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13.3: Force: The Cause of Motion

Difficulty Level: At Grade Created by: CK-12
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Mass is the amount of stuff in an object and we give it the symbol m. The more atoms, molecules, or whatever there is in an object, the more mass it has. Mass used to be called inertia because it is hard to change the motion of a large mass. Mass likes to continue moving like it is moving and it takes a force to change its motion.


Force is a push or a pull. When an elevator accelerates upward, the motor exerts a force on the cable attached to the elevator to pull it up. Even when the elevator is going down, the motor must exert a force on the cable to keep the elevator from falling too fast. When you stand on the floor, the floor exerts an upward force on you to keep you from falling through the floor. When you stand on a scale, the floor exerts an upward force on the scale and the scale in turn exerts an upward force on you, which is read on the scale. Complicated - isn't it?


Now that you understand the basics of motion, we can discuss an important force in many cases involving motion. All masses attract each other. We call that gravity. The Earth is a very large mass equal to about a trillion billion elephants. The Earth attracts the rocket towards its center and this often causes the rocket to return back to Earth. The acceleration of a freely falling body near the surface of the Earth is given the symbol g. How would things change on the Moon or other planets? You can explore that later.


When you stand at rest on the floor, the floor pushes up on you to keep you from falling through. The value of this force is your mass times the acceleration of gravity and we call the value of this force your weight W = mg. This is what you actually feel because of Earth's gravity.

Newton’s Laws: Putting It All Together

Isaac Newton (1643-1727), one of the greatest minds ever, discovered that force was directly related to acceleration. The more total force you exert on an object, the greater its acceleration F = ma, where F stands for the total force on the mass in question. In many cases there can be many objects acting on one another, so using Newton's idea can be complicated. In our case the rocket motor and gravity will be acting on the rocket in the simplest case. We will also consider the air drag acting on the motion (wind can also act on the rocket, but we won’t model that). Thanks to Newton, we now have the reason for acceleration and thus motion. See Laboratory Activities in this book for experiments on Newton’s laws.

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