TrueTears question thread

[dejan91]

So if a force of 1000N is applied and there is a safeft factor of 2, is the max allowable force 500N or 2000N?

[TrueTears]

Thanks guys!!! I got it

Safety factor:

A multiplier applied to the calculated maximum force or a number that the tensile strength is divided by in order to determine the safe working load.

Both will give same answer

Ok there's another question but using what I said before it doesn't work...

A sample of rope fractures under a tension at 50 000N. The builder wishes to place a safety factor of 2.5 on the rope. The cross section area is m^2. the recommended maximum stress for the rope would be ?

Using

A multiplier applied to the calculated maximum force

stress

But answer is

However if you do

a number that the tensile strength is divided by in order to determine the safe working load.

You get the right answer. Ie:

stress =

then .

Why doesn't both ways work here?

Is there a bullet proof way to do these safety factor questions?

Why did both ways work for this question?

The yield strength of mild steel is Pa. what is the minimum diameter of mild steel rod that can support a 75kg man with a safety factor of 3 before breaking?

[arthurk]

well the second way is definitely correct seeing as the maximum stress coinciding with a safety factor must be below the stress at the fracture point which method 1 clearly did not provide.

[TrueTears]

well the second way is definitely correct seeing as the maximum stress coinciding with a safety factor must be below the stress at the fracture point which method 1 clearly did not provide.

Yeah, so how do I know which method? What's the difference in the 2 problems which didn't allow both methods to work?

[Damo17]

Not sure if this is correct but maybe the difference is that they are asking for maximum stress so you use the second method that gets , the value is the ultimate stress that the rope can withstand. I don't know if this is correct or not.

[kamil9876]

 Remember that recommended stress is obviously less than maximum allowable stress. The more below maximum stress you are the more SAFE you are, hence greater SAFETY factor.By maximum stress, I mean the maximum PHYSICAL stress you can apply before the laws of physics imply that the thing breaks This psychological intuition can be quantified as:



Quite obviously, twice as much Maximum stress means its twice as safe, and half as much applied stress also means twice as safe. The more stress you apply the less safe it is.

In this situation you are after the denominator (Actual stress you plan to apply)

[kamil9876]

It's quite confusing, you have to be able to distinguish between "Maximum Physical Stress" and "Maximum Recommended Stress". THe latter is basically "How muich stresss you plan to apply in order to not drop below a certain Safety". And obviouosly how much you plan to apply goes in the denominator since as you apply more you get less safe.

[TrueTears]

OH YES I totally get it now, thanks everyone and kamil!

[TrueTears]

Explain what would happen to the light globes (Lighter, brighter, short circuit, burnout ?) if only is switched on.


Thank you all once again!

[naved_s9994]

Voltage drop will become equal, as it will transform into a parallel (provided it is has equal resistance).
I.e, if resistance equal - then therefor brightness equal.. Switch A will NOT make a difference. The Flow of CURRENT does.

So it all in the end means , it transforms into a parralel circuit and it all depends on the resistance of each globe

[Mao]

To clear things up with stress/safety-factor in a nutshell:

ULTIMATE stress: the point when the material breaks
MAXIMUM stress: ULTIMATE/safety factor

[Mao]

When SB is closed, it is in parallel with L3. Since a switch has no effective resistance, it will short along the switch, current will not pass through L3 (and it is dark).

The intensity of the other globes, however, depend on the type of source (fixed or variable current), and the type of globe (fixed current drawing or non-fixed).

[TrueTears]

Thanks Mao and naved

[TrueTears]

(Soz for such a bad diagram, but all the sin curves are the same, they are simply just shifted up, the top right and bottom left diagram have exactly the same sin curve except the axis are different, one is brightness vs time the other is voltage vs time.)

The top left diagram is the signal that is being sent into W

At X the signal would be the top right diagram.

At Y what would the signal be like?

The answer is the bottom left signal. But I thought it should just be the original, ie, top left? Why is it the bottom left signal? Wouldn't that have a higher max voltage then the original?

Isn't the laser just using light as the carrier wave and transporting the original signal across the optical wires? So why does the entire original signal get shifted upwards (as indicated in the bottom left diagram)?

[Mao]

Bias of the diode prevents current flow from smoothly alternating (due to switch-on voltage). Hence to preserve the signal, the demodulated signal at Y is the bottom left.

Modulation/demodulation is not a process where the input signal is 'exactly copied' to the output device. Only the frequency and amplitude are preserved.