gravity

[Maryann Zahm]

How can I explain to my first graders where gravity comes from? From inside the earth pulling things down or from somewhere else?
We have conducted an experiment with balls of equal size but different masses and they have hit the ground at the same time. But why?

[ChrisG]

Hi Maryann,
Welcome to the Ask an Expert forum. I have tried to explain this concept in an understandable way to my young son, so I can appreciate the difficulty of your task. All objects with mass have gravitational attraction to all other objects with mass. The force of the attraction is proportional to the mass of the objects, so "big" objects (like the Earth) have strong gravitational attraction that we can observe, while "little" objects (like marbles, buses, and buildings) have so little gravity that we don't notice it in our everyday experience. Every particle of water dirt and stone in the Earth attracts us (as does every particle in the air and the buildings around us). Because all of the Earth's mass is centered around its core, the average force that we experience is towards the center of the planet.

Here is a thorough discussion of objects with different masses falling with the same acceleration:
http://www.grc.nasa.gov/WWW/K-12/airplane/ffall.html
For a first grade class, maybe it would suffice to point out that (1) bigger objects require more force to move (as we know from kicking or throwing a heavy ball) (2) gravity pulls harder on bigger objects (which we experience as one ball weighing more than another) (3) pulling harder on a more massive object makes it move similarly to pulling half as hard on a half-as-massive object. I suppose there could be a demonstration where the kids are gravity and they pull on different objects with different weights.

Of course, things don't always fall with the same acceleration when there are other forces at work, like air-friction or buoyancy, but it sounds like ball demonstration has craftily avoided these possibly confusing exceptions.

Gravity really is like magic.

[Craig_Bridge]

Chris gave you one approach to attempt to explain something that simply IS. In other words, this is an observed physical property of our world. Why? Well that turns into a philosophical discussion. Not everything that IS can be decomposed or modeled to universal satisfaction. All of the complicated mathmatical modeling of gravity as gravitational fields and attempts at unified field theories that people have tried historically have simply been complex attempts at modeling what IS and so far haven't been very satisfying or enlightening compared simple acceptance of this is the observed behavior and acceptance that perception is part of our individual philosophical reality.

Chris's arguements and those of others are circular in nature. Start with something that IS observed, come up with mathmatical models that are consistent with what IS observed, and then look at propeties of the mathmatical model to explain the starting point, that which was taken as a given to start with. A great exercise when those doing it obtain a greater appreciation as a result of doing the exercise, but totally confusing for those who don't follow it, and boring to those who follow but gain nothing.

Greater minds than mine have pondered this area for years and not come up with much more interesting than Galileo's refuting Aristotle's previously accepted idea that heavier objects fell faster. Galileo's dropping of a cannonball and a musketball simultaneously from a tower and observing that they hit the ground at nearly the same time is a simple demonstration of what IS an observable property in our world.

Try several approaches and hopefully all you students will understand at least one of the approaches is one way to teach it.

I personally think this is one of those experiments that you can use as a great opportunity to intruduce the scientific method. Ask the question of which falls faster? Then ask each member of the class to write down their hypothesis on a piece of paper and turn it in (restricted to heaver is faster, lighter is faster, and equal). Then do the experiment several times with multiple observers or beter yet with video camera and frame by frame playback. In other words, use this as a great introduction to scientific investigation methods. Don't start with trying to explain it, use scientific investigation to discover what IS. This is a great opportunity for learning that you don't have to guess correctly and that scientifically disproving your hypothesis is often a more interesting scientific result that offers opportunities to learn more.

It also gives you the opportunity to teach the history of how great philosophers like Aristotle can get it wrong and great scientists like Galileo can disprove things (e.g. be an early mythbuster). You can then mention that scientists today are still trying to come up with a better understanding of gravity.

If you sow the seeds of scientific curiosity at this age and it grows in even a small percentage of your class, you will have done very well.

[Craig_Bridge]

If you have some really bright inquisitive students, you might want to do some enrichment from
http://www.vias.org/physics/bk1_05_01.html
which gives a brief explaination of why Galileo was a great scientist and early experimentor with the properties of fluid effects. It also has a great quote Richard Feynman attributes to his father:
"Nobody knows why things fall. It's a deep mystery, and the smartest people in the world don't know the basic reason for it."

[ChrisG]

I agree with Craig's eloquent observation that we do not know "why" gravity exists or why it behaves as it does. My own experience with very young scientists has been, however, that "why" often means "tell me everything you know about...", and that was more the nature of the assistance I was trying to provide. Apologies if I misinterpreted the question.