Gravity: First Physics
In several posts, we have used the term “gravity” without explaining or exploring what it is. Children, even infants, have a sense of what gravity is and isn’t. They know that when they drop things, they fall to the ground. They know that if they jump up, they come back down. They are aware that snowflakes, rain, and leaves drop to the earth. If thrown and not caught, a ball will tumble down. Everyday experiences inform our understanding of gravity. And here is your chance to label a common phenomenon, a label that can and should be used frequently.
But wait; there are things that stay aloft. A bird, a bee, a kite, or a jet can stay in the air, apparently defying gravity. Clouds do not fall. The moon stays in its orbit. So, what is going on?
The official definition of gravity or gravitational force is “The force of attraction between two objects.” We tend to think of it in terms of things dropping back to earth or being pulled to the ground, which according to the definition refers to the attraction between that object and the massively large earth. But gravity is a universal force, keeping the planets in their orbits and playing a role in star formation. Gravity doesn’t function in isolation, as it needs to work with the mass of the object. (Mass is not the same as weight. Mass refers to the material an object is made of. Weight is the product of gravitational force).(For help with these concepts if you need a reminder go to:
The bigger an object’s mass (think of the Sun), the more gravitational pull it has. Compared to other forces, gravity is described as “weak” because it takes a large mass to make it work.
Gravity is invisible, so for young children and even many adults, it is a difficult concept to grasp. Understanding the specific ways it works may have to wait for more mature cognitive strategies to develop, but there are daily events your child witnesses that deserve a label, and you can readily provide it, “it’s gravity.”
Feel free to use scientific terms with young children. Those terms may not sink in right away, or be repeated readily, but that child is developing an understanding that will have benefits later on. But if using a label, use it correctly. In this instance, don’t confuse gravity with other forces, such as magnetic or electronic. An applied force refers to the force applied to an object by another object or a person, such as pushing or throwing.
Remember that these explorations are on-going. Parents, you can capitalize on each observation or question about gravity when it comes up, which should be frequent, to extend learning. A little here, a little there, and over time, knowledge builds. Mastery will come only with exposure.
Pull together a collection of objects in your home, including toys or objects that can be dropped to the floor and won’t break. Invite your child to hold them out and let go, what happens? Is there a pattern?
Keep your eyes out for things that fall or drop to the ground because of gravity. As noted, rain, snowflakes and leaves fall to the earth. Are there other things your child notices while driving in the car, taking a walk, or even watching a movie or show on television. Can you make a game of this, keeping score of who notices the most things that gravity pulls back to earth?
Can you hear gravity? There is the sound of things hitting the floor, but is that the sound of gravity? (N0).
But just for fun, you might want to listen to these recent recordings of the newly discovered gravitational wave. The first part of the recording at the end of the article is best for you the adult. But don’t let your child miss the amazing noises played at the end!!!
For another great explanation of this recent phenomenal discovery here is a famous physicist describing these events to Stephen Colbert.
Compare and Contrast
Compare gravity to other forces such as throwing or pushing. That is, invite your child to hold an object out and just let it go. Now ask her to hold it out and push or throw it down. It will move faster in the second instance as she has combined two forces or gravity and an applied force such as a push.
We will do a separate post on magnets, but if your child has some magnets, play with them, contrasting how magnetism works to hold an object in place despite gravitational pull.
Drop things of varying weights. Ask your child to pick out two objects, one that is obviously heavier than the other. Now hold the objects out with palms facing down and arms or palms at the same height. Drop the objects at the same time. Do they fall to the earth at the same speed? Alternatively, did they hit the ground at the same time? (All objects should hit the ground at the same time unless there is air resistance. A feather, for example, may float to the ground more slowly, as it is more susceptible to air resistance).
Check out what happens when different objects are dropped on the moon. Review the post on the moon (http://guidingcuriosity.com/magical-mystery-moon/) and you will find that there is not air resistance there, so it is the perfect place to test gravitational pull on objects of varying sizes and weights. Also, recall, however, that the force of gravity on the moon is 1/6th of what it is here on earth. https://www.youtube.com/watch?v=5C5_dOEyAfk
Which sports rely on gravity? They all do ultimately, but some more obviously than others, such as sky-diving, skiing, ski jumping, gymnastics and any number of XGames events. Football, soccer, basketball and baseball all depend on the ball going up and coming back down. Talk about gravity and figure skating, ice hockey, volleyball, and diving. What is your favorite sport and how does gravity play a role?
How has gravity shaped the earth? Water in streams and rivers flows downhill. Avalanches and rock slides shape the contours of mountains by falling downward. Can your child identify the effects of gravitational force as you drive around?
Experiment and Measure
Where is the center of gravity? Every object has a center of gravity. Try and balance a fork or pencil on your finger. Once it is balanced, you have discovered its center of gravity!
Choose objects of varying characteristics. One light and one heavy, one small and the other large, one solid and one that is hollow. Now match these up and drop them at the same time from a constant height keeping track of which hit the ground first. For example, at the same time drop the heaviest and lightest objects, then the heaviest and the smallest, and then the heaviest versus the hollow object. Create other pairings and drop those objects.
A good idea is to stand on a chair or the sofa to drop objects so your child has a little more height allowing for more time to compare the speed of the fall. Wherever your child is dropping the object from, be certain that he or she knows that nothing or no one is below, including siblings and pets.
In conducting these experiments, it might also be a good idea to have an extra person around to observe when the objects hit the ground.
You can weigh the objects and time how quickly they fall for added precision.
Keep track of which object fell the fastest. What characteristic might be responsible for falling the fastest? Which object fell the slowest and why? In a vacuum or a space with no air, objects will fall at the same speed no matter what shape or weight they are. See the video above with astronauts dropping objects on the moon. Here on earth, because of air resistance, weight and the shape of the objects matter. Really light objects and objects with a lot of surface area slow down because they hit air on the way to the ground.
As an example of this fact, drop a flat sheet of notebook paper at the same time that you drop a sheet of notebook paper crumpled into a ball. The “ball,” with less surface area, will fall faster. Another example is to identify objects of the same shape but different weights such as a ping-pong ball and golf ball. The heavier of the two will hit the ground first, but not because of differences in gravitational pull, but because of the air resistance.
Invite your child to keep playing with those ramps and rolling things down. See our post on Kids and Things in Motion (http://guidingcuriosity.com/kids-things-motion-primary-physics/). What objects roll the fastest, slowest, or just come to a stop? Why? Could it be mass is working with gravity to accelerate an object? (Coming to a stop is the result of friction. Newton tells us that objects in motion stay in motion or maintain their velocity or objects that are stopped, will stay that way unless a force is exerted on them. But, because of friction and/or air resistance, the velocity of objects is slowed and sometimes stopped).
Why do birds remain up in the air despite the pull of gravity? The answer is “lift”. Birds are light and they also have powerful chest muscles. A bird’s wings press down and forward at the same time when they are flapping. When taking off, the bird has more lift from air under its’ wings than from air pressing down on top. This is because of the shape of the wing and the angle the air hits it. The higher pressure against the bottom creates lift and the continued flapping keeps the bird aloft and moving forward. If the bird stopped flapping its wings, it would glide to earth.
A cloud stays aloft because the water particles that it is made of are very light which in combination with other factors (air currents) makes it difficult for gravity to pull them down.
Elaborate and Glossary
Check for understanding and then expand. Ask your child what is gravity or how she knows that gravity exists. If she seems to have a basic understanding, then continue to explore the topic in more depth. Here are some ways to dig a little deeper.
Find a book on Galileo and his original experiments dropping items from the Tower of Pisa.
Find a book on Isaac Newton and discover his amazing ideas about gravity and many other scientific topics.
Discuss what would life on earth be like if there was no gravity?