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Forces
Force – A push or a pull upon an object resulting from the object's interaction with another object. When the
interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.
A force causes an object to move, change speed, change direction, or stop.
People often think of force as something you apply using your muscles. When you push or pull on an object, you
apply a force on it. You also apply force when you throw a baseball or kick a soccer ball, or sit on a soccer ball.
These forces are examples of contact forces – are those types of forces that result when the two interacting
objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces,
tensional forces, normal forces, air resistance forces, and applied forces.
Field forces, or non-contact forces, are another class of forces. These forces do not involve physical contact
between the agent and the receiver, but act through space. The force of gravity, namely the gravitational
attraction between two masses such as the earth and you, or the sun and the planets, or between stars, is one such
force. Another is the electric force, often observed as static electricity, which causes objects that are similarly
charged to repel each other (and oppositely charged ones to attract). Yet another is the magnetic attraction
between magnets and steel.
Examples of contact and field forces are listed in the table below.
Contact Forces
Frictional Force
Tension Force
Normal Force
Air Resistance Force
Applied Force
Spring Force
Action-at-a-Distance Forces
Gravitational Force
Electrical Force
Magnetic Force
Type of Force
Description of Force
(and Symbol)
Applied Force
Fapp
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.
Gravity Force
(also known as Weight)
Fgrav
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. The force of gravity on earth is always
equal to the weight of the object as found by the equation:
Fgrav = m * g
where g = 9.8 N/kg (on Earth)
and m = mass (in kg)
Normal Force
Fnorm
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 surface, then the
surface is exerting an upward force upon the book in order to support the weight of
the book. On occasions, 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.
Friction Force
Ffrict
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 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. The maximum amount of friction force that a surface can
exert upon an object can be calculated using the formula below:
Ffrict = µ • Fnorm
Air Resistance Force
Fair
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.
Tension Force
Ftens
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.
Spring Force
Fspring
The spring force is the force exerted by a compressed or stretched spring upon any
object that is attached to it. An object that compresses or stretches a spring is
always acted upon by a force that restores the object to its rest or equilibrium
position.
Force is a quantity that is measured using the standard metric unit known as the Newton. A Newton is
abbreviated by an "N." To say "10.0 N" means 10.0 Newton of force. One Newton is the amount of force required
to give a 1-kg mass an acceleration of 1 m/s/s. Thus, the following unit equivalency can be stated:
1 Newton = 1kg 
m
s2
A force is a vector quantity, which means it has both a magnitude (size) and direction.
Because a force is a vector that has a direction, it is common to represent forces using
diagrams in which a force is represented by an arrow. The size of the arrow is reflective
of the magnitude of the force and the direction of the arrow reveals the direction that
the force is acting. (Such diagrams are known as free-body diagrams) Furthermore,
because forces are vectors, the effect of an individual force upon an object is often
canceled by the effect of another force. For example, the effect of a 20-Newton upward
force acting upon a book is canceled by the effect of a 20-Newton downward force
acting upon the book. In such instances, it is said that the two individual forces balance
each other; there would be no unbalanced force acting upon the book.
Other situations could be imagined in which two of the individual vector forces
cancel each other ("balance"), yet a third individual force exists that is not balanced
by another force. For example, imagine a book sliding across the rough surface of a
table from left to right. The downward force of gravity and the upward force of the
table supporting the book act in opposite directions and thus balance each other.
However, the force of friction acts leftwards, and there is no rightward force to
balance it. In this case, an unbalanced force acts upon the book to change its state of
motion.
If either all the vertical forces (up and down) do not cancel each other and/or all horizontal forces do not cancel
each other, then an unbalanced force exists. The existence of an unbalanced force for a given situation can be
quickly realized by looking at the free-body diagram for that situation. Free-body diagrams for three situations are
shown below. Note that the actual magnitudes of the individual forces are indicated on the diagram.
In each of the above situations, there is an unbalanced force. It is commonly said that in each situation there is a
net force acting upon the object. The net force is the vector sum of all the forces that act upon an object. That is
to say, the net force is the sum of all the forces, taking into account the fact that a force is a vector and two forces
of equal magnitude and opposite direction will cancel each other out. At this point, the rules for summing vectors
(such as force vectors) will be kept relatively simple.
Observe the following examples of summing two forces:
A net force (i.e., an unbalanced force) causes an acceleration.
Combine your understanding of acceleration and the newly acquired knowledge that a net force causes an
acceleration to determine whether or not a net force exists in the following situations.
GRAVITY
Newton realized that everything pulls on everything else. He discovered that a force of gravity acts between all
things–involving only mass and distance. Every mass attracts every other mass with a force that is directly
proportional to the product of the two interacting masses. This statement is known as the Law of Universal
Gravitation.
Expressed in symbol shorthand,
m1 and m2 are the masses, and d is the distance between their centers. Thus, the greater the masses m1 and
m2, the greater the force of attraction between them. The greater the distance of separation d, the weaker is
the force of attraction–weaker as the inverse square of the distance between their centers.
Gravitational Force – attraction between two or more objects that have mass.
a. Always attracts objects, never repels.
b. The closer the objects are, the stronger the force.
 Gravity gets weaker with distance the same way a light gets dimmer as you move farther away
from it.
The greater the distance from the Earth’s center, the less the gravitational force on an object. In using Newton’s
equation for gravity, the distance term d is the distance between the centers of the masses of objects attracted to
each other. Note in Figure 7.6 that the girl at the top of the ladder weighs only 1/4 as much as she weighs at the
Earth’s surface. That’s because she is twice the distance from the Earth’s center.
c. The more massive the object, the stronger the force.
d. Has the ability to occur over great distances.
Mass is the amount of stuff (atoms or molecules) used to make-up some piece of matter.
Mass is the amount of matter in an object.
We can measure the force of gravity. The amount of gravitational force between two objects is called Weight.
Weight is the force due to gravity that acts on an object’s mass.
Mass remains the same regardless of location, but weight varies with location due to changes in the gravitational
force.
If the gravitational force cause weight than the following is true:
Weight is Force (of gravity) and Force (of gravity) is Weight
Suppose you tie a string around a 2-pound bag of sugar and hang it on a weighing
scale similar to the scales found in grocery stores. A spring in the scale stretches
until the scale reads 2 pounds. The stretched spring experiences a “stretching
force” called tension. The same scale in a science lab is likely in units of newtons.
This scale will show the weight of the bag of sugar as 9 newtons rather than 2
pounds. Both pounds and newtons are units of weight. Units of weight in turn are
units of force. The bag of sugar is attracted to the Earth with a gravitational force
of 2 pounds–or equivalently, 9 newtons.
We measure all forces in a unit called NEWTONS. 1 Newton = ¼ pound.
1 Kg = 9.8 N
A book lies at rest on a desk. What forces act on the book to make up this zero net force? One is the force of
gravity–the weight of the book. Where is the upward force coming from? It is coming from the desk that is
supporting the book. We call this upward force the support force, or the normal force. * The support force must
equal the weight of the book.
Hold a stone above your head and drop it. It accelerates during its fall. When air
resistance doesn’t affect the motion of a falling object, we say the object is in free fall.
Interestingly, the amount of acceleration is the same for all freely falling objects in the
same vicinity. We find that a freely falling object gains speed at the rate of 9.8 m/s each
second (~10 m/s per second).
In the figure to the right, we imagine a freely-falling boulder with a speedometer
attached. As the boulder falls, the speedometer shows that the boulder goes 10 m/s
faster each second. This 10 m/s gain each second is the boulder’s acceleration.
To calculate the force of gravity on an object we use the equation: F = mg.
Force (weight) = mass x acceleration of gravity.
F
m
g
F = Force (N)
m = mass (Kg)
g = acceleration due to gravity (9.8m/s2 on earth)
All free falling objects have the same acceleration.
The weight of a 1-kg stone (or 1 kg of anything) is 10 N at the Earth’s surface. The weight of 10 kg of matter,
such as the boulder, is 100 N. The force acting on a falling object is its weight. The acceleration of the stone is
and for the boulder,
For free fall the downward net force is weight. Only weight! But when air is
present, the downward net force = weight – air drag.
What happens to the net force if air drag builds up to equal weight?
 Then acceleration becomes zero. Does this mean the object
comes to a stop? No! What it means is the object no longer
picks up speed. We say the object has reached terminal
velocity.
Tides
1. The revolution of the moon around the Earth and the rotation of the earth cause the tides
a. Due to the moons gravity
2. There are two high and two low tides per day
3. Although the Sun is bigger the moon has a greater effect on tides due to its proximity
a. Sun is too far and does not have as great of an effect as the moon but still does have some effect.
The part of the Earth beneath the crust is molten–fluid. Because of this we have Earth tides–actual
rises and falls in the Earth’s crust. Earth tides, however, are much smaller than ocean tides. There
are also atmospheric tides. These regulate the cosmic rays that reach the Earth’s surface. The
greatest fluctuation of tides between high and low (ocean, Earth, or atmospheric) occurs during the
alignments that make a new and full moon. The fluctuations in atmospheric tides produce changes
in the intensity of cosmic rays reaching the Earth’s surface–which in turn affects some life forms.