Download Forces Reference Sheet - Charles A. Dana Center

Forces Reference Sheet
A force is a push or pull upon an object resulting from the object's interaction with another
object. Whenever there is an interaction between two objects, there is a force upon each of
the objects. When the interaction ceases, the two objects no longer experience the force.
Contact Forces
A contact force results when the two interacting objects are perceived to be physically
contacting each other.
Air Resistance Force
The air resistance is a special type of frictional force that acts upon objects as they travel
through the air. The force of air resistance is often observed to oppose the motion of an
object. This force will frequently be neglected due to its negligible magnitude (and due to
the fact that it is mathematically difficult to predict its value). It is most noticeable for
objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with
large surface areas.
Applied Force
An applied force is a force that is applied to an object by a person or another object. If a
person is pushing a desk across the room, then there is an applied force acting upon the
object. The applied force is the force exerted on the desk by the person.
Frictional Force
The friction force is the force exerted by a surface as an object moves across it or makes
an effort to move across it. There are at least two types of friction force—sliding friction
and static friction. Though it is not always the case, the friction force often opposes the
motion of an object. For example, if a book slides across the surface of a desk, then the
desk exerts a friction force in the opposite direction of its motion. Friction results from the
two surfaces being pressed together closely, causing intermolecular attractive forces
between molecules of different surfaces. As such, friction depends upon the nature of
the two surfaces and upon the degree to which they are pressed together.
Normal Force
The normal force is the support force exerted upon an object that is in contact with
another stable object. For example, if a book is resting upon a table, the table surface is
exerting an upward force upon the book in order to support the weight of the book. On
occasion, a normal force is exerted horizontally between two objects that are in contact
with each other. For instance, if a person leans against a wall, the wall pushes
horizontally on the person.
Tension Force
The tension force is the force that is transmitted through a string, rope, cable or wire
when it is pulled tight by forces acting from opposite ends. The tension force is directed
along the length of the wire and pulls equally on the objects on the opposite ends of the
wire.
The Charles A. Dana Center at the University of Texas at Austin
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Reference Sheet: Forces
Spring Force
The spring force is the force exerted by a compressed or stretched spring upon any
object that is attached to it. A force that restores the object to its rest or equilibrium
position always acts upon an object that compresses or stretches a spring.
Action at a Distance Forces
Action at a distance forces are those types of forces that result even when the two
interacting objects are not in physical contact with each other, yet are able to exert a push or
pull despite their physical separation.
Gravitational Force
The force of gravity is the force with which the earth, moon, or other massively large
object attracts another object towards itself. By definition, this is the weight of the object.
All objects upon earth experience a force of gravity that is directed "downward" towards
the center of the earth.
Magnetic Force
A magnetic force is the attraction or repulsion that arises between electrically charged
particles because of their motion. A magnetic force is the basic force responsible for the
action of electric motors and the attraction of magnets for iron.
Electromagnetism is an interaction between electricity and magnetism, as when an
electric current or a changing electric field generates a magnetic field, or when a
changing magnetic field generates an electric field.
Inertia
An object’s tendency to resist changes in its state of motion is described as inertia.
If a force does not act on an object in motion, the object will continue in motion. If an object
is at rest, it will continue in that resting position unless a force acts it on. The resistance to a
change in motion varies with mass, and the greater the mass of an object, the greater is its
tendency to resist a change in motion. Therefore, an objects’ inertia is tied to its mass.
Force Diagrams
For this session, diagrams will be used to show the relative magnitude (strength) and
direction of all forces acting upon an object in a given situation.
The length of the arrow in the force diagram reflects the relative magnitude of the force. The
direction of the arrow shows the direction of the force.
Each force arrow in the diagram is labeled to indicate the type of force. Represent the object
of interest with a black dot and draw the force arrow from the dot.
The forces acting on an object determine the object’s motion. If the forces are balanced, the
object will continue doing what it is doing. If the forces are unbalanced, the object’s motion
will change depending on the direction of the unbalanced force.
The Charles A. Dana Center at the University of Texas at Austin
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Reference Sheet: Forces
Example A: Object resting on a table.
Normal Force
Object
Force of Gravity
Balanced Forces:
force. The object will not move.
The normal force is equal to the gravitational
Example B: Object resting on a table.
Lifting Force
Force of Gravity
Object
Unbalanced Forces: The lifting force is greater than the gravitational force. The object moves
upward.
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Reference Sheet: Forces
Example C: Object resting on a table.
Normal Force
Object
Force of Gravity
Unbalanced Forces: The force of gravity is greater than the normal force. The object moves
downward.
Example D: Object being pushed across a table.
Normal Force
Applied Force
Resistance Force
Force of Gravity
Object
Unbalanced Forces: The applied force is greater than the resistance force. The object moves to the
right.
The Charles A. Dana Center at the University of Texas at Austin
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Reference Sheet: Forces
Example E: Object being pushed across a table.
Normal Force
Applied Force
Object
Force of Friction
Force of Gravity
Unbalanced Forces: The applied force is greater than the resistance force. The object moves to the
left.
The Charles A. Dana Center at the University of Texas at Austin
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