Everyday Forces
Holt Physics
Chapter 4 Section 4
Pages 141-148
Everyday Forces
 Weight
– the magnitude of the force
of gravity acting on an object
Everyday Forces
Fg = mg
Fg = force due to gravity (in Newtons)
m = mass of object (in kilograms)
g = acceleration due to gravity (-9.81 m/s2)
* Mass ≠ Weight
Everyday Forces
 Normal
Force – (Fn) – a contact force
exerted by one object on another in
a direction perpendicular to the
surface of contact. On a horizontal
surface, Fn = mg
Everyday Forces
 Normal
force diagrams-
Everyday Forces


Frictional Forces – Contact forces which
oppose applied forces
These are the surfaces in contact at a
microscopic level.
Everyday Forces
1. Static friction - (Fs) - force exerted
on a motionless object by the
environment to
resist an
external force
attempting to
cause a change
in motion
Everyday Forces
2. Kinetic friction – (Fk) – force
exerted on a moving object that
opposes
motion
Everyday Forces
*
Fk < Fs due to surface adhesion
between two surfaces in contact
 Coefficient of friction (μ) – a ratio of
frictional force to normal force
between two objects
Coefficient of Friction
 The
coefficient of friction is a ratio of
force of friction to the normal force
acting between two objects.
 This completely depends on the
objects in contact.
 Different experimental values have
been calculated based on the
materials in contact (page 144: table 4-2)
Coefficient of Friction Equations
 Coefficient
of static friction
μs =
 Coefficient
Fs,max
Fn
of kinetic friction
μk =
Fk
Fn
Everyday Forces
3. Air resistance is
a form of friction
 as velocity
increases so does
air resistance