Physics 2211: Lecture 9

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Physics 2211 Lecture 9

• Quantitative Dynamics
• Static Dynamic Equilibrium

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Review Newton's Laws

• Newton 1 An object subject to no external
  forces is
• at rest or moves with a
  constant velocity
• Newton 2 FNET ma where FNET ??F
• .

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EQUILIBRIUM

• If the net force on an object is zero, the
  object is in equilibrium
• STATIC EQUILIBRIUM object is at rest
• DYNAMIC EQUILIBRIUM object is moving along a
  straight line

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Go back and forth between representations as
needed.
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Problem 1
The three ropes in the figure are tied to a
small, very light ring. Two of the ropes are
anchored to walls at right angles, and the third
rope pulls as shown. What are T1 and T2, the
magnitudes of the tension forces in the first
two ropes?
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Free-Body Diagram
Newton
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A 20 kg loudspeaker is suspended 2.0m below the
ceiling by two 3m long cables that angle outward
at equal angles. What is the tension in the
cables?
1 20 N 2 102 N 3 147 N 4 211 N 5 367
N
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• A car with a weight of 15000 N is being towed up
  a 20 degree
• slope at constant speed. The tow rope is rated at
  6000 N
• maximum tension. Will the rope break?

1 Yes 2 No
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Rope will not break
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• Nonzero acceleration
• Newtons 2nd Law
• Mass and Weight

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Using Newton's 2nd Law

• Law 2 For any object, FNET maWhere FNET
  ??F
• The objects trajectory can be determined
• using a in the equations of kinematics.

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A 1500 kg car is pulled by a tow truck. The
tension in the tow rope is 2500 N. A 200 N
frictional force opposes the motion. If the car
starts from rest, what is its speed after 5.0sec?
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Mass and Weight

• Mass is an intrinsic property
  of matter measured in kg.
• Weight is the gravitational force
  exerted on an object by a planet.

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Measuring mass and weightthe spring scale
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A block of mass m rests on a spring scale. The
scale sits on the floor of an elevator moving up
at constant speed. How is the magnitude of the
measured spring force F related to the weight w?

• F gt w
• F w
• (3) F lt w

Correct Answer is 2
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A block of mass m rests on a spring scale. The
scale sits on the floor of an elevator that is
slowing down as is moves upward. How is the
magnitude of the measured spring force F related
to the weight w?

• F gt w
• F w
• (3) F lt w

Correct answer is 3
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Weightlessness

• Elevator cable breaks, and elevator
  free-falls.

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Recap
Newtons 2nd Law Equilibrium (Static and
Dynamic) Non-zero acceleration
problems Mass and Weight
Next Time
Recitation