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Name: Sharon
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 8/6/2004

We are trying to develop an assessment for a science unit based on curriculum from the National Science Resource Center and the Smithsonian. One of the key concepts of this unit states: "A force is any push or pull on an object. An unbalanced force is needed to make a resting object move, to bring a moving object to rest, or to change the direction of a moving object." We do not find a coherent explanation of what an unbalanced force is, or why it is important.


An unbalance force is one that is not opposed by an equal and opposite force operating directly against the force intended to cause a change in the object's state of motion or rest. Consider this little illustration:

Object, O is at rest and subjected to a force from the left as shown:

Let ====> represent the force to change the object's state of motion or rest on object. O

This unopposed (unbalanced) force will cause the object to move to the right. ====> O

Let O <==== represent an opposing force of equal magnitude operating on object O.

When the forces are opposed and impinging on the object ====> O <====, the object will not move because each force is balanced by an equal and opposite force.

However, if the forces are unbalanced and aligned thus, ====> O <========, the larger force coming from the right is unbalance by the one from the left. Thus, the object will move toward the left.

The picture is more complicated that I can illustrate here because an opposing force my be impinging on the object from an angle. Overall, it is the "net" unbalanced force that will cause the object to move or change its state of motion.

ProfHoff 901


Newton's second law of motion defines force as mass times acceleration. Thus a force acting on an object will induce an acceleration assuming the mass stays constant. In a frictionless environment, a body can be considered at rest when it is not being subjected to an acceleration. That means that the object can either be completely still or moving at a constant velocity with respect to the observers frame of reference.

To my knowledge, in physics, there is no such thing called an unbalanced force. I can however, explain what they may be talking about in this manner...

Our world has friction. It is a force acting on everything that moves. We do not see it or create it but it is there and generally causes a negative acceleration on moving objects like cars and bikes. To counteract this force of friction, we must generate a force in the opposite direction of the frictional force. If these two forces are equal and opposite, then the net acceleration of the object is zero and the object (car or bike) maintains a constant velocity. Now, if we wish to change this velocity, we must "unbalance" the forces. If we increase the force pushing the object forward, it accelerates in that direction and vice versa.

Bob Hartwell

Dear Sharon,

An example that I used with my high school physics students is a "tug-o-war". If both sides pull with equal and opposite forces, the flag at the center of the rope does not accelerate to the left or to the right. There are certainly forces being applied to the rope -- team "A" pulling to the right and team "B" pulling to the left -- but there is no "net force", that is, no "unbalanced" force -- so the flag does not accelerate left or right.

Likewise, an airplane flying at a constant speed (say 400 mph) in a straight line (say due north) and at a constant altitude (say 30,000 feet) has no unbalanced forces. The forward thrust of the planes engines exactly equal the backward air resistance (drag) and the upward lift of the plane's wings exactly equals the downward pull due to gravity. The plane has forces applied, but no net forces (no "unbalanced" forces). To accelerate to a faster speed, the planes thrust would have to be greater than its drag (thus applying a net force forward). To slow down, the plane's drag would have to be greater than the plane's thrust (applying a net force backward). To go to a higher altitude, the plane's lift would have to be greater than the plane's weight. And so on.

This is important because of Newton's second law, F=ma, states that an object will only accelerate when a net force is applied. If there is no net force, the object will not accelerate.

Todd Clark, Office of Science
U.S. Department of Energy

You have confronted one of the fundamental problems in physics that is not confronted in most presentations and/or physics texts. The problem is that of the "undefined terms". If you examine the standard definitions of the fundamental terms in physics -- for example: "force", "energy", "mass"-- you will find their definitions are circular. One being defined in terms of the other and the other way around. Circular definitions are logically unacceptable, so how can the circle be broken? The dilemma is resolved by recognizing that "physics" is an "effective theory". The term "effective theory" is used in a non-standard specialized way. An "effective theory" is one that recognizes that there are certain fundamental elements of the theory that cannot be defined in terms of elements of the theory.

In physical science and mathematics all theories are "effective" (at least all I have come across). Some examples:

Euclidean geometry deals with "points" and "lines" but those terms are not defined within the lexicon of Euclidean geometry. They are the fundamental elements whose BEHAVIOR geometry treats.

Thermodynamics deals with "work" and "heat", "energy", "temperature". It is my favorite example of circular definitions: According to Van Nostrand's Scientific Encyclopedia (a typical source): "Heat. The agency whose addition to or removal from a physical system is the cause of thermal changes of several types. These include rise and fall of temperature..." "Temperature. That property of systems which determines whether they are in thermodynamic equilibrium. Two systems are in equilibrium when their temperatures...are equal." "...This led to the comparison of the states of thermal equilibrium of two bodies in terms of a third body called a thermometer. The temperature scale is a measure of [the] state of thermal equilibrium, and two systems are at thermal equilibrium must have the same temperature." > If that does not drive you up the wall I do not know what will!!! What is this "agency"? Temperature is that "property"? What is that "property"?

The problem is too large to discuss here, but paraphrasing Richard Feynman, he says of "energy" words to the effect -- There is this number I know how to calculate (energy) using certain formulas. When I change a system in some experimentally defined way, I always calculate the same value from the formulas. I will call this result of calculating using the formulas "the change in energy". There are other formulas that give rise to quantities that do not change too. One of them I might call "momentum".

So there are quantities that are defined operationally by some instrument or another. The definition of the quantity is what the instrument says it is. Not everyone will agree with me, but I do not see any other "out" to obscure circular definitions.

Vince Calder


What is meant here by an "unbalanced force" is a force on an object that is not balanced by another force on the same object. An object with an unbalanced force is an object with a non-zero net force, an object not in equilibrium.

The statement about an unbalanced force is saying that the motion of an object in equilibrium (all forces adding up to zero) cannot change.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

I would prefer to call it the NET force. If you take the vector sum of all the forces acting on an object, that sum is the net force, F, that goes into Newton's 2nd Law: F = ma. m is the mass of the object and a is the acceleration of the center of mass of that object. Note that if the net force is non-zero, the object will accelerate. This could cause it to start moving (if it had been at rest) or to speed up or slow down and/or to change direction.

Remember that force is a vector quantity and to find the net force, you must take the VECTOR sum of all the forces acting on the object.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

An unbalanced force is any force that is not opposed by some other force. For example, gravity exerts a force on your body that is perfectly opposed by the force the ground exerts on the bottoms of your feet. As a result, you do not sink into the ground or fly up into the air, even though there are forces exerted on you that would, if unopposed, cause these things to happen.

Tim Mooney

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