
- Extra: Sit in a chair without armrests and a straight back with your back straight up, feet flat on the ground and hands on your thighs. Now try to stand up, keeping your back vertical, hands on your thighs and feet on the ground. Can you do it? Why would this be so hard?
- Extra: This challenge requires empty space behind and in front of you. Bend over to grab your toes, with your knees slightly bent. Now try to jump, first forward then backward. Which one is easier, and why would this be the case? Hint: Observe how your center of mass (or your mass distribution) shifts when doing a regular jump forward or backward. Think of how your mass distribution is different when bending over to hold your toes compared with standing up. How would this impact the location of your center of mass? How could this make specific jumps difficult?
Observations and results
Was it surprisingly difficult to accomplish some of these seemingly straightforward challenges?
Every person (and every solid item) has a center of mass. Where this point is located determines whether you (or the solid item) is balanced or will fall over. When you stand against the wall your center of mass is above your feet so you are balanced. When you bend over the mass of your head and torso moves forward. As a result your center of mass moves forward and away from your feet; you feel you are about to lose your balance and fall over. You can save yourself by moving a foot forward so your center of mass falls in between your two feet—but that is not allowed in the challenge. It is not possible to pick up that candy! The laws of physics prevent it. Without the wall behind you, you could probably pick up the candy. Did you notice that you pushed your behind backward for counterbalance while bending over? This kept your center of mass above your feet and helped you with your balance.
A similar thing happened when you tried to stand on one foot. As you probably noticed, without the wall you could lift your left foot with ease. When you stood with your right side against the wall and tried to lift your left foot, however, you could not do it. Your center of mass was above the area spanned by your two feet. With the wall to the right of you, you had no way to shift your center of mass to the right by leaning to the right so it would be above your right foot when you lifted your left foot.
If you tried the extra challenges, you were probably stuck in your chair and unable to make a single jump forward—although backward jumps were possible. These challenges are set up so certain shifts in your center of mass are not allowed or force you to lose your balance. You were not allowed to bend forward while standing up from your chair, and leaning forward before doing a forward jump makes you lose your balance.
More to explore
Circus Science: How to Balance Anything, from Scientific American
Center of Gravity, from ExplainThatStuff
An Experiment on the Center of Gravity: Kids Using Gravity to Outperform Adults!, from Bright Hub Education
“X” Marks the Spot: Finding the Center of Mass, from Scientific American
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies