Warm-Up
?
Introduction to Forces
Lesson
Question
Lesson Goals
Describe how forces
affect
the motion of an object.
Determine how net
Identify and describe
various forces.
W
2K
Analyze free-body
diagrams.
forces
affects
the motion of objects.
Words to Know
Fill in this table as you work through the lesson. You may also use the glossary
to help you.
an action that has the ability to change an object’s
force
of motion
friction
a contact force that
newton
the SI unit of
motion
the support force a surface exerts on an object, which is
normal force
tension
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angle to the surface
always at a
a force from a string or cable that
or pulls
1
Warm-Up
W
2K
Introduction to Forces
Words to Know
air resistance
a contact force between an
resists motion
equilibrium
the state in which an object has a net force of
free-body diagram
a diagram that uses
and the air that
to show the external
forces acting on an object
a contact force that opposes motion between
kinetic friction
surfaces
net force
the sum of
static friction
a contact force that exists between
objects, preventing motion
forces acting on an object
Motion
• Motion can be described in in terms of:
•
speed
.
• velocity.
• acceleration.
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2
Instruction
Introduction to Forces
Slide
2
Forces
• A force is an action that has the ability to change an object’s state
of motion
.
• Forces are vectors.
• Vectors are assigned as positive or negative based
on direction
.
• Forces are measured in
• 1 newton = 1 kg ·
4
Newtons
(N).
m
s2
Common Types of Forces
• There are various types of forces.
Force
push or pull
Symbol
Fp
Description
a force that is applied
to an object by a
person or another object
gravitational
force
normal force
Tension
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Fg
FN
FT
a force that is always directed toward the center
of Earth; the attractive force between all matter
in the universe
the support force that a surface
exerts
on an object; always at a 90-degree angle to the
surface
a force from a string or cable that stretches or
pulls; pulls outward along the string or cable
3
Instruction
Introduction to Forces
Slide
4
• These forces act to prevent or stop the motion of an object.
Force
Symbol
Ff
Description
a contact force that resists motion
a contact force that exists between
static friction
kinetic
friction
Ffs
stationary
objects, preventing motion
a contact force that opposes motion between
Ffk
moving
surfaces
a contact force between an object and the
air resistance
7
Fair
air
that resists motion
Free-Body Diagrams
• A free-body diagram is a drawing that shows the size and direction of all
forces acting on an object. vectors
are used to represent the forces.
Fair
Fg
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4
Instruction
Introduction to Forces
Slide
7
Constructing a Free-Body Diagram
EXAMPLE
Two horses pull a sleigh over snow to the right. Construct a free-body diagram
of the forces acting on the sleigh.
Fp1
Fg
Friction of any kind always acts in the
OPPOSITE
direction of the motion
or of the pull.
9
Free-Body Diagrams
• Construct a free-body diagram of the forces acting on one of these cars.
Include the force of static friction.
θ
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5
Instruction
Introduction to Forces
Slide
12
Net Force
• The net force is the SUM
−
of all
+
forces acting on an object.
• Only PARALLEL forces can be
added together.
• Directions need to be assigned as
either positive or negative before
adding forces together.
• Right and up are POSITIVE
50 N − 75 N = −25 N
.
• Left and down are negative.
14
Force Components
• When NON-PARALLEL forces
act on an object, one or more of the
vectors need to be separated into an
x-component and a y-component.
• Trigonometric functions are used
to determine the value of any
components.
• sinθ =
•
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COS
OPPOSITE
hypotenuse
=
adjacent
hypotenuse
6
Draw a circle around the vector
representing gravitational force.
Instruction
Introduction to Forces
Slide
14
Net Force with Force Components
Determine the net force of a box on a ramp inclined at an angle of 20° with the
following forces acting on it.
Fnet y =
FN −
F gy
= FN − ( Fgy )(cos θ )
=
460 N − 490 N(
COS20
= 460 N − 460 N
= 0N
Fnet x =
Fg(sinθ ) −
Ff
= FN − ( Fgy )(cosθ )
=
490 N(sin20°) −
168 N
=
=
16
118 N
Balanced and Unbalanced Forces
• The application of a force does not always
result in a change of motion.
• Balanced forces are equal in
MAGNITUDE and act in opposite
directions.
• Unbalanced forces occur when forces
are not of equal magnitude or they do
not point in THE SAME
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directions.
7
− 50 N
50 N
)
Instruction
Introduction to Forces
Slide
16
The Effect of Balanced Forces on Motion
• When forces acting on an object are
exactly equal and OPPOSITE
, then:
• the object is in a state of
EQUILIBRIUM.
• the net force is 0
.
• there is no change in motion.
• Forces at ANGLES
need to be
broken into x and y components before
it can be determined if NET
equals zero.
force
The Effect of Unbalanced Forces on Motion
• When forces acting on an object are either
UNEQUAL or not acting in opposite
directions, then:
• net force is either POSITIVE
or negative.
• there is a change in motion, and that
change is in the same
as the net force.
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DIRECTION
8