Center of Gravity - Gravity Defying Hammer (Debbie Brenner, Jovanny Navarro)
Author
Jovanny Navarro, modified by Debbie Brenner
Principle(s) Illustrated
Center of Gravity
Fulcrum
Standards
NGSS Science & engineering standards
Asking questions
develop and using models
NGSS Cross-cutting concept standards
Cause and effect
systems and system models
energy and matter
stability and change
NGSS Disciplinary core idea standards
From molecules to organisms (HS-LS1)
ecosystems: Interactions, energy and dynamics (HS-LS2)
Earth and human activity (HS-ESS3)
Matter and Its Interactions (HS-PS 1)
Questioning Script
Materials
- Hammer
- Rubber Band or String
- Ruler
- Unopened soda can
- Empty, rinsed soda can
- 150 milliliters of water
Prior knowledge & experience:
Students may be familiar with the notion of gravity.
Students have some understanding of force, mass, and acceleration.
Gravity pulls objects downward.
Root questions:
Challenge #1:
Why doesn't the apparatus that includes the ruler and hammer fall?
Analyze where the center of mass might be.
Where is the balance point?
What is the heaviest part of a hammer?
Challenge #2:
Why is it easier to balance the empty can partially filled with liquid, compared to the completely empty can as well as compared to the full can?
Target response:
Challenge #1:
The center of gravity is directly over the tip of ruler, not at the center of the ruler.
Challenge #2:
If the can has too little water, the weight of the can itself will be the most significant factor in the center of mass, causing the center of mass to be too far away from the base. In this case, the “base” is that very small edge that the can is balancing on.
If the can has too much water, the center of mass will again be to one side of the balancing point, causing it to topple over.
If the can has just the right amount of water in it, the center of mass will be directly over the base.
Common Misconceptions:
The center of gravity is at the middle of an object. This results from the fact that most objects are symmetrical.
Instructions
Challenge #1: Balance a ruler with a hammer
This balancing act looks like it should not be possible. The hammer hangs precariously underneath the ruler, looking like it should make the ruler fall off the table. But there it hangs, perfectly balanced!
Helpful hints:
1) Take the rubber band or string and make a loose loop around the hammer and ruler, as shown in the picture.
2) Make sure the end of the hammer is touching the ruler, and then position the ruler at the edge of a table, as shown. (You might have to reposition the string/rubber band a few times to get it just right.)
Challenge #2: Balance a soda can on its edge
You will need an empty can of the type shown in this picture. The can must have a bottom that looks like this one, with an angular section between the side and the flat bottom. You’ll also need some water that you can gradually pour into the can. (You could also do this trick by gradually drinking the soda until the can will balance.)
Helpful hints:
1) Try to balance the empty can
2) Try to balance the full can
3) Pour water into the empty can until it is about 1/3 of the way full. Try to balance it again.
4) If it does not balance, add or subtract a little bit of water until the can balances. If you have balance the can well, you will be able to push the can gently and get it to roll around in a circle!
Real-World Connections
Center of mass is important in many branches of science and engineering.
Engineers who design vehicles are very concerned about center of mass. Cars that have their center of mass closer to the ground are less likely to flip over (a special concern for racing cars that go around turns at very high speeds).
If an airplane has its center of mass too far forward, it will tend to be less maneuverable and be difficult to control during take-off and landing. A plane with a center of mass too far back will be very maneuverable but will be less stable during flight.
A helicopter must be designed so that the center of mass can shift backward when the pilot wants to go forward. (This is why a helicopter looks like it is tilting nose down when it is flying forward-- the center of mass moves behind the rotor.)
In sports, high-jumpers learn to bend in such a way that their bodies go over the bar even though their center of mass does not. Long jumpers shift their center of mass as efficiently as possible in order to maximize the power of their jump. Athletes shift their center of mass all the time as they run and jump, though they don’t think about what they are doing in scientific terms.
Scientists called kinesiologists study body movement and develop equipment or therapies based on the science and math of body movement.
In astronomy, center of mass is very important. When a moon orbits a planet, or a planet orbits a star, they are actually both moving around a central point called the “barycenter.” This point can be inside one of the bodies, or at a point in space. The Earth and the moon orbit each other at a point that is about 1,710 kilometers (1062 miles) below the surface of the Earth. It is this barycenter point that orbits around the Sun.