Free Body Diagram: Explaining 3 Cord Tensions for Block C

In summary, the FBD of Block C would involve three cord tensions if the bottom pulley is considered part of the system, but it is also perfectly acceptable to only consider Block C separately and create a separate FBD for the bottom pulley. This saves a bit of time, but ultimately both methods are valid.
  • #1
princejan7
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Homework Statement



http://postimg.org/image/5h0hvdi4l/
Can someone explain why the FBD of Block C would involve three cord tensions?

Homework Equations





The Attempt at a Solution

 
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  • #2
princejan7 said:
Can someone explain why the FBD of Block C would involve three cord tensions?
Whoever is claiming that is probably considering the bottom pulley, which is connected to block C, as part of the block C system. Perfectly OK.

But you certainly do not have to! If you consider block C separately, you'll also need a diagram for that bottom pulley.
 
  • #3
Can someone explain why the FBD of Block C would involve three cord tensions?

It can't. There are only two cords directly connected to block C.

Now, if you were to make a horizontal cut between the two smaller sheaves, you would have the tension in three cords to consider.
 
  • #4
Doc Al said:
Whoever is claiming that is probably considering the bottom pulley, which is connected to block C, as part of the block C system. Perfectly OK.

But you certainly do not have to! If you consider block C separately, you'll also need a diagram for that bottom pulley.
http://postimg.org/image/oxb74l6t1/

so here they've considered the block + the bottom pulley as the system?
is there any reason for why they did that instead of just considering Block C as the system?
 
  • #5
princejan7 said:
http://postimg.org/image/oxb74l6t1/

so here they've considered the block + the bottom pulley as the system?
Yes, for the purpose of that FBD they consider 'block + bottom pulley' as a single unit.

is there any reason for why they did that instead of just considering Block C as the system?
Just saves a bit of time. If you just considered Block C alone, you'd have to do another FBD for the bottom pulley to figure out the tension in the cable connecting it to the block. No big deal; either way is fine.
 

FAQ: Free Body Diagram: Explaining 3 Cord Tensions for Block C

1. What is a free body diagram?

A free body diagram is a visual representation of the forces acting on an object. It includes all the forces, such as weight, tension, and friction, that are acting on the object.

2. What is the purpose of a free body diagram?

The purpose of a free body diagram is to help us understand the motion of an object by visualizing the forces acting on it. It allows us to analyze the forces in a systematic way and make predictions about the object's movement.

3. How do you draw a free body diagram?

To draw a free body diagram, you first need to identify all the forces acting on the object. Then, draw a dot to represent the object, and draw arrows to represent the magnitude and direction of each force. Make sure to label each force and include the units of measurement.

4. What are the 3 cord tensions in a free body diagram for block C?

The 3 cord tensions in a free body diagram for block C refer to the three forces acting on the block: the weight of the block, the tension in the cord pulling the block up, and the tension in the cord pulling the block to the side.

5. How do you explain the 3 cord tensions for block C in a free body diagram?

In a free body diagram for block C, the weight of the block is represented by an arrow pointing straight down, the tension pulling the block up is represented by an arrow pointing upwards, and the tension pulling the block to the side is represented by an arrow pointing to the right. The magnitudes of these forces depend on the mass of the block, the angle of the cords, and the acceleration of gravity.

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