# What is gravity?

Gravity:  A heavy topic!

Today, we talked about how strange and interesting gravity is. We know so much about gravity already, but there is so much we still need to learn. What scientists have learned about gravity has allowed us to travel into space and to the Moon. It’s allowed us to send probes out into the solar system and beyond, and even to the surface of other planets, including Venus and Mars!

We learned that gravity depends on the mass of an object, and that all objects have some amount of gravitational force. Students were introduced to Isaac Newton’s law of universal gravitation and explored the equation’s underlying principles through a series of activities. With a demonstration using balls on a blanket, we showed how massive objects bend the fabric of space-time, causing other objects to gravitate towards them. With just the right velocity and trajectory, a smaller object can go into orbit around a larger object…but if the smaller object is moving too slowly, it can fall and crash into that larger object. It’s a good thing the Earth doesn’t slow down in frictionless space, or it could crash into the Sun!   Gravity is obviously important for astronomers and cosmologists to understand.

We examined how two objects with comparable air resistance will fall to the ground at the same time, regardless of mass. We now know that the same thing will happen if we were to drop the objects on the Moon!  We also compared the weight of an empty bottle with other bottles that weighed as much as the empty bottle would weigh if it were on the moon, Jupiter, or the Sun. The bottle filled with sand is what the empty bottle would feel like on the Sun, where it would weigh more than 27 times as much as it weighs on Earth!

Finally, we studied the relationship between gravitational force and the distance between the two masses using a repulsive magnetic force as a model. It turns out that the force is proportional to 1/(distance)^2; so if you increase the distance between two objects by 2, the gravitational force between them goes down by 4! Gravitational force drops off very quickly as objects move away from one another! This relationship, very common in physics, is called “The Inverse Square Law.”

Author

## Dr. Maureen Griffin

Maureen earned a Ph.D. in Chemical and Biomolecular Engineering from the University of Pennsylvania in 2003. While at Penn, she developed a novel micro-mechanical technique called micropipette peeling to investigate the role of muscle cell adhesion in normal and diseased skeletal muscle cells. After graduating, Maureen worked full time as a post-doctoral researcher and then a staff scientist a SelectX Pharmaceuticals. She joined the teaching staff in 2008 and was excited to be made an executive staff member in 2009. Maureen also continued to consult part time for SelectX until her daughter's birth in 2009; now she is focused on Science from Scientists and, of course, her children. Maureen uses her spare time to read, blog, cook, and renovate her house.

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