Gravity is the force that acts on objects with mass. Let's take the earth as an example since it is familiar to all of us. Objects on the earth fall and tend to remain on the ground because the gravitational force of the earth is pulling on them. That's why, for example, your books stay sitting on your desk instead of floating up into the air. You might be wondering what gravitational force depends on. To figure that out, let's look at another example, the moon. I'm sure you've seen pictures before of astronauts on the moon jumping really high -- it doesn't seem as if there is as much gravitational force pulling on them, right? That's because gravitational force depends on the masses of the objects involved. The moon is a lot less massive than the earth, so there is less gravity on the moon than there is on earth.
There's one more thing that gravitational force depends on. When you're at the earth's surface, if you jump up, you come back to the ground. However, if youíre an astronaut and take a rocket into space you would notice that the farther you get from the earth, the weaker the force of gravity would be. This is because the gravitational force between two objects not only depends on the masses of both objects, but also on the distance between the objects.
Now that you know that gravity depends on mass and distance, we can answer your question about how gravity affects acceleration. Newtonís second law of motion states that force equals mass times acceleration. Letís say for example that you want to find the acceleration of objects on different planets. If you set up an equation and do some math, you will find that near the ground of the planets, the acceleration of objects depends on the mass of the planet and the radius of the planet. Gravitational acceleration increases as planet mass increases and planet radius decreases.
Thereís a lot that can be said here, and there are lots of different situations that we can look at, but itís hard to do that without using a bunch of equations. If you want to find more information and the math behind it all, ask us a follow up!
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(published on 10/22/2007)