Drawing Free-Body Diagrams: Rock Suspended by Two Strings

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In summary: The tension in the left string would be attributed by the triangle's vertices (the left string, the rock, and the right string) being pulled in different directions.
  • #1
fiziksfun
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My teacher asked us to draw a free body diagram for the following scenario:

(A rock is suspended by two strings. The right string is supposed to be longer than the left one. Static)

(looks like this:)
http://i23.tinypic.com/28u7ccn.jpg

My free-body diagram looks like this:

http://i21.tinypic.com/ojmhc1.jpg

Is this right? I don't understand why the tension in the left string would be more. Help!

(P.S. how do I insert a picture?)
 

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  • #2
Tension arises from the weight of the object that is being held by the string, the more vertical the string the more that downwards weight contributes to the tension, alternatively let's imagine a ball in the middle of a 2d square, attatched by 4 strings at 90 degree angles, the string at the top of the square has the highest tension, as it is holding the ball in its suspended position and its force is upwards (the force from the ball is downwards) the lowest tension string is at the bottom since the balls force is in the same direction that the string would apply its tension, unless the ball was pulling upwards due to some elastic streaching by the top string.

The side strings should potentially have no tension unless both strings were attempting the deviate the ball from a perfectly vertical position (angle them) and then the strings tension would be attributed by the strings deviating the ball from a free fall

This is just my guess on the subject though, I've never done free body diagrams.
 
  • #3
If the object is in static equilibrium, the vector sum of all of the forces on the object will equal zero. This can be interpreted by adding those vectors tail-to-tip, and finding that the last tip meets the first tail, ... resulting in a [closed] polygon. In your case, you should find that the vectors (constrained by their directions) added tail-to-tip forms a triangle.
 

1. What is a free-body diagram?

A free-body diagram is a visual representation of all the forces acting on an object. It helps to simplify complex systems and understand the net force acting on the object.

2. How do I draw a free-body diagram?

To draw a free-body diagram, start by identifying the object and all the forces acting on it. Then, draw a dot to represent the object and arrows to represent each force, making sure to label each arrow with the type of force and its direction.

3. What forces should I include in a free-body diagram for a rock suspended by two strings?

You should include the weight of the rock (acting downwards), the tension in each string (acting upwards), and the force of gravity (acting downwards).

4. Why is it important to draw a free-body diagram?

A free-body diagram helps to visualize and understand the forces acting on an object. It also allows for the calculation of the net force and acceleration of the object, which can be used to predict its motion.

5. What are some common mistakes to avoid when drawing a free-body diagram?

Some common mistakes to avoid when drawing a free-body diagram include forgetting to label forces, drawing forces in the wrong direction, and including forces that do not act on the object. It is also important to ensure that all forces are drawn to scale.

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