06 - forces i.pptx
TRANSCRIPT
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Forces I: The Basics
Ms. Mikaela Irene Fudolig
Physics 71
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What is a force?
A force is a push or a pull.
Force is a VECTOR.
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Some Physics 71 Forces
Weight
Tension
Normal ForceFriction
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Weight
Earth exerts a force on any object
Weight points towards the center of the
Earth (DOWNWARDS)
Mass vs. Weight
ˆ
w mg w mg mgj
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Exercise 1: Mass and Weight
A man weighs 50kg. What is his weight on
Earth?
ˆ
w mg
w mg mgj
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Weight
w=mg
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Tension
Strings PULL on objects
For each INDIVIDUAL string, there is a
separate tension.
A SINGLE massless and taut string has
the SAME MAGNITUDE of the tension all
throughout its length.
There is NO TENSION whenthere is NO STRING.
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Tension
T
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Tension
T T
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Normal Force
Force exerted by SURFACE on an object
Points NORMAL to the surface.
There is NO NORMAL FORCEwhen there is NO SURFACE.
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Normal Force
N
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Normal Force
N
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Normal Force: 2 or more objects
Exerted by one object on another.
Fby Y on B
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Friction
Usually acts OPPOSITE the direction of
motion
Usually causes deceleration
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Exercise 1: May the force be with
you…
What are the forces
acting on the box in
this picture?
w=mg
N
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Exercise 2: May the force be with
you…
What are the forces
acting on the box in
this picture?
w=mg
N
T
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Exercise 3: May the force be with
you…
What are the forces acting on the box in
this picture?
w=mg
NT
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What does a force do?
Newton’s 1st Law of
Motion
Newton’s 2nd Law of
Motion
Every body continues inits state of rest, or of uniform motion in a
straight line, unless it iscompelled to change
that state by forcesimpressed on it.
F ma
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What does a force do?
Newton’s 1st Law of
Motion
Newton’s 2nd Law of
Motion
A body acted onby no NET force
moves with
constant velocity(which may bezero) and zeroacceleration.
F ma
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Inertia
The tendency of a body to remain at rest,
or to keep moving in a straight line once it
is set in motion, results from inertia.
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Conceptual Exercise 5a
A box is initially sliding to the right at
1m/s, but after 5 seconds, it stops. What
can you conclude about the direction of the net force in this system?
FRICTION
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Conceptual Exercise 5b
A box is initially sliding to the right at aconstant speed of 1m/s, but after 5
seconds, its speed suddenly keepsincreasing. What can you concludeabout the forces in this system?
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Conceptual Exercise 6
A box is initially at rest. If you want
to keep it moving to the left, what
should you do?
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Conceptual Exercise 7
If you do apply a force to the left, and
you stop applying that force, will the box
also stop moving immediately? Assumethe surface is frictionless.
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Conceptual Exercise 8
A box is initially at rest on a tabletop
(x0=0). You push it to right for 5 seconds
until it reaches a speed v0. After 5
seconds, you release the box.Draw the v vs. t diagram.
Draw the x vs. t diagram.
Draw the a vs. t diagram.
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Inertial Reference Frame
Reference Frame where Newton’s 1st Lawis valid.
Reference frame moving at constant
velocity. If A and B are both inertial reference
frames,
object relative to A object relative to Ba a
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Conceptual Exercise 8
Which of the following are inertial
reference frames?
Earth
accelerating airplane
train moving at constant velocity
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Newton’s 2nd Law
F ma
unit of force: Newtons (N)
1N=1kg-m/s2
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Numerical Exercise 9
If you push a 2.5kg box with a force of 5N
to the right
what is the acceleration (direction and
magnitude) of the box?
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Numerical Exercise 10
If you push a 5kg box with a force of 5N to
the right
what is the acceleration (direction and
magnitude) of the box?