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Physics Fact Sheet
Chapters 4 - 5
FORCE & NEWTON’S LAWS
Force Equations:
Newton’s 2nd Law:
FNet  m  a
or
W  m g
Force of Friction:
F f   K  FN
or
F f   S  FN
Pressure from Force:
P
F
A
Conversions:
Force (Weight)
Mass
1 lb = 4.45 N
1 N = .22 lbs
1 slug = 14.59 kg
1 kg = .0685 slug
Name
Symbol
Unit
Notes
Force
Force (Net)
Force (Weight)
Force (Normal)
Force (Friction)
Force (Applied)
Force (Tension)
F
FNet
W
FN
Ff
FA
T
N
N
N or lbs
N
N
N
N
Mass
m
kg
Acceleration
Acceleration (gravity)
a
g
m/s2
m/s2
Coefficient of Friction (Static)
Coefficient of Friction (Kinetic)
S
K
-----
Pressure
Area of contact
P
A
Pascals (N/m2)
m2
- 9.8 m/s2
*** NOTE: 1 Newton = 1 kg  m/s2 ***
Helpful Kinematics Equations:
a=
vf vi
t
or
v f  v i  at
d
1
a t2
2
or
a
2d
t2
Types of Forces
Force: a push or pull acting upon an object as a result of its interaction with another object.
Contact Forces: Frictional, Tensional, Normal, Air Resistance, Applied and Spring.
Non-Contact or Action-at-a-Distance Forces: Gravitational, Electrical and Magnetic.
Force (Weight)
W
N or lbs
The force of gravity is the force at 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 which is directed "downward" towards the center of the earth. The
force of gravity on earth is always equal to the weight of the object.
Force (Normal)
FN
N
The normal force is the support force exerted upon an object which 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 which are in contact with each other.
Force (Friction)
Ff
N
The friction force is the force exerted by a surface as an object moves across it or makes an effort
to move across it. The friction force opposes the motion of the object. For example, if a book moves
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.
Force (Applied)
FA
N
An applied force is a force which 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.
Force (Tension)
T
N
The tension is the force which is transmitted through a string, rope, or wire when it is pulled tight
by forces acting from each end. The tensional force is directed along the wire and pulls equally on the
objects on either end of the wire.
Force (Spring)
Fspring
N
The spring force is the force exerted by a compressed or stretched spring upon any object which is
attached to it. An object which compresses or stretches a spring is always acted upon by a force
which restores the object to its rest or equilibrium position. For most springs (specifically, for those
which are said to obey "Hooke's Law"), the magnitude of the force is directly proportional to the
amount of stretch or compression.
Force (air)
Fair
N
The air resistance is a special type of frictional force which acts upon objects as they travel
through the air. Like all frictional forces, the force of air resistance always opposes the motion of
the object. This force will frequently be neglected due to its negligible magnitude. It is most
noticeable for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects
with large surface areas.