Physics 2211a Kulp Class 22: How does a rope work

1
Physics 2211a (Kulp)Class 22 How does a rope
work?

• W. D. Kulp
• william.kulp at physics.gatech.edu
• Office W501b
• Office Hours MF 11-12 AM, Tu 9-10 AM, by appt.
• www.physics.gatech.edu/academics/Classes/fall200
  6/2211/a/

2
Where were we?

• Last Class Class 21
• Distinguish between the interacting objects in
  the system of interest and the environment that
  influences the objects through external forces.
• Identify force pairs and the objects on which
  they act, and determine the magnitude and
  direction of each force.
• For a system of objects constrained to accelerate
  together, determine the forces acting on the
  system and solve for the motion of the objects
  within the system.

3
Where are we?

• Last Class Class 21
• Distinguish between the interacting objects in
  the system of interest and the environment that
  influences the objects through external forces.
• Identify force pairs and the objects on which
  they act, and determine the magnitude and
  direction of each force.
• For a system of objects constrained to accelerate
  together, determine the forces acting on the
  system and solve for the motion of the objects
  within the system.

• Today Class 22
• Analyze a system of objects connected by a rope
  and determine the tension in the rope.
• Analyze the motion of system of objects connected
  by a string which passes over massless pulleys.

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String theory
?
y

• A 1 kg mass is suspended by a massless string.
  The string is tied to the wall after passing over
  a massless, frictionless pulley. Assuming that
  the string does not stretch, what is the tension
  in the horizontal section of the string?

1 kg
5
String Theory (continued)
?

• A 1-kg mass is hung from each end of a massless
  string which passes over two massless,
  frictionless pulleys. What is the tension in the
  horizontal section of the string? (the string
  does not stretch)

1 kg
1 kg
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With a cord
A

• Block A, supported by a frictionless table, is
  connected via a massless string and massless,
  frictionless pulley to block B, which is hanging
  over the edge of the table.
• What is the acceleration of block A? Of block B?

B

• The blocks must have the same acceleration if the
  string does not stretch.

7
Mush math

• A sled dog pulls two sleds over snow (?k 0.10).
  
• The tension in rope 1 is 150 N what is the
  tension in rope 2?

8
Show the constraint

• What is the acceleration of the 2.0 kg block?

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The massless string approximation

• A string of mass mS is used to accelerate a block
  of mass mB across a frictionless table.

• Pull with tension T1, what is T2?

T2
T1
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Whats next?

• Friday, 10/13 Class 23 How do we analyze
  interactions?
• Use Newton's laws of motion to analyze a system
  of interacting objects, construct simultaneous
  linear equations based on that analysis, and
  solve the equations for unknown forces and
  accelerations, then determine the motion of the
  objects.

• To-do list
• Homework 22 before class on Friday
• Read assignment (8.5)