# Find the forces exerted by the two cables

• petal5
In summary: I'll just stick to the equation.In summary, you need to calculate T2, the equation for which I posted in post #10.
petal5
I'd appreciate help with this problem as I don't know how to start it:

A beam with a mass of 1500 kg and a length of 2.0 m hangs from two cables,
such that it makes an angle of 65degrees with the vertical. Both of the cables are completely
vertical; one is attached at the lower end of the beam, while the other is attached
0.40 m from the higher end of the beam. (a) Find the forces exerted by the two
cables.

Have you any thoughts on the problem?

I'm thinking F1+F2=Mg but I'm not sure

petal5 said:
I'm thinking F1+F2=Mg but I'm not sure
That is true, and it will be useful later in the problem. How about taking moments about the lower end of the beam?

Sorry but what do you mean by 'talking moments'?

petal5 said:
Sorry but what do you mean by 'talking moments'?
Moments is equivalent to torques. Examine the torques on the beam using the lower end as the axis of rotation.

Thanks for your help,I really appreciate it!
Do you mean to work out the following: T1=r1f1sinx , which would be T1=(0.4m)(F1)(sin25)

petal5 said:
Thanks for your help,I really appreciate it!
Do you mean to work out the following: T1=r1f1sinx , which would be T1=(0.4m)(F1)(sin25)
You have the right idea, perhaps you have the benefit of the diagram, but from the text I disagree with your angle and distance. If you do have a diagram, would it be possible for you to post it on Imageshack or something similar?

I've had another look at it and the angle and distance I posted above are incorrect.I've now come up with T1=r1f1sin65, T1=(0m)(F1)(0.9063).This is with using the lower end to take torques.However,I'm a bit confused as F1 turns out to be 0.
For T2 I've gotten (1.6m)(F2)(0.9063) therefore T2= -(1.45)F2
(I don't have a diagram by the way!)

Of course T1 should be zero since you are taking torques about this point, this is your pivot point. Your on the right track but not there yet, you need to consider both the torque due to the tension and the weight of the beam. Thus;

$$T_{2}\cdot (1.6)\sin(65) - (1500\cdot g)\cdot (1)\sin(65) = 0$$

Do you understand why?

In my last post I said that F1=0 and then you replied that T1=0.Are both F1 and T1 equal to 0?
Is the formula you mentioned above the formula for F2?

petal5 said:
In my last post I said that F1=0 and then you replied that T1=0.Are both F1 and T1 equal to 0?
That was poor practise from me, I apologise. You are taking torques about the base the beam, yes? Therefore, the tension is being applied through the axis of rotation, yes? Hence, the distance from the axis rotation to the applied force is zero. Does that make more sense?
petal5 said:
Is the formula you mentioned above the formula for F2?
Yes, in words the above equation is "the magnitude of the torque exerted by the tension in the second string minus the magnitude torque exerted by the weight of the beam must be zero.

Do you follow?

Hootenanny said:
You are taking torques about the base the beam, yes? Therefore, the tension is being applied through the axis of rotation, yes? Hence, the distance from the axis rotation to the applied force is zero. Does that make more sense?

Ye,the above makes sense to me.However,I am still unsure as to how to work out F2

petal5 said:
Ye,the above makes sense to me.However,I am still unsure as to how to work out F2
You do not need to calculate F2, you need to calculate T2, the equation for which I posted in post #10. Can you solve that for T2?

But part (a) of the problem is: Find the forces exerted by the two cables.Then part (b) is:What are the torques exerted by each of the two cables and by the centre of mass of the beam about a pivot point at then lower endc of the beam.So I seem to need F2??

Last edited:
petal5 said:
But part (a) of the problem is: Find the forces exerted by the two cables.Then part (b) is:What are the torques exerted by each of the two cables and by the centre of mass of the beam about a pivot point at then lower endc of the beam.So I seem to need F2??
The force exerted by the cable is the tension in the cable i.e. T2

Thanks again for all your help.Filling in for the equation in post #10 I get T2= 13321.274

Now, you can use your equation;
petal5 said:
I'm thinking F1+F2=Mg
To calculate T1. (I'm not checking your arithmetic though )
petal5 said:
Thanks again for all your help
My pleasure.

## 1. What is the purpose of finding the forces exerted by two cables?

The purpose of finding the forces exerted by two cables is to determine the amount and direction of the forces acting on an object or structure due to the tension in the cables. This information is important in designing and analyzing structures, as well as predicting their stability and potential failure points.

## 2. What factors affect the forces exerted by two cables?

The forces exerted by two cables are affected by the tension in the cables, the angle at which they are attached to the object, and the weight of the object itself. Other factors such as wind or external forces acting on the object can also affect the tension and therefore the forces exerted by the cables.

## 3. How do you calculate the forces exerted by two cables?

To calculate the forces exerted by two cables, you will need to know the tension in each cable, the angle at which they are attached to the object, and the weight of the object. You can use trigonometric equations and vector analysis to determine the magnitude and direction of the forces exerted by each cable.

## 4. What are the different types of forces that can be exerted by two cables?

The two types of forces that can be exerted by two cables are tension and compression. Tension is the force that pulls the object towards the cable, while compression is the force that pushes the object away from the cable. The magnitude and direction of these forces can vary depending on the angle and tension of the cables.

## 5. How can the forces exerted by two cables be optimized for stability?

To optimize the forces exerted by two cables for stability, the tension and angle of the cables must be carefully considered. The cables should be under enough tension to support the weight of the object and any external forces acting on it, while also being at an optimum angle to evenly distribute the forces and prevent any potential failure points. Additionally, the materials and construction of the cables should also be taken into account to ensure their strength and durability.

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