Common Misconceptions: Force
By Christopher MooreI will begin writing short posts each week about common misconceptions students have about basic ideas in physics. This week I’d like to address the concept of force.
Most students think they know what a force is after finishing an introductory course in physics. But most students carry away several misconceptions about the concept.
In a typical class, force is introduced as a push or a pull. Although, in a sense this is correct, this view of force leads to several misconceptions. By viewing force as a push, students connect the concept of force with the actual act of pushing.
Teacher: If John pushes Jason to the ground, then who is using force?
Student: John, of course.
A force is not an act or a tangible thing, or a property of an object. A force is an interaction between two objects. It is correct that the act of pushing involves a force, but the act itself is not the force. Beginning a discussion on force by describing the concept as an interaction may result in a different exchange between teacher and student:
Teacher: If John pushes Jason to the ground, then who is using force?
Student: Well, in a sense both use force. But force isn’t actually a thing that can be “used” so your question is poorly formed. The force is the interaction between both Jason and John.
Refering to force as a push or pull also leads to misconceptions about Newton’s Third Law. A typical exchange might go as follows:
Teacher: If a book that weights 10 N sits on a desk, what is the equal and opposite reaction to the weight.
Student: The table pushes up on the book with 10 N of force.
Teacher: How many forces are involved?
Student: There are two forces. The book pushes down on the table and the table pushes up on the book.
Although the student’s statements are correct, they do not correctly answer the teacher’s questions. By treating force as a push or a pull, the student can naturally read Newton’s Third Law as: “For every force there is an equal and opposite force.” This is incorrect. It is: “for every action there is an equal and opposite action.” Believeing force to be an “action” leads to this misconception. Force is an interaction between two objects, and Newton’s Second Law describes this interaction quantitatively. Newton’s Third Law couples the objects, and descibes force qualitatively.
A student well versed in force-as-interaction may answer as follows:
Teacher: If a book that weights 10 N sits on a desk, what is the equal and opposite reaction to the weight.
Student: Weight is an interaction between an object and the Earth, so if the Earth exerts 10 N of force on the book, then the book must exert 10 N of force on the Earth.
Teacher: Why is that so?
Student: Because they are the same force. The Earth “pulling” the book is the same interaction as the book “pulling” the Earth. Force does not causes an equal and opposite force. The force causes an equal and opposite action. Newton’s Third Law simply says that a force is an interaction between two objects. So if that interaction affects one of the objects, then it has to affect the other.
Teacher: How many forces are involved?
Student: For a book on a table there are two forces. The interaction between the book and the Earth and the interaction between the book and the table.
Another common misconception about force is the belief that a force is an inherent property of an object. This misconception usually arises due to a failure to adequetly distinguish between mass and weight. And example is as follows:
Teacher: An elephant weights 2000 N. How much force does the elephant have?
Student: 2000 N.
Our well trained student would answer differently:
Teacher: An elephant weights 2000 N. How much force does the elephant have?
Student: That’s a stupid question. An elephant can’t have force. Force is an interaction between two objects. The interaction between the Earth and the elephant can be quantified as 2000 N. But an elephant does not have weight.
Issac Assimov in The History of Physics
