Torque Problem: Finding the weight of a horizontal bar

In summary, the ball is hanging from the beam by a rope. The tension in that rope is around 4000N so the tension in the main rope must be >4000N.
  • #1
DracoMalfoy
88
4

Homework Statement


n the diagram below, the horizontal bar has a length of 4m. The force due to the wire angled at 30 degrees is 1200N. What is the weight of the horizontal bar?
Chapter8_Quest2.png


Homework Equations


T=F⊥r

The Attempt at a Solution



I know that I need to add the three( the bar, the wire, and weight, but I'm not sure how to do that. I tried...

0= Tbar+Twire+Tweight
0= -Tbar+-Twire+-Tweight
0= Tsin30+-1200sin30+-400sin30⋅9.8

I'm not sure if I did this correctly. The options for answers are

A) 6640N
B) 1600N
C) 3320N
D) 4200N
E) 2750N

The first answer that i got using another method was 3320. That was wrong.
 

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  • #2
Are you sure that the force due to the wire (presumably its tension) is 1200N? Is there possibly a zero missing?
 
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  • #3
gneill said:
Are you sure that the force due to the wire (presumably its tension) is 1200N? Is there possibly a zero missing?

Thats what the question states. but i think that I've found the answer. hopefully.

0=1200sin30⋅4-400⋅9.8⋅4+Tsin30⋅4
0=2400-15680+2T
-2T=-13289
T=6640
 
  • #4
DracoMalfoy said:
0=1200sin30⋅4-400⋅9.8⋅4+Tsin30⋅4
I don't understand this equation. The last term appears to be the same as the first but with T standing for the tension of 1200N. Where does the weight of the bar feature?
 
  • #5
I don't see why there would be two terms with components that depend upon the angle of the wire. Both the bar and ball have forces that point straight down (gravity).

If I look at just the weight of the ball, ##400~kg \cdot g = 3923~N##, then no component of a 1200 N tension could counteract that. That's why I suspect that the given tension is suspect.
 
  • #6
gneill said:
I don't see why there would be two terms with components that depend upon the angle of the wire. Both the bar and ball have forces that point straight down (gravity).

If I look at just the weight of the ball, ##400~kg \cdot g = 3923~N##, then no component of a 1200 N tension could counteract that. That's why I suspect that the given tension is suspect.

So the question is flawed? lol
 
  • #7
DracoMalfoy said:
So the question is flawed? lol
It happens more often than is generally comfortable :smile:

Try the tension at 12000 N and see if a one of the given answers results.
 
  • #8
DracoMalfoy said:
Thats what the question states. but i think that I've found the answer. hopefully.

0=1200sin30⋅4-400⋅9.8⋅4+Tsin30⋅4
0=2400-15680+2T
-2T=-13289
T=6640
I agree with this answer.
Your method of balancing torques is interesting - I mean the term (Tsin30⋅4).
Usually, one would include the force at the pivot point of the bar and then eliminate this
force when balancing the translational forces.
You have apparently done this indirectly.
 
  • #9
J Hann said:
I agree with this answer.
Your method of balancing torques is interesting - I mean the term (Tsin30⋅4).
Usually, one would include the force at the pivot point of the bar and then eliminate this
force when balancing the translational forces.
You have apparently done this indirectly.
Disagree. Forces at the pivot point do not contribute to torques.
 
  • #10
J Hann said:
Your method of balancing torques is interesting - I mean the term (Tsin30⋅4).
It still does not make sense to me (see post #4).
As @gneill has posted, the tension should be 12000N. With the data as given the answer is negative!
 
  • #11
+1

The 400kg ball is hanging from the beam by a rope. The tension in that rope is around 4000N so the tension in the main rope must be >4000N.
 

1. What is torque and how does it relate to weight?

Torque is a measure of the twisting force that is applied to an object. The weight of an object can affect its torque because the further an object's weight is from its pivot point, the stronger the torque will be.

2. What is the equation for calculating torque?

The equation for torque is: Torque = Force x Distance. This means that the torque is directly proportional to the force applied and the distance from the pivot point to where the force is applied.

3. How can I find the weight of a horizontal bar using torque?

To find the weight of a horizontal bar, you will need to know the distance from the pivot point to where the bar is being supported and the amount of force being applied to the bar. Using the equation for torque, you can rearrange it to solve for weight: Weight = Torque/Distance.

4. What units are used to measure torque?

Torque is typically measured in units of newton-meters (N*m) or foot-pounds (ft-lb). These units represent the amount of force applied at a specific distance from the pivot point.

5. Can torque be negative?

Yes, torque can be negative. Negative torque indicates that the rotational force is in the opposite direction of the positive rotation. This can occur if the force is applied in the opposite direction or if the weight is closer to the pivot point than the applied force.

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