Misconceptions
SED 690
Dean Papadakis
November 2007
Thedefinition of a misconception as found in Websters dictionary is as follows:
mis·con·cep·tion(mĭs'kən-sěp'shən)
n. A mistaken thought, idea, or notion; a misunderstanding: had many misconceptions about the new tax program.
What is the misconception?
The student misconception that I have chosen is that of CENTRIFUGAL
FORCE.
When is it observed?
The reason that I have selected this particular physics misconception is that it is experienced in everyday life very frequently, such as when one is driving in an automobile, and the automobile turns a corner. The person in the car will experience some kind of push on their body, to the right, when the car turns left or they will experience some kind of push on their body, to the left, when the car turns right. This push that they experience is called centrifugal force. Centrifugal force is defined as an outward-seeking or center-fleeing force. So if a person in a car is driving in a circle, the push that they feel will be directed outward or away from the center of the circle. However, physicists define a centrifugal force as a pseudo force or a “fake or unreal” force. They define a centrifugal force this way because there isn’t any real object that is the cause of the push or force. The problem that occurs is that what they feel seems to be a push on their body, in the opposite direction they are turning, but if the passenger is sitting in the middle of the car seat, and they are not next to anything like a door, they will realize that there is nothing next to them that is actually pushing on their body. The reason that people mistakenly consider centrifugal force a real force is that they really do “feel” something when they turn a corner in a car.
Another example of a centrifugal force is with the washing machine. The cylindrical tub inside a washing machine has tiny holes in it that allows the water to drain, especially when the tub is rotating during the spin cycle. As the tub rotates at a high speed, there is some kind of “force” that is pushing the water outward through the holes so that the clothing is as dry as possible before it is placed into the dryer. Again, this centrifugal force that pushes the water out from
the clothing, is a pseudo force that has nothing physically pushing on the water. Another example of centrifugal force is found at Six Flags Magic Mountain Amusement Park in Valencia, California, or at many amusement parks. The ride called “Spinout” has a large rotating cylinder that people stand in and it begins to rotate faster and faster. Then all of sudden, the floor is lowered from underneath the feet of the people in the ride and they do not fall down, because of the centrifugal force acting on their body, pushing them against the walls of the cylinder. They clearly feel that something is pushing on them, but they can also clearly see that there is nothing in front of them, pushing on their bodies.
The Origins of this Misconception
So, the origins of this misconception of centrifugal force as being a real force, emanate from the fact that a person can really feel the force, and therefore, people will ask how can a physicist state that it is not real. According to a physicist, there must be some object that is responsible for the force that is described. Just because we have given this kind of force, a name, does not mean that it is real. An example of a real force would be the force of gravity that we all feel pulling on our body. The object that is the cause of the force is the earth pulling downward on our body. Another example, going back to cars, would be when a car is involved in a car crash. It is very easy to know what object caused the large
dent in the body of our car. It was the other car exerting a large force on our car.
Teaching Techniques to help address this Misconception
In order to understand why centrifugal force is not a real force, we need to first understand what really causes centrifugal force. The answer is inertia. Inertia is a property that all objects have. Galileo and Newton were the first to describe and understand how inertia works. It began with asking the question, does an object require a force to act on, it in order to keep it moving? In our everyday experience, we might say that the answer to this question is YES. However, the answer is really NO. For example, if you roll a smooth steel ball on a flat smooth table, you will see that the ball continues to roll at a constant speed for some distance, without the need of any outside force keeping it rolling. In a car, where there is a lot of friction between the wheels of the car and the road, we know that we need to keep pushing on the gas pedal in order to keep the car moving. However, if there were no friction, we would not need to do that. Another important aspect of inertia is that the steel ball rolling on the table, will not only continue rolling at a constant speed but it will also continue to roll in a straight line, at a constant speed, unless some outside net force acts on it. This is a property that all objects have, including our bodies. So if we return back to the car problem, we can explain what we feel when the car turns, in terms of inertia. Here is a demonstration that can be done in the classroom to help students understand that centrifugal force is not a real force. Swing a bucket of water in a circle. Or as I like to do, to swing a cup of water on a wooden board with rope attached to it. The students see that the water does not leave the cup of water even when it is upside down, and yet there is nothing above the water that is holding it in the cup. Again, it is the inertia of the water that keeps in moving in a straight-line path and prevents it from falling downward. Returning to the car problem. A person is sitting in the back seat of a car, next to the right side door. The car is traveling forward in a straight-line path. Our bodies are also moving in a straight line path and the property of inertia that our bodies possess, because we have mass, will want to keep our bodies moving in a straight line path unless some outside net force acts on our body. When the car starts to make a turn to the left, the inertia of our bodies keeps us going in a straight line forward. As the car turns to the left, the sensation of being pushed to the right is really the sensation of the car’s door pushing left into our body as our body goes straight ahead. The same explanation can be applied to the water being forced out of the washing machine tub. The water that is moving in a circular path, at any point in the motion will have a tangential motion straight ahead and so the water will continue to go in a straight-line path. The holes in the tub will allow the water to travel through them as the water really travels straight ahead.
In the book called “Physics Begins with an μ . . .Mysteries, Magic and Myth, the author describes the mystery of how a person will stay in an amusement park ride when it is traveling upside down. Of course, the people in the ride have a bar that holds them in, but they really do not need the bar, because they would stay in the car, without it. However, for added safety, the bar must be used. The myth behind this mystery is that people will say that it is “centrifugal force” that keeps them in the ride. Since centrifugal force acts outward or away from the center of the circle, what is the object that is pushing the person into the seat? The answer is that there is nothing that is over their head and body that is pushing on them to hold them in the seat and so centrifugal force cannot be the explanation. This is the misconception that people have. Again, it is the inertia of the person that causes them to continue moving forward in a straight line and not to fall vertically out of the car.
In the article “What Do Kids Know-and Misunderstand About Science?” the author states that having in-class discussions and careful assessment, can unmask misconceptions and build student understanding. When teachers present new material to students, the teacher may not realize that students understand the material differently, depending on the personal misconceptions that each student brings to the lesson. She also states that when we give multiple-choice tests to students, and grade them, a student may get a particular question correct, but that does not mean that the student really completely understands the concept tested by the question. Also, when the student gets a particular question wrong, we still don’t know what exactly the student is thinking regarding the concept in the question. Therefore, testing must take many forms, and when we ask a student to explain something in writing, on a test, we get a much better sense of what the student understands. It is best if we can “help students to clarify, probe, and challenge their current ideas-before the test”.
References:
Crockett, C., “What Do Kids Know – and Misunderstand – About Science?” Educational Leadership, 34-37 (2004).
Driver, R., Squires, A., Rushworth, P., Wood-Robinson, V., “Making Sense of Secondary Science (Research into children’s ideas), Introduction, pp.1-13. (Year?)
Jewett, J. W., “Physics Begins with an μ . . .Mysteries, Magic and Myth”, Chapter on Forces, 1994