Centripetal Forces and Centrifugal Effects
Understanding the Physics of Uniform Circular Motion
Jul 6, 2009
People often think centrifugal forces fling objects outward when they travel in circular paths. This is incorrect. So-called centrifugal forces are not real forces. They are an example of inertial forces, which are effects caused by an object’s inertia.
Newton’s First Law and Inertial Forces
According to Newton’s first law, an object’s inertia causes it to continue its constant velocity motion until an outside force acts on it. Velocity includes direction, so constant velocity motion must be in a straight line. Directional changes are changes in velocity and require external forces. In addition, objects at rest have a constant velocity of zero.
Any change in velocity, whether it is an increasing speed, a decreasing speed, or a changing direction, is an acceleration and requires an outside force.
When an object is in or on something else that is accelerating, physicists say that it is in an accelerating reference frame. Objects in accelerating reference frames will experience apparent forces caused by their inertia.
For example, when a car stops suddenly, passengers who are not wearing seatbelts fly forward. No force pushed them in the back. Rather their reference frame, the car, changed its velocity (accelerated), but the passengers’ inertia caused them to continue moving at the same velocity until the windshield, seatbelt, or airbag applied a stopping force to the passengers.
These passengers are experiencing what physicists call inertial forces. Despite their name, inertial forces are not real forces. They are effects caused by inertia in an accelerating reference frame.
Circular Motion
Because of their inertia objects always tend to travel in a straight line. Therefore any object following a circular path must have a force constantly acting on it to continually change the direction of its motion. A force acting in the same or opposite direction as the velocity will change the speed, but not the direction of the velocity. A force acting perpendicular to the direction of the velocity will change the direction of the motion but not the speed.
For an object to travel in circular motion at a uniform speed, a force must be acting so that it is always perpendicular to the velocity. The force must also point inward towards the center of the circle. Physicists call such inward forces centripetal forces.
Passengers in a car going around a curve feel like they are being pushed sideways because their inertia makes them continue in a straight line, while the car is accelerating to follow a circular path. Just as when the car stops or speeds up, these passengers experience an inertial force, the centrifugal effect, because they are in an accelerating car. However the real force acting on these passengers is the inward centripetal force provided by the car, which forces them to follow the car’s path around the curve rather than continue in a straight line.
When twirling a ball on the end of a string, the tension on the string provides an inward centripetal force on the ball. If the string breaks, the ball flies in a straight line. There is no outward centrifugal force flinging the ball. Rather when the inward centripetal force stops acting on the ball, it continues its natural straight line motion as dictated by Newton’s first law.
Contrary to what many people think, objects following a circular path do not experience an outward centrifugal force. The centrifugal effect is an apparent inertial force caused by the object’s inertial tendency to travel in a straight line. The real force acting on objects traveling in uniform circular motion is the inward centripetal force which changes the objects path from straight line to circular motion.
 |
 |
