Center of Mass Challenge

By Jeremy Johnson

In my 13 years of classroom teaching experience, I’ve learned a few tricks to keep my students from becoming overwhelmed—or bored—by their science textbooks.  One of my favorite tricks is to get my kids up on their feet, doing science instead of reading about it.

When I teach about gravity and center of mass, for example, I like to shake things up by turning our classroom into an impromptu biokinetics lab.  I challenge my students to perform a series of seemingly simple physical tests, described below.  Lift a chair?  Raise your leg?  Pick up a quarter?  No problem!  (Or so they think…)

In the end, these physical experiments allow my students to grasp the basic concepts of gravity and center of mass in a far more compelling way than they’d ever learn from any textbook.  The results are hilarious and unforgettable!  As an added bonus, these challenges often bring my students’ families together as the students go home and challenge their siblings and parents to do them as well.

Just imagine:  science bringing families together, one discovery at a time!  I love it!

Let’s start with a quick vocabulary lesson.

Gravity is a term used by scientists to describe the force of gravitation given on an object or near the surface of a celestial body such as the Earth, the moon, or another terrestrial planet.  This force pulls on ALL mass.  All mass—from the smallest sub-atomic particle to the largest star in the universe—exerts some amount of gravitational force.  The more mass an object has, the more gravitational force is exerted.

Center of Mass is a term used by scientists to describe that point in an object where the object’s mass or weight seems to be concentrated.  This point could be [on???] a physical object or it could be a point outside of the object.  In a round/spherical object, the equally distributed mass is the center of mass for the sphere.  Finding the center of mass in an irregular object (such as the human body) takes a little more work to figure out… and a lot more fun.  That’s where these challenges come into play.

Center of Mass Challenge 1:  The Thumb Press

  1.  Place a chair against the wall so that it cannot slide backward.
  2. Sit in the chair with your feet flat on the floor in front of you. (Your feet may not be angled or slanted to the side.)
  3. Have a partner gently place a thumb in the middle of your forehead.
  4. Now try to stand up without forcing your partner’s hand back.

Center of Mass Challenge - Educational Innovations Blog

WHAT HAPPENED?   The reason it’s so hard to stand up is because your center of mass is located over the seat of the chair rather than over your feet, which are in front of you.

Center of Mass Challenge - Educational Innovations Blog


Center of Mass Challenge 2:  The Chair Lift

  1. Place a chair sideways along a wall.
  2. Stand next to the chair.  Make sure your feet are beside the chair, not under the chair.
  3. Bend over the chair, creating a 90-degree angle with your body.
  4. Place your head against the wall.
  5. Now try to lift the chair while keeping your head pressed against the wall.

Center of Mass Challenge - Educational Innovations Blog

WHAT HAPPENED?   Results of this challenge may vary by gender.  Interestingly enough, many girls can successfully complete this challenge, while most boys cannot.  Why?  Again, it’s all about the center of mass.  The center of mass for most girls is lower to the hips, while the center of mass in boys is much higher.  Therefore, for most girls, the center of mass while bent over the chair is above their feet, while the center of mass for most boys is above the chair.

Center of Mass Challenge - Educational Innovations Blog


Center of Mass Challenge 3:  The Quarter Grab

  1. Stand with your back and feet against the wall. (Your legs must be straight and feet together.)
  2. Have a partner place a quarter between your feet, in front of your toes.
  3. While keeping your legs straight, bend over and pick up the quarter without falling over.

Center of Mass Challenge - Educational Innovations Blog

WHAT HAPPENED?   The reason this challenge is so difficult is because of our anatomy—specifically, our bottoms.  As you bend over to pick up the quarter, your rear end naturally extends backward to help keep your body balanced.  Since it’s pressed against the wall, your bottom has no where to extend.  This causes the center of mass to shift forward, resulting in falling forward—at least for most people.

Center of Mass Challenge - Educational Innovations Blog

 

Center of Mass Challenge - Educational Innovations Blog

 


Center of Mass Challenge 4:  The Leg Raise

  1. Stand with your right shoulder and right leg pressed against the wall.
  2. With both your shoulder and leg touching the wall, “simply” raise your left leg.  Your goal is to stand only on your right leg—the one that is touching the wall.

Center of Mass Challenge - Educational Innovations Blog

WHAT HAPPENED?   This challenge is nearly IMPOSSIBLE.  For the same reason as before: because the wall is in the way, your body can’t counter-balance the mass distribution.  Your center of mass is over BOTH legs, not just one.

Center of Mass Challenge - Educational Innovations Blog

 


Watch my students in action!

If you don’t have time to perform these challenges in your own classroom—or if you’d like to get a quick look at how they can be set up—take a look at the video below, which was put together “with a little help from my (classroom) friends.”  Enjoy!

 

 

 

Jeremy Johnson has taught science in elementary and middle school for more than 13 years.  He is also the founder of the YouTube channel “SciOnTheFly.”  https://www.youtube.com/user/scionthefly

 

 

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One Response to Center of Mass Challenge

  1. robert farber says:

    Excellent job. The pictures and explanations will be useful to teachers at all levels.

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