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
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
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.
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
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.
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
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
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).
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. That property of systems which determines whether they are in
thermodynamic equilibrium. Two systems are in equilibrium when their
"...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.
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
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
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
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.
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Update: June 2012