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Introducing Forces
Chapter 4 (MHR Physics)
Forces
Essentially thought of as pushes or pulls
on our everyday level
 Are responsible for motion
 Cause objects to accelerate
 Vector quantity-has magnitude & direction
 Units are Newtons, (N) dynes, pounds

Common types of forces
Gravitational
 Frictional
 Electric
 Magnetic
 Mechanical

Key Terms
Inertia
 Kinematics
 Dynamics
 Mechanics
 Force
 Inertial Mass
 Gravitational Mass

Inertia (4.1)
Is the tendency of an object to remain in
its current state of motion. The amount of
an object’s inertia is directly related to its
mass.
 Examples you don’t want to get out of bed
on a Saturday am
 A granite boulder rests on a rocky beach

Inertia (4.1)

Galileo’s thought experiments led to our
understanding of inertia-objects rolling down
inclines will speed up, those rolling up inclines
will slow down and those rolling along a
horizontal surface will continue to roll. His work
built on Aristotle’s and Buridan who believed
respectively that constant force produced
constant speed and that objects remained
moving because they had “impetus” inside
The study of motion
Kinematics is the branch which describes
an object’s motion in terms of
displacement, velocity, and acceleration
 Dynamics explains why objects move i.e
respond to forces
 Mechanics = Kinematics + Dynamics

Inertia (4.1)



Inertial mass is the property of matter causing it
to resist a change in its motion
Gravitational mass is the property of matter that
determines the strength of the gravitational
force. The more mass, the stronger the
gravitational attraction for that mass.
Einstein showed that these two are essentially
the same.
Common Forces (4.2)
Forces which involve direct physical
contact are contact forces e.g. pushing,
pulling, and frictional
 Forces which do not involve direct physical
contact but which act over a distance are
called non contact forces e.g. electric,
magnetic, gravitational

Common Forces (4.2)



Weight (gravitational force) is the force of gravity
acting on a mass. We can jump higher on the
moon than on earth because the moon’s gravity
is about 1/6 th that of the earth.
Fg = mg where F is force in N, m is mass in kg
and g is acceleration due to gravity
Since g varies with location on the earth, your
weight (not mass) will vary by location
Common Forces (4.2)







Friction is a force which opposes motion. It is
measured in N and is a contact force
Two kinds of friction:
Static friction involves no net motion
Kinetic friction involves movement
The coefficient of friction is a pure number which
indicates how much friction exists between
surfaces.
The symbol is: μ
The formula for friction is: μ = Ff /FN or Ff = μFN
Friction


Depends on nature of the surfaces-rougher
surfaces have more friction than smoother ones
Actually an electromagnetic interaction at the
atomic level. Surface atoms of objects in contact
interact and form attractive forces leading to a
“stick and slip” process when one object slides
over another e.g. marker on whiteboard, chalk
on chalkboard, squealing tires
Friction
The coefficient of friction is a “stickiness”
value for specific combinations of surfaces
 Friction also depends on the magnitude of
the forces pressing the surfaces together.
 The force that a surface exerts back on an
object is called the normal force. It acts
perpendicular to the surface.

Friction
Force of friction is independent of velocity
 Force of friction is independent of the area
of contact (only if mass is evenly
distributed). The surfaces must be flat.
 Certain plastics and rubbers have natural
properties which do not agree with the
standard model of friction e.g. “icegripping” tires, adhesive tape

Free Body Diagrams (FBD)
Represent all the forces acting on an
object.
 Only contains forces and angles at which
they act.
 Draw them roughly to scale so a bigger
force will have a bigger line.

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