Motion and Its Causes
Force and Newton’s Laws of Motion
What can exert a force?
The first answer that might come to mind is that a person can exert a force. He or she can throw a ball, pull the string on a bow to shoot an arrow, push a chair across the floor, or pull a wagon up a hill. Many animals can do the same thing, so one might say that living organisms can exert forces.
What if you place a rock on a table? Does the table exert a force on the rock, or does it just block the rock’s natural motion toward the floor? If you put a heavy rock in your hand, it would sag downward because you would have trouble exerting the upward force on the rock. The same happens with a table. If the table is made of thin wood, it will also sag. What would happen if the table were replaced with a sheet of paper? The paper might hold small stones, but with a heavy rock it would tear because it could not exert a strong enough force. The heavier the rock, the greater the force the table or floor exerts. In summary, inanimate objects can also exert forces.
Forces like these, exerted by humans or tables that touch the object, are called contact forces. What are other contact forces? You might think of rubber bands on sling shots, roads on the wheels of your car, or ropes pulling carts. Water and air can also exert a force. Think of a stick moving down a stream, or what you feel when you stick your hand out the window of a fast-moving car.