Physics 2211: Lecture 21 Notes

1
Physics 2211 Lecture 14

• More on Free Body Diagrams
• Ropes and Pulleys

2
Tools Ropes Strings

• Can be used to pull from a distance.
• Tension ( T ) at a certain position in a rope
  is the magnitude
• of the force acting across a cross-section of
  the rope at that
• position.
• Tension is the force you would feel if you cut
  the rope and grabbed the ends.

3
Tools Ropes Strings...

• An ideal (massless) rope has constant tension
  along the rope.
• A real rope has mass, and the tension generally
  varies along the rope
• For example, a heavy rope hanging from the
  ceiling...
• We will deal mostly with ideal massless ropes.

T Tg
T 0
4
Tools Ropes Strings...

• The direction of the force provided by a rope is
  along the direction of the rope

T
w
5
Tools Pegs Pulleys

• Used to change the direction of forces
• An ideal massless pulley or ideal smooth peg will
  change the direction of an applied force without
  altering the magnitude

F1
F1 F2
F2
6
A pulley/peg changes the direction of a
force without changing its magnitude
Three choices for the system
7
A w 20 N block is attached to a scale. The
scale reads 20 N when the scale is attached to a
wall. What does the scale read when it is instead
attached to another w 20 N block? (1) 0 N
(2) 10 N (3) 20 N (4) 40
N
Reads 20 N
8
Solution

• Draw a Free Body Diagram of one of the blocks!!

• Use Newtons 2nd Lawin the y direction

T
a 0 since the blocks are stationary
FTOT 0
w
T - w 0
T w 20 N.
9

• The scale reads the tension in the rope, which is
  
• T 20 N in both cases!

T
T
T
T
T
T
T
10
Correct Answer is 4
11
Correct Answer is 2
12
Correct Answer is 5
13
Recap
Free Body Diagrams Pulleys Ropes
Next Lecture
Quantitative Form of Newtons Laws Chapter 5