# Centrifugal Forces Explained

Centrifugal Forces Explained

Attach a string to a ball and whirl it about your head at a constant speed. You must exert a force on the string to keep the ball from going off in a straight line. Think of David and his sling when he felled Goliath. At the appropriate time he let loose the stone and sent it flying into the giant’s forehead. The force exerted by the string/sling to keep the object circling your head is called a centripetal force. The word centripetal comes from the Latin and means to seek the center. Note that you are pulling it into or toward the center of the circle where you are standing. But what kind of force sends the ball/stone flying toward the giant? Release the string and you remove the centripetal force leaving the object free to travel in a straight line in a direction tangent to the circular path of the ball (ignoring the force of gravity).

Say you are riding in a car, traveling in a straight line at a constant speed. Suddenly the driver turns sharply to the left. You lean to the right and a force seems to be pushing you into the door. That is outward from the center of rotation of the turn. You were traveling in a straight line and suddenly the car body enclosing you changes direction and gets in the way of your travel. According to Newton’s first law, a body at rest or in a state of uniform motion continues in that state unless a resultant force acts on it to change its motion.

In your stationary frame of reference, the door is pushing in on you. And a force is pushing you into the door. Newton’s third law of motion says, to every force there is an equal and opposite reaction force. The reaction to the centripetal force of the door against you is called the centrifugal (Latin for center-fleeing) force. In magnitude it is just equal to the centripetal force. But this is a fictitious force. Indeed it is not even a force because it is your body’s inertia that tends to force you to fly off on a tangent to the circular path. You might even call it the centrifugal reaction. Centripetal force is a real force because it is due to the influence of an object (the door). A fictitious force would be noticed only when a system is looked at as a accelerating frame of reference. Someone on the street corner, a non-accelerating frame of reference, as the car passes would not be aware of the centrifugal force.

Note also the centripetal force does no work. By definition work is the product of a force and the displacement in the direction of motion.