chapter 5 two dimensional forces equilibrium an object either at rest or moving with a constant...
TRANSCRIPT
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Chapter 5
Two Dimensional Forces
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Equilibrium An object either at rest or moving
with a constant velocity is said to be in equilibrium
The net force acting on the object is zero (since the acceleration is zero)
0F
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Equilibrium cont. Easier to work with the equation in
terms of its components:
0 0x yF and F
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Inclined Planes Choose the
coordinate system with x along the incline and y perpendicular to the incline
Replace the force of gravity with its components
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Forces of Friction When an object is in motion on a
surface or through a viscous medium, there will be a resistance to the motion This is due to the interactions
between the object and its environment
This is resistance is called friction
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More About Friction Friction is proportional to the normal
force The force of static friction is generally
greater than the force of kinetic friction The coefficient of friction (µ) depends on
the surfaces in contact The direction of the frictional force is
opposite the direction of motion The coefficients of friction are nearly
independent of the area of contact
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Applications Involving Friction, InclinesOn a microscopic scale, most surfaces are rough. The exact details are not yet known, but the force can be modeled in a simple way.
For kinetic – sliding – friction, we write:
is the coefficient of kinetic friction, and is different for every pair of surfaces.
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Kinetic Friction, ƒk
The force of kinetic friction acts when the object is in motion
ƒk = µ n Variations of the
coefficient with speed will be ignored
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4-8 Applications Involving Friction, Inclines
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Static Friction, ƒs
Static friction acts to keep the object from moving
If F increases, so does ƒs
If F decreases, so does ƒs
ƒs µ n
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The static frictional force increases as the applied force increases, until it reaches its
maximum. Then the object starts to move, and the kinetic frictional force takes over.
Applications Involving Friction, Inclines
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Block on a Ramp, Example
Axes are rotated as usual on an incline
The direction of impending motion would be down the plane
Friction acts up the plane
Opposes the motion Apply Newton’s Laws
and solve equations
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Atwood Machine
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Atwood Machine Let’s take a look
at the forces on each mass…
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Atwood Machine
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Atwood Machine
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Static Equilibrium Condition of an object when
net forces equal zero Object is motionless
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mg
1
T1
2
T2 Since the sign is not accelerating in any direction, it’s in equilibrium. Since it’s not moving either, we call it Static Equilibrium.
Thus, red + green + black = 0.
Hanging sign f.b.d.
Free Body Diagram
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T2
mg
T1
1 2
T1 cos 1
(x component)
T2 cos 2 (X component)
T1 sin 1
(y component)
T2 sin 2
(Y component)
T1 cos 1 = T2 cos 2
Horizontal:
Vertical:T1 sin 1 + T2 sin 2 = mg
Components & Scalar Equations
If in Equilibrium……..the following would be
true
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Sample ProblemA mother and daughter are outside playing on the
swings. The mother pulls the daughter and swing (total mass 55.0 kg) back so that the swing makes an angle of 40.0° with the vertical (50.0 ° from horizontal)
A. 703N
What is the tension in each chain holding the swing seat and the daughter?
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Sample Problem…
Is this a case of equilibrium?
Calculate the magnitude of the net force
45°40 °
150N160N
75N
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Equilibrium Example – Free Body Diagrams