Help engineering hsc MC questions (1 Viewer)

[ABBAS38]

How is the answer 2000kg??
For Q. 9

 

[ABBAS38]

I added the question

 

[ABBAS38]

Is anyone working it out just let me know pls thx

 

[blyatman]

Max stress is 400 MPa.

Stress = F/A so F = A*stress = (1/4)*pi*(8e-3)^2 * 400e6 = 20,106 N.

So max load it can sustain is 2000 kg.

 

[ABBAS38]

But isn’t the max stress 800MPa not 400

 

[StudyOnly]

Let's do this first,
Max stress = 800 MPa = 8 x 10^8 Pa
diameter = 8mm = 0.008m
FoS = 2

We know:
Allowable Stress = Max Stress/FoS
8 X 10^8 / 2 = 4 x 10^8 Pa

Stress = F/A
4 x 10^8 Pa = F/(pi x 0.004^2)
F = 20106.19N

F = ma
20106.19N = m x 10
m = 2000kg (roughly)

 

[ABBAS38]

Ok got it but I thought the formula has yield stress in it and max stress not allowable and max allowable. Max allowable stress equals yield/FoS

 

[blyatman]

Max stress is 800 MPa. This is what it was designed to - it will fail if it exceeds this. So a FoS is added in to cap the allowable stress at 400 MPa.

Not sure why you're getting caught up on the yield stress.

 

[StudyOnly]

Keep this in mind:
You know the formula is stress(allowable) = stress(yield)/FoS for ductile materials

Now stress(yield) is the REAL point of failure, for safety engineers will add a FOS to make sure that the maximum load does not come near to stress(yield) because there is a risk of failure there. As a result they divide by a FOS thus decreasing the maximum (stress(allowable)) so that there is a safety net from failure. In the question it says "A factor of safety of 2 was set by...." which means that that "maximum allowable stress" of 800Mpa is the stress(yield) from the formula. This is why we divide it by 2 to get the stress(allowable).

This is an example of why you should understand what those formulas mean. Use this as a lesson to understand the formulas before the final exams.

I'm sorry if this explanation was crap/confusing but I am in a bit of a rush

 

[ABBAS38]

Nah this explanation is really good thanks heaps I get it now!

 

[StudyOnly]

Nah this explanation is really good thanks heaps I get it now!

No problem

 

[ABBAS38]

How is the answer B??

 

[ABBAS38]

No problem

Why is the answer B?? Thanks

 

[StudyOnly]

w = Fs
w = (90cos30 - 30) x 3
= 143.8kJ .......... B

w = Fs
where:
F = force applied in direction (parallel to) of movement
s = distance moved

 

[ABBAS38]

How is the answer C????

 

[ABBAS38]

w = Fs
w = (90cos30 - 30) x 3
= 143.8kJ .......... B

w = Fs
where:
F = force applied in direction (parallel to) of movement
s = distance moved

I have another question about the pulley I posted it just now

 

[ABBAS38]

How is the answer C????

Anyone

 

[StudyOnly]

How is the answer C????

load = 120 x 10 = 1200N
vr = 4 (cut number of ropes)
ma = load/effort = 1200/400 = 3

Efficiency = (ma/vr) x 100
= 3/4 x 100
75% ....... C

 

[ABBAS38]

load = 120 x 10 = 1200N
vr = 4 (cut number of ropes)
ma = load/effort = 1200/400 = 3

Efficiency = (ma/vr) x 100
= 3/4 x 100
75% ....... C

I get it but I don’t really get the velocity ratio part how is there 4 cut ropes and how do I just find them in general for a pulley system(VR)??