dino's questions leading up to unit 3 exam, 2009.

[dino]

Hey guys,

Glad I've found this place. =]

With only 2 days until the exam, I am still having some difficulties that I can't shake off.

Mostly, these problems are to do with Gravitation and Structures.

With structures, I am having a lot of difficulty comprehending the equilibrium stuff, especially the "painter" type questions. σ-ε though is fine.

With gravitation, no matter how hard I try, I also eff it up nicely. Any tips for when to use the formulae and how and blah blah blah?

Heres a few questions I just got wrong from the 2007 tsfx exam and there arent explanations for. If someone could help, that'd be much appreciated:

[img]http://img.skitch.com/20090607-dg6r47yn1q4ba9wtk86np9jp3c.preview.jpg[/img]
This question made me go crazy before, I spent a good 10 minutes looking at it, trying to find an indication of an angle and such. Nada. In the end, I made up an assumed value so I could do the other 3 questions that were consequential.

[img]http://img.skitch.com/20090607-ny9anbpif6mk382qp2if7ufmwh.preview.jpg[/img]
I suspect I got the second one completely wrong because I got the first one wrong, but I dunno? I applied the v=sqrt(GM/r) formula to find the first question, and then v=2πr/T for the second. Wrong?

[img]http://img.skitch.com/20090607-1dm5cm2akwxk7jj7chfuhqa41n.preview.jpg[/img]
[img]http://img.skitch.com/20090607-gqjkgghskp12nw8pcrkr9d2dds.preview.jpg[/img]
And then we get to this question. I have absolutely no idea as to how to even start it. HELP!


That's about it for now, I'm sure I'll have some more after a few more practice exams.

I'll be very grateful if anyone can help me with just one of these questions. I'm stressing out here!

Ok, now time to go and do another exam. grr

[vamsiaus]

for q1 let
x=5m
a=-9.8
v=0
u=?
and use the formula v^2= U^2 +2ax and transpose to find the initial velocity "u"

[Flaming_Arrow]

 9) u=? d = 5 a= -10 v= 0

solve for u

[dino]

I knew it would be that easy. I just couldn't see the v=0 part. Massive let down.

[vamsiaus]

for q2 use the formula v= sqrt (GM/R) and for the radius add the radius of earth plus the altitude

[vamsiaus]

q15 transpose the formula of (r^3 / t^2) = GM / (4pi^2) and solve for T which is the period

[dino]

q14 answer on my calc is 7882.17 ms-1, whereas, by the answer book, it is 3.08x10^3 ms-1.
wtf? I've applied that formula.

[vamsiaus]

lol i jst did it with the calculator and i got exactly 3075.84. Maybe the correct use of brackets may help.
((6.67x10^-11) x(5.98x10^24)) / 42160000
and then sqrt the whole func

[dejan91]

Use radius = (35786X1000) + 6.37x10²⁴

You might've used wrong value for radius, as your larger answer suggests.

[IntoTheNewWorld]

q14 answer on my calc is 7882.17 ms-1, whereas, by the answer book, it is 3.08x10^3 ms-1.
wtf? I've applied that formula.

I got the same as the book, are you adding the radius of the earth to the distance above the earth? (which is given in kms btw, have to convert)

I swear the TI-30XB Multiview (green one) is the best calculator for these things

[dino]

q14 answer on my calc is 7882.17 ms-1, whereas, by the answer book, it is 3.08x10^3 ms-1.
wtf? I've applied that formula.

I got the same as the book, are you adding the radius of the earth to the distance above the earth? (which is given in kms btw, have to convert)

I swear the TI-30XB Multiview (green one) is the best calculator for these things

Boom. there we go. the km -> m. and I do spesh... =s

[arthurk]

would u happen to have a copy of this exam you could upload

[dino]

I'm really stuck on these structures questions. The painter ones. How the hell do I do them? this is my main problem at the moment, and its starting to get too late. shit shit shit. help?

[kamil9876]

The children on the beam question(I'm assuming this is what you refer to as "painter question"):

Q3:
Sorry, but are the two lengths in figure3 1.0 and 3.5? I will assume so:
TO calculate the net torque I took downwards forces as negative, upwards as positive. My x axis had it's origin at the point where the pole touches the vertical beam and it goes from left to right.

The net torque is:



Where is the Force acted by the wall.
Now this net torque simlifies to:


However the net torque must be zero if they are not rotating, hence:

30g-2.5F=0
30g=2.5F
F=12g N

The fact that F is positive indicates that the forces is upwards, since i chose up to be positive. So the force of the wall on the beam is positive, however the force of the beam on the wall is in the opposite direction(by Newton's third law) hence downwards, so B is the answer to q3 and u got the asnwer to 4

question 5:

Now in this case the you want the force the pole gives, that means x=0 cannot be at the pole, since 0*F=0.
Hence we will know make our origin at the wall:

Now the net torque is:

However the net torque must be zero. Hence:

 

and now solve for F.

question 6:

TO solve this question you have to look at how the x-value of the 40kg dude affects the net torque(which must be zero, hence and will vary)
Looking at our equation for that was done in question 4:


We see that as x increases(he moves more towards the pole(hence to the right(which is positive))) the Net torque would decrease if F stayed constant. However we require the net torque to be zero so in order for that to happen F must decrease. So we know decreases in magnitude.

Now to see what happens to , look at our equation from the previous question:


In here, if x increases, the net torque would decrease if F stayed constant. However we want it to get back to 0, and in order for the to happen F must increase, so we know increases, Hence the correct answer is C.

Edit: Fixed up the mistakes I made on question4 and 3 as stated in my latter post.

[arthurk]

Just adding on to what Kamil said
Answer to 3 was B
answer to 4 was 120N
Answer to 5 was 580N
answer to 6 was C

i chose A for question 3 as well but was shattered haha
maybe im misinterpreting the question or something but the rest of it was easy

Question 4:
300x1 + 2.5F = 400x1.5
2.5F= 300
F = 120N
Don't know how kamil got 6N but other than that her solutions are correct