Physics / mechanics help pls

coffeehound

pbartlett said:

 the tension in the ropes decreases as a function of cos(theta), which is 1 when the ropes are vertical (hence then support the full 100kg), to zero when they are horizontal - in that case the rope could be made of cotton as there is no tension in it.

I think it's the other way around pb:  because it's dividing by Cos (theta), with the ropes at 90 degress horizontal, it would be dividing by zero, which would make the force on the ropes infinitely high, so they would break before you could ever get them horiztontal!

pbartlett

have you added a beam into the system eg like here? ie in the first case you have a rope with a weight and in the second case you have a 30deg rope but have added a beam?

Force and Tension in Rope due to Angle (engineeringtoolbox.com)

[Deleted User]

coffeehound said:

pbartlett said:

 the tension in the ropes decreases as a function of cos(theta), which is 1 when the ropes are vertical (hence then support the full 100kg), to zero when they are horizontal - in that case the rope could be made of cotton as there is no tension in it.

I think it's the other way around pb:  because it's dividing by Cos (theta), with the ropes at 90 degrees horizontal, it would be dividing by zero, which would make the force on the ropes infinitely high, so they would break before you could ever get them horizontal!

Yes, exactly this, tension increases with angle and approaches infinity as it gets nearer to 90 degrees.
I did a quick spreadsheet to show the range of values, you can see it increase massively as it approaches 90 degrees

getmore4less

pbartlett said:

have you added a beam into the system eg like here? ie in the first case you have a rope with a weight and in the second case you have a 30deg rope but have added a beam?

Force and Tension in Rope due to Angle (engineeringtoolbox.com)

that link supports the tension in the rope increases with angle.

getmore4less

An alternative way to show that the tension increases is to start with one rope vertical and one horizontal
tension in vertical all the mass 
tension in horizontal Zero(using massless ropes)
as you move the vertical rope further away the tension in the horizontal rope increases as the angle of the vertical rope.

Mickey666

coffeehound said:

I was thinking of the edge case where if you tried to pull on the ropes to bring the load towards horizontal, you need to use increasing force.  So therefore the ropes must see a higher weight as you increase the angle?  

You need to use increasing force because the ropes begin to pull against themselves.  Imagine there was no weight there at all just a ring connecting the two ropes.  As you pull on the ropes they become increasingly horizontal until you are effectively pulling two ends of the same rope with increasing force until the ropes (or the ring) break.

coffeehound

[Deleted User] said:
Yes, exactly this, tension increases with angle and approaches infinity as it gets nearer to 90 degrees.

I did a quick spreadsheet to show the range of values, you can see it increase massively as it approaches 90 degrees

Thanks that illustrates it nicely