08 - forces iii.pptx
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Applying Newtons Laws:
Frictionless Surfaces
Mikaela Irene Fudolig
Physics 71
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Review
1. Newtons 1st and 2nd Law
netF ma
1. Newtons 1st and 2nd Law
0
0
netF
a
1. Newtons 1st and 2nd Law
0net
net
F
Fa
m
1st Law 2nd Law
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Review
2. Newtons 3rd Law
For any two objects A and B:
A on B B on AF F
A on B B on Aand comprise an
ACTION-REACTION PAIR.
F F
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Review
2. Newtons 3rd Law
The action force and the reaction force act on
DIFFERENT bodies.
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Review
3. Free-body diagrams
Identify the object.
Identify the forces acting on the object. Weight
Normal force
Tension
Friction
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Review
3. Free-body diagrams
Identify the direction in which those
forces act.
Weight points downwards.
Normal force points perpendicular to the
surface, pushing the object.
Tension pulls the object.
Friction is against the direction of motion.
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Review
3. Free-body diagrams
Draw the coordinate axes.
Identify clearly where +y and +x directions
are.
Apply Newtons 2nd Law PER AXIS.
x xF may y
F ma
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Review
3. Free-body diagrams
For inclined planes, TILT the axes such
that one axis isparallelto the surface,
and the other axis isperpendicularto
the surface.
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Example 1a: The Weighing Scale
A weighing scale does NOT measure the
WEIGHT.
It measures the NORMAL FORCE it
exerts on an object.
Alternatively, it measures the reaction force to
the normal force it exerts.
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Example 1a: The Weighing Scale
You are standing on a weighing scale
inside an elevator at rest. If your weight is
500N, what is the reading on the weighing
scale?
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Example 1b: The Weighing Scale
You are standing on a weighing scale
inside an elevator going up at a constant
speed of 2m/s. If your weight is 500N,
what is the reading on the weighing scale?
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Example 1c: The Weighing Scale
You are standing on a weighing scale
inside an elevator going up and
accelerating at 1m/s2. If your weight is
500N, what is the reading on the weighingscale?
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Example 1d: The Weighing Scale
You are standing on a weighing scale
inside an elevator going down and
accelerating at 1m/s2. If your weight is
500N, what is the reading on the weighingscale?
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Example 1e: The Weighing Scale
You are standing on a weighing scale
inside an elevator going down and
decelerating at 1m/s2. If your weight is
500N, what is the reading on the weighingscale?
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Example 2: A girl pulling a box
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Example 2: A girl pulling a box
If the girl pulls on the rope connected to
the 20-kg box with a force of 150N at 25
from the horizontal, find
the acceleration of the box (magnitude and
direction)
the normal force exerted by the surface on the
box.Note: Neglect friction.
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Example 3: Box on an inclined plane
A 1-kg box slides down with aninitial speed of 10cm/s on a
frictionless ramp 5m long inclined
at 30 from the horizontal.What is the normal force on the box?
What is the acceleration of the box?
What is the velocity of the box upon
reaching the end of the ramp if it
starts from the top of the ramp?
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Exercise 4: Box on an inclined plane
A 1-kg box is on a frictionless rampinclined at 30 from the horizontal.
You push it up the ramp with a
force of 1N.What is the acceleration of the box?
If it initially starts from rest:
in what direction will it move?
how far has it traveled after 1s?
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Things to know about STRINGS
Assume strings are massless and taut.
Strings ALWAYS PULL on objects
The magnitude of the tension in the stringis the same everywhere on the string.
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Things to know about STRINGS
N
mg
T
TB on G = TG on B T
(magnitude)
N
mg
ThandT
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Example 5
If the object (mass m)below is at rest,
what is the tension in the string?
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Exercise 6
If the object (mass m)below is at rest,
what is the tension in the string?
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Conceptual Exercise 7
A boy rides an
improvised swing as
shown. If the boys
weight is 400N andthe rope used has a
breaking point of
300N, why doesnt
the rope break?
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Exercise 8
A picture frame
weighing 8N is
supported by two
different wires withtension T1 and T2
respectively. Find
each tension.
60 30
1T
2
T
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Two or More Objects
If object A and object B are connected:
the magnitudes of their accelerations are the
SAMEthe magnitudes of their velocities (a.k.a.,
speed) are the SAME
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Two or More Objects
Draw free-body diagrams foreachbody.Dont forget to include the action-reaction
forces!Foreach object, write a separate
equation foreach component ofNewtons 2nd Law.
Identify the unknown and givenquantities.
Solve!
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Example 9
If the motor,
which is at rest,
has a mass m,and the O-ring
has negligible
mass:Find T1, T2, and
T3.
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Example 10
You push on a box of mass m1 with a force F.
The box is in direct contact with a second box of
mass m2.
What is the acceleration of the boxes?What is the magnitude of the force exerted by one box
on the other?
F
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Exercise 11
Two boxes of the same mass m=1kg are
connected in the manner shown below. If
you pull the gray box with a force of 5N,
what is the acceleration ofthe gray box?
the blue box?
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Example 12
In what direction
will the masses
(initially at rest)
move if:=15
m1=2kg
m2=1kg
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Example 13: Atwood Machine
Find the acceleration
of each mass
(magnitude and
direction).
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