HSC 2015 mechanics discussion (1 Viewer)

[coolman]

Hey guys, making a thread to see what people got for the mechanic question in the test.
The .85 efficiency one I got 153N
Youngs modulus I think I made a mistake in my values not my method, got 343 gpa
Slope one I got around 1042N
The piston force I got was 350N and how much movement I got 8.57
The truss analysis, I had no idea. I wrote the roller joint was zero reaction because all the forces acted perpinducular to it (roller can only act up). Got 1950 for Rb

Lift drag ratio I got was 5.67, forgot to put the : 1

 

[keepLooking]

202 GPA.
Slope, I fucked up.
Piston Force, I got 350.
Movement I didn't finish ;(
Got the same Lift/Drag ratio as you and I didn't put : 1 either.
I got 1950 for Rb too.
For 0.85 efficiency, I got around 300.


For the roller support, it acts horizontally in this case. It acts where it is facing haha.

 

[coolman]

You put 0 aswell for the roller joint?

Howd you get 300 for efficiency Btw? L/E = Distance effort/distance load x .85. Distance load was 50 (i used radius of the winch) and distance effort 350, gives you vr of 7, so 7 x .85 = 910/F

 

[keepLooking]

You put 0 aswell for the roller joint?

Howd you get 300 for efficiency Btw? L/E = Distance effort/distance load x .85. Distance load was 50 (i used radius of the winch) and distance effort 350, gives you vr of 7, so 7 x .85 = 910/F

No. The roller joint is 3540. You mean the first part of the question right?

 

[coolman]

Yeah, if it acted horizontally, it would be zero because all of the forces were vertical

 

[D94]

Do you have the paper? I could answer some questions.

 

[keepLooking]

This? If so, that is definitely not 0.

Do you have the paper? I could answer some questions.

Apparently BOSTES is enforcing that no students will be allowed to take out even question booklets. At our school, they were pretty anal about it.

 

[coolman]

Do you have the paper? I could answer some questions.

Nope, but I do remember some

Piston A has area 100mm sq and B has 700mm sq, if piston experience force 50N how much does piston B experience, the 2nd part was if piston moves 60mm, how much does piston b move up.

The slope one was a 150kg block pushed up by winch at at angle of 25 degrees, find the tension in the wire

2nd part of it was efficiency, the tension is 910N, diamter of the winch machine is 100, and the pedal length is 350. Efficiency is .85, what is the minimum force required to move it up

The truss one was against a wall, all forces acted downwards, i wrote at the roller joint it was zero reaction because its reaction acted horizontal but there is no horiztonal forces

 

[coolman]

Yeah if roller acts horizontal and all the forces are vertical, wouldnt it be zero?

 

[keepLooking]

Yeah if roller acts horizontal and all the forces are vertical, wouldnt it be zero?

Nope.

 

[coolman]

Nope.

Fx = 0

But Reaction at A is the only force acting in the x plane, doesnt that mean RA is zero?

EDIT unless you consider reaction at B, but that would mean it would be acting downwards?

 

[keepLooking]

Fx = 0

But Reaction at A is the only force acting in the x plane, doesnt that mean RA is zero?

The question was like

this right?

I'll post my solutions in a few minutes (brb)


Let the pin joint be B.
(Clockwise +)
Mb = 0
Mb = -1.5(1200) - 4.5(750) + 1.5A
A = 3450

 

[coolman]

The question was like

this right?

I'll post my solutions in a few minutes (brb)



Let the pin joint be B.
(Clockwise +)
Mb = 0
Mb = -1.5(1200) - 4.5(750) + 1.5A
A = 3450

Yeah I know what you mean, I did that aswell, but I considered sum of Fx=0, howd you get Rb, sum of y forces?

EDIT: I think the other x plane force is The horizontal component of Rb, Rb should have been root(1950 sq + 3450 sq)

 

[keepLooking]

Yeah, I got what you mentioned for the second part which was finding the pin joint.

For the ROLLER joint, it was 3450, I'm pretty sure. I did it by taking moments around the PIN joint.

If you meant finding the member AC, I don't remember that one.

 

[D94]

The question was like

what are the distances?

 

[720YoloSwagScope]

what are the distances?

1.5m between the two joints and each section of the beam thingy

 

[D94]

The question was like

Assuming this diagram is correct, and let the roller joint be A, and pin joint be B, then:

Taking moments about the pin joint,

Sum M = 0 (take anti-clockwise as pos, and Bx pos in the right direction)
Bx*(1.5) - 750*(4.5) - 1200*(3) = 0
So Bx = 4650 N

Ay (up) is obviously 1950N
Ax = 4950N (in opp direction to Bx)

and By = 0 since roller joint.

Standard x-y Cartesian plane.

 

[D94]

Nope, but I do remember some

Piston A has area 100mm sq and B has 700mm sq, if piston experience force 50N how much does piston B experience, the 2nd part was if piston moves 60mm, how much does piston b move up.

Fb = 350N
X = 8.57mm (opp direction to A)

The slope one was a 150kg block pushed up by winch at at angle of 25 degrees, find the tension in the wire

2nd part of it was efficiency, the tension is 910N, diamter of the winch machine is 100, and the pedal length is 350. Efficiency is .85, what is the minimum force required to move it up

any coefficient of friction? winches pull, not push, was there a diagram?

 

[synx]

Hey guys, if anyone remembered how to correctly draw their sectioned drawing, can u just pic here? thanks

 

[vflaash]

For the youngs module question I think it's a trick question as it asked for GPa and KN/mm^2 is MPa and I got 202MPa so I put in 0.202 GPa

Forgot to put :1 do you think we. Will lose a mark?