Centrifugal Force

Centrifugal is not to be confused with centripetal. Centrifugal means away from the center or outward.

The use of or at least the familiarity with this word centrifugal, combined with the common sensation of an outward lean when experiencing circular motion, often creates or reinforces a common misconception. The common misconception, believed by many, is the notion that objects in circular motion are experiencing an outward force.

A well-meaning student may think, "I can recall vividly the sensation of being thrown outward away from the center of the circle on that roller coaster ride. Therefore, circular motion must be characterized by an outward force."

As we have already covered, the motion of an object in a circle requires that there be an inward net force - the centripetal force. This is an inward-directed acceleration that demands an inward force. Without this inward force, an object would maintain a straight-line motion tangent to the perimeter of the circle. Without this inward or centripetal force, circular motion would be impossible.

Learned physics types would admit that circular motion leaves the moving person with the sensation of being thrown outward from the center of the circle. Before you draw a hasty conclusion that there definitely is an outward force when experiencing circular motion which leaves you feeling like you're being thrown in an outward direction, ask yourself three probing questions:

•Does the sensation of being thrown outward from the center of a circle mean that there was definitely an outward force?

•If there is such an outward force on my body as I make a left-hand turn in an automobile, then what physical object is supplying the outward push or pull?

•And finally, could that sensation be explained in other ways that are more consistent with our growing understanding of Newton's laws?

Group Discussion: Does the outward sensation that comes with moving in a circular motion mean that there must be a bonafide outward force? If yes, what is this outward force? If no, what can be considered the cause of the sensation felt?

If you can answered the first of these questions with "No" then you have a chance in neglecting the idea that there is an outward force acting on a body moving in circular motion. If you quickly concluded that the outward feeling means there is an outward force, then you must admit that your conclusion is contrary to all that has been discussed in previously with regards to Newton's Law of Inertia.

The sensation of being thrown outward is attributable to the idea of inertia, rather than the idea of force. When making that left-hand turn in the car, your tendency to be thrown rightward across the seat (that would be outward or away from the center of the circle) was not due to a force. It was due to your tendency to travel in a straight line while the car seat was making its turn. In fact, you were not thrown rightward at all; you moved in a perfectly straight line.

Finally, your body hits the door on the right side of the car and the door provides an inward push on your body to cause your body to begin moving in circular motion. But until hitting the door, your body's tendency was to follow its inertial path.

Scale Readings and Weight

Technically a scale does not measure your weight. Technically, the scale is measuring the upward (external) force applied by the scale to balance the downward force of gravity acting upon the object (or scale). When an object is in a state of equilibrium or balance (either at rest or in motion at constant speed), these two forces are balanced. If you're standing still on the scales, the upward force of the scale upon the person equals the downward pull of gravity and your weight.

The scale reading (that is a measure of the upward force) equals the weight of yourself as you stand on it.

What happens to the reading when you start moving or you start jumping up and down?

If you stand on the scale and bounce up and down, the reading will change. Bouncing motion = acceleration of your body. During the acceleration periods, the upward force of the scale is changing. Therefore the scale reading is changing.

Is your weight changing? You will continue to weigh as much (or as little) as you always do. In summary, the scale measures the external contact force (provided by the scale) that is being applied to your body.

Practical Examples

Otis L. Evaderz is conducting an elevator experiments.

He stands on a bathroom scale and rides an elevator up and down.

As he is accelerating upward and downward, the scale reading is d_________ than when he is at rest and traveling at c____________ speed. When he is accelerating, the upward and downward forces are not b_________. But when he is at r________ or moving at

c___________ speed, the opposing forces b__________ each other. Knowing that the scale reading is a measure of the upward n__________ force of the scale upon his body, its value could be predicted for various stages of motion.

For example, the value of the net force (Fnet) on Otis's 80-kg body could be predicted if the acceleration is known. The acceleration in this case is 50m/s/s. We can use Newton's second law to determine the varying contact forces on the elevator ride. Look at the diagram's on the board. Otis's who weighs 80-kg.

What is Otis' speed and direction when:

Traveling with constant speed (A),

Accelerating upward (B),

Accelerating downward (C),

Accelerating upward but the cable snaps so is free falling downward (D)


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