Newton and Gravity

• Newton realized the force that pulled an apple to the ground is the same as the force that holds the moon in orbit around the Earth.Photos.com/Photos.com/Getty Images
  
  Like so many fundamental principles of physics, the concept of gravitational force goes back to Isaac Newton. He showed that any body with mass will exert an attractive force on other bodies with mass, and the acceleration of one body towards another is along the line connecting the center of the two bodies and is proportional to the mass of the attracting body. This actually means if you jump up in the air, the Earth will accelerate toward you as you fall back to the ground. Don't feel too powerful, though --- the Earth's acceleration toward you is an infinitesimal fraction of yours toward the Earth.
  
  

Gravitational Acceleration

• The Earth is constantly pulling everything towards its center. The mathematics and the measurements of this acceleration show it is 9.8 meters per second^2. That is, every second an object falls, the Earth will pull it towards the center with a velocity that increases by 9.8 meters per second. So, if a bird drops a turtle from the sky, the instant the bird opens its mouth, the turtle has a downward speed of zero. One second later, the turtle's speed is 9.8 meters per second. Another second later, the turtle's speed is 19.6 meters/second. And by the third second, the turtle is rushing to the ground at 29.4 meters/second, which is about 66 miles per hour.
  
  

Speed and Velocity

• It doesn't matter if the ball is thrown by a little leaguer or a pro --- the time it takes to fall only depends on its maximum height.Comstock Images/Comstock/Getty Images
  
  Physicists try to be very specific in their language, because the world works according to very specific rules. To physicists, "speed" means the total rate at which an object is moving, and "velocity" means the rate at which an object is moving in a specific direction. For example, if you throw a baseball across a field, part of its speed will be in a vertical direction. Gravity will change that speed, pulling the ball down 9.8 meters a second with every second that passes. But part of the baseball's speed is horizontal, and gravity will not change that at all. So if you throw a ball to a certain height, it will take the exact same length of time to return to the ground, whether you throw it straight up or across the baseball field. Falling does nothing to an object's horizontal velocity.
  
  

Terminal Velocity

• The maximum speed of an object through the air depends upon air density and the size, mass, and streamlining of the object.Ablestock.com/AbleStock.com/Getty Images
  
  A body moves in response to the forces acting on it, and gravity is always acting on objects near the Earth. There is one more force on a falling object, and that is air resistance. If you've ever stuck your hand out the window of a moving car, you've felt that force. You've also noticed the force changes depending on the shape of your hand. In most cases, air resistance won't do much to a falling object --- when you throw a baseball, for example, it would be really hard for you to notice air resistance. But all objects create some amount of air resistance, and at some point, the air resistance will balance the force of gravity and the object will not speed up any more. This is called terminal velocity. For a baseball, it's about 100 mph.