# Engineering Marathon (1 Viewer)

#### Kaos1

##### Member
my question is how does a commutator work?
a commutator is a part of an electric engine, and acts as a conductor for the bruches and the magnets... as to exactly how it goes about doing that (and for that matter, any additional information about a commutator) i do not know

#### arman

##### Member
hey arman wat nash r u btw, r u persian?? ur name sounds like it
na man, i'm aussie (european background) 'arm' are my initials

a commutator is a part of an electric engine, and acts as a conductor for the bruches and the magnets... as to exactly how it goes about doing that (and for that matter, any additional information about a commutator) i do not know
there are two types of commutator

split ring and slip ring

a split ring commutator is connected to an external DC circuit. as the motor rotates, the current passes through the coil of the rotor (or armature) and as the coil becomes vertical, the current stops (due to the gap in the 'split ring') the coil continues to turn due to inertia and when the brushes touch again, current is reversed in the coil and so the coil/rotor continues to rotate (much easier to explain with a diagram and with a good understanding of physics and magnetic fields)

a slip ring commutator maintains electrical contact with the brushes at all times. this is connected to an AC circuit and so the current will alternate through the ccoil every half turn (as long as the motor is roating at the same frequency as the supply AC, ie 50Hz, 100 rpm [changes each half-turn])

hope that helps.. probs just confuses you lol

#### arman

##### Member
o h and heres a question to keep us going

which of the following is a semiconductor:
a) capicitor
b) NPN transistor
c) potentiometer
d) resistor

#### yz125

##### Member
o h and heres a question to keep us going

which of the following is a semiconductor:
a) capicitor
b) NPN transistor
c) potentiometer
d) resistor
answer = b (i only know this due to physics)

Question: Explain the special properties produced by composite materials

#### arman

##### Member
answer = b (i only know this due to physics)
agreed

Question: Explain the special properties produced by composite materials
Lightweight
High strength to weight ratio
chemically inert

thats more of an outline, can anyone explain?

Question: Compare the different signals for black and white tv and for colour tv

btw yz125 hey? that what u ride, i was on a kx100, now got a ktm250 - was going to take it away for schoolies but can't fford it now

#### CMCDragonkai

##### New Member

Question: In the following diagram; determine force and nature of force in member AB (HINT: determine reactions first)

I managed to get AB as 16.28822036 kN tension.

A brief description of how I did it.

I got the angle under the bridge 12.52880771 (the line from RB to B and the angle between that to horizontal). Using this angle, I then got the vertical distance from the horizontal to B by using trig with the 3m that is given. This distance was 2/3 m.

Now having this distance, I could get the hypotenuse which is RB to B and also the distance of the vertical bar from B. RB to B is square root of (85/9). The distance of the vertical bar from B is 2 and 1/3 m. (3m subtract 2/3 m).

Now cutting it and looking at the RHS, I summed it at B (canceling AB and the hypotenuse I was talking about), and got the top bar's force. I got this as 87 1/7 kN in tension. (This will be useful later on)

Now I look at the LHS. Using the hypotenuse distance square root of (87/9) and also the 3m given I get the area of the triangle RB - A - B. I do this by using 1/2*a*b*sinC. C is worked out by 90 degrees minus the angle under the bridge. (90 - 12.52880771 = 77.47119229). (I get area as 4.5m^2)

Now this area has to equal the other area equation, 1/2*base*height. The height is always perpendicular, so this is what I want, as it is the perpendicular distance from AB to RB. So I need to base, I calculate the base by using a^2 = b^2 + c^2 - 2bc*cos angle. The a is the base, and the rest are form the triangle. I calculate the base as 3.80058475m. Thus height is 2.368056652m. (This is the perpendicular distance between the line AB and the point RB).

Now I sum it at RB, since I sum at RB, the horizontal component of RA is canceled and the force on RB-B is also canceled. This leaves the vertical component of RA (100 kn), AB and the top bar (87 and 1/7 kn in tension).

Thus, 0 = (-100) * 3 + (87 1/7) * 3 + AB * 2.368056652.

AB = 16.28822036kN tension.

I have no idea if this is right. This process is so convoluted that there is a high chance of error throughout the process. What do you guys think?

There must be a better and simpler way than this.

#### yz125

##### Member
agreed

Lightweight
High strength to weight ratio
chemically inert

thats more of an outline, can anyone explain?

Question: Compare the different signals for black and white tv and for colour tv

btw yz125 hey? that what u ride, i was on a kx100, now got a ktm250 - was going to take it away for schoolies but can't fford it now
Yea i ride a yz125. Although I havn't riden it in about 4 months. Nice, ktm 250, how often and whereabouts do u ride?

And in answer to the 'explain' part I dunno either lol thats why i asked it. All i got is that composites consist of 2 or more materials mixed together to take advantage of each materials best properties (is that right?).

Last edited:

#### Kaos1

##### Member
as far as the exam goes, we will not be asked a question as complex as this, simply because we do not have the time to figure out all of the information. we will get a simple trussed bridge or cantilevered sign, and told to figure out the reactions at the attachment points in one question, and then a force and nature of one of the members. the max amount of marks of this type of question would be 4 or 5, depending on the question.

as long as you know what to do, and you get a relatively correct answere, then all should be good...

Semiconductor? isnt that a ceramic resistor or something? i got no idea what a transistor is

yes. a composite material is a material that combines the properties of 2 different materials, to make a stronger, lightweight, chemically inert material... to an extent

reinforced concrete is a composite, because it combines the concretes compressive strength with the steels tension strenght...

carbon fiber is a composite, because it combines the carbon fibers tension strength (aswell as flexibility, impact resistance, etc) with the resigns's compressive strength (and flexibility, impact resestance, ease of construction, etc, etc.)

BUT!! an aluminium alloy is NOT a composite. it is a new material. this is because it does not combine the properties of the material, it combines the properties of the chemials. slight difference

if you dont quite understand that, i can explain it in a more simplistic way if necessary

#### arman

##### Member
CMCDragonkai thanks for the working

Semiconductor? isnt that a ceramic resistor or something? i got no idea what a transistor is
a semiconductor is made of semiconducting material (eg: silicon is used mostly today) it has less conductivity than a metal but more than a non-conductor
semiconductors are used for transistors, diodes, microchips, etc

a transistor is the electronic component that is the fundemantals to todays computers - it basically takes in one input and can give out 1 of two values (0 or 1), making it useful for switching with varying voltages

#### Kaos1

##### Member
aaaah k then, i get it. thanks for that clrification

#### arman

##### Member
Yea i ride a yz125. Although I havn't riden it in about 4 months. Nice, ktm 250, how often and whereabouts do u ride?
yeah I havn't been for a proper ride in ages (like since january). hey just re-did clutch in the KX last holidays and found a reciept for a part I got when we first pulled her down and it was dated in April ! lol

I've taken the KTM for a run around the paddock for like an hour, but I need to do up the forks before I take it out (just bought it and found out forks are leaky and soft) , but when i do go riding, i go up state forest near mine on mid-north coast near taree - was going to mudgee (louee) 4 schoolies but now i have to spend money on my bike, i cant afford it

Anyways, for a Question

Define and explain crack theory

#### bossleymaths

##### Member
crack theory is essentially reducing the chances of propagation of dislocation. it is where a crack appears and it continues to propagate throughout the whole material causing dislocations within the internal structure.

QUESTION

suggest how integrated systems in a hybrid vehicle may achieve improved energy efficiency compared with conventional petrol vehicles. (who ever answers this correct is a legend, i am having difficulty)

#### Kaos1

##### Member
suggest how integrated systems in a hybrid vehicle may achieve improved energy efficiency compared with conventional petrol vehicles

key word here is 'suggest' ... which means its up 4 interpritation, and basiically anythin can be the right answere, as long as you put up a good arguement, and have proof...

hybrid vehicles can achieve improved efficiency by simply incoroprating the key advantages of different forms of power (eg, prtrol, electric, hydraulic, etc). this concept is similar to that of composite materials, in that it incorporates the properties of two different materials, btu the main difference is that this is to do with propulsion, not strength in materials etc. when compared to conventional vehicles, hybrid vehicles use less fuel, are safer, and more reliable then reciprical engines, because there is no chance of a breakdown on the side of the road.

overall, hybrid vehiclesare much more efficient then convention petrol engines, because they utilise the properties and advantages of various different sources or propulsion.

give reasons as to why a block and tackle lifting system is more effective then an inclined plane

#### mahdi

##### Member
I got RB = 130Kn and RA = 100Kn

Then i cut through AB and the members above and below it.
Then i took moments about the top right hand corner (this eliminates the two forces from the sectioned members).

Then using trig i found the perpendicular distance from my point to AB.
I got that it was 33.88KN in tension.

Question: Describe TWO ways of preventing Concrete Cancer (spalling)

Last edited:

#### Bdogz

##### Member
I got RB = 130Kn and RA = 100Kn

Then i cut through AB and the members above and below it.
Then i took moments about the top right hand corner (this eliminates the two forces from the sectioned members).

Then using trig i found the perpendicular distance from my point to AB.
I got that it was 33.88KN in tension.

Question: Describe TWO ways of preventing Concrete Cancer (spalling)
Finally someone got what i got

Way 1: Galvinising the steel with zinc to act as a sacrifical anode

Way 2: Vibrate the concrete into position to reduce porosity

Q: Compare and contrast how thrust is produced in the Turbojet and Turboprop engine

Last edited:

#### tashisthebest

##### Member
Q: Compare and contrast how thrust is produced in the Turbojet and Turboprop engine[/quote]

Turbojet : a series of fans act like a compressor drawing the air in. the air is then compressed and thus it heats up, it is then mixed with fuel n then ignited. this air then expands rapidly. this expanding air, follows the law of conservation of momentum and newtons third law. as the air expels out of the nozzle it forces the plane to move forward. the speed of the aircraft is proportional to the speed the gasses are existing the nozzle.

Turboprop : A large fan, shaped as an airfoil (following Bernoulli principle). A set of compressors pressurizes and heat the air, then mixed with fuel n then ignited. this then expands pass the turbine that is attached to the propeller. the the trust is generated through the movement of the propeller through the air.

Ouestion : Name two types of corrosion and how they occur, giving examples?

#### CMCDragonkai

##### New Member
crack theory is essentially reducing the chances of propagation of dislocation. it is where a crack appears and it continues to propagate throughout the whole material causing dislocations within the internal structure.

QUESTION

suggest how integrated systems in a hybrid vehicle may achieve improved energy efficiency compared with conventional petrol vehicles. (who ever answers this correct is a legend, i am having difficulty)
The picture of the question really helps.

Regenerative Braking...
Transmitting energy to the efficient electric motor rather than the inefficient gearbox drive train.
Using the electric motor at slow speeds which is more efficient than a petrol motor and using a petrol motor when driving at high speed. This means you're not overkilling when driving at slow speeds or underkilling when you're driving at high speeds.

#### tommykins

##### i am number -e^i*pi
I'll name RA and RB C and D respectively.
There is a reaction Cx, Cy and Dy.

Fx = Cx+10 = 0
Cx = -10 kN

Fy = Dy+Cy - 30 = 0
Dy+Cy = 30

Mc = 3Dy - 30*12 - 3*10 = 0 -> Dy = 130 kN
Thus, Cy = -100 kN

Using method of sections (cbf having the diagram here) -

The angle RB is atan(3/14) = 12.1*

The height of the triangle RB-B is 3tan(12.1) = 9/14
Thus the triangle AB has a height 3-(9/14) = 33/14

angle at AB is arctan((33/14)/3)) =38.16*

Now analysing the section -

Fy = -ABsin(38.16)+RBBsin(12.1) =-30
Taking the moment at A -
Ma = -(-100)*3+(-10)*3+RBBcos(12.1)*3 = 0

so RBB = -92.045 kN

Solving above
AB = (RBBsin(12.1)+30)/(sin38.16) = 17.327 kN so AB is in tension

Last edited:

#### mR sinister

##### Member
crack theory is essentially reducing the chances of propagation of dislocation. it is where a crack appears and it continues to propagate throughout the whole material causing dislocations within the internal structure.

QUESTION

suggest how integrated systems in a hybrid vehicle may achieve improved energy efficiency compared with conventional petrol vehicles. (who ever answers this correct is a legend, i am having difficulty)
I would talk about hybrid vehicles.. eg electric
and how an AC induction motor can be used to improve energy efficiency

btw, this question is really vague, as its asking for 'improved energy efficiency'...
in reality electric, hydro Solar etc.. at this moment, can now where near provide enough energy as a petrol based.

i think its just basically asking you to outline: cost, practicality, and pollution

#### 00iCon

##### Member
Ouestion : Name two types of corrosion and how they occur, giving examples?
Stress corrosion cracking: occurs when a material is subject to tensile stress and a corrosive environment where either condition may not have caused a crack, both conditions together can.
Exfoliation corrosion: occurs when an electrolytic solution causes grain boundaries to react with the grains. this often happens where end grian is exposed. Grain layers then begin to peel off as an advanced form, and this is exfoliation corrosion.