View Full Version : Moment question. - Physics
lolwatever
12-03-2006, 06:58 AM
:sl:
doin sum rev... this question i completely got no idea how 2go about it... ne help would b appreciated :)
The spring loaded follower
A bears against the circular portion of the cam until the lobe of the cam lifts the plunger. The force requried to lift the plunger is proportional to its vertical movement
h from its lowest position. For design purposes determine the angle
theta for which the moment of the contact force on the cam about the bearing
O is a maximum.
In the enlarged view of the contact, neglect the small distance between the actual point
B and the end
C of the lobe.
i think my problem is trying to model the motion of the lobe... after that it's just a matter of solving for the maxima of the function.
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Ameeratul Layl
12-04-2006, 12:36 PM
salam,
Have you managed to solve it? I'm not sure where to begin. Are we meant to consider Trig?
wasalam
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lolwatever
12-05-2006, 07:15 AM
format_quote Originally Posted by
Ameeratul Layl
salam,
Have you managed to solve it? I'm not sure where to begin. Are we meant to consider Trig?
wasalam
salams sis,
yeh in almost all the questions trig is involved.... Here's an idea how to go about it, (my problem is i don tknow how to transfer this idea into numbers)
1. Determine equation of motion fo the lobe.
2. Relate that motion to Force.
3. We know that Moment is maximum when the force is 90 degrees to the line between the force and moment center.
4. Come up with equation "M= Fd" (where F is an equation of force)... and find the maximum value of the function.
im sure das how u suppose 2go about it.. my problem is part 1 and 2.
jazaks :D :w:
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Ameeratul Layl
12-05-2006, 10:34 AM
salam,
I find it too confusing.
wasalam
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lolwatever
12-08-2006, 10:02 AM
:sl:
so do i :cry:
:w:
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Malaikah
12-08-2006, 10:09 AM
:sl:
It's moments like this I'm so glad that I never ended up doing eng. :D
LOL I just realised my above comment had a double meaning. ;D Either way, that question looks evil. :uuh:
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lolwatever
12-08-2006, 10:15 AM
format_quote Originally Posted by
Malaikah
:sl:
It's moments like this I'm so glad that I never ended up doing eng. :D
assuming meaning 1: did these questions make u cry? lol
meaning 2: u got questions like tht in ib/phys A :?
LOL I just realised my above comment had a double meaning. ;D Either way, that question looks evil. :uuh:
it's actually the last question in the chapter xercsies.. so i guess dere's some sort of twist to it :heated:
neway atleast im sure i wont be gettin ne xam question like this :D
:w:
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Malaikah
12-08-2006, 10:17 AM
:sl:
No, we didnt have anything that evil looking for moments, but we had some pretty evil questions though...
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lolwatever
12-08-2006, 11:03 AM
:sl:
lol ok i think this is ittt...
Let l = 80 mm be the distance OC, and r = 40 mm be the cam radius.
The force F imposed by the follower A on the cam at B is vertical. The moment arm from O to the line of action of F is:
d = l cos Q
(where Q is the angle theta, we can ignore the small horizontal distance from B to C).
The compression of the spring is
h = l sin Q - r
so that the spring force is
F = k h = k l (sin Q - r).
Sooo... the moment of F about O is
M = k l cos Q (l sin Q - r) = k l r cos Q (2 sin Q - 1)
since:
l / r = 2.
This will have a stationary value when
dM/dQ = 0
i.e....
k l r [-sin Q (2 sin Q - 1) + cos Q (2 cos Q)] = 0.
That is,
4 sin^2 Q - sin Q - 2 = 0
==>
sin Q = (1/8)[1 +/- sqrt(1 + 32)]
= 0.843 or -0.593.
Hence Q = 57.5 deg.
aha! :D
:w:
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Ameeratul Layl
12-08-2006, 12:41 PM
salam,
oh bravo! Even if you've got it wrong ATLEAST you tried. :)
I think that is a A grade type question.
wasalam
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MusLiM 4 LiFe
12-08-2006, 11:06 PM
momentz *faints* had a end of unit ting yh WHOA woz dat hard or wha.. :-\ :offended: i bet i do reali cr*p on it!
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lolwatever
12-08-2006, 11:53 PM
format_quote Originally Posted by
Ameeratul Layl
salam,
oh bravo! Even if you've got it wrong ATLEAST you tried. :)
I think that is a A grade type question.
wasalam
:sl:
jzks sis :D im pretty sure its correct inshalah.. coz its the same answer as the top corner of that pic i posted.
the hard bit was the first 2 equations... coz i kinda was confused about the trig behaviour of the motion...
:w:
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lolwatever
12-08-2006, 11:54 PM
format_quote Originally Posted by
MusLiM 4 LiFe
momentz *faints* had a end of unit ting yh WHOA woz dat hard or wha.. :-\ :offended: i bet i do reali cr*p on it!
post ur questions here inshalah sis, moments is like every second thing an engineer thinks of :p
:w:
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MusLiM 4 LiFe
12-09-2006, 07:20 PM
lol im dun doin it.. im soo happy.. now il hav 2 wait 4 da exam :offended: n i bet i get non of it corect :p
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Ameeratul Layl
12-12-2006, 12:45 PM
i have a mock for Physics on thursday. pleasssssssse make dua.
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lolwatever
12-12-2006, 06:34 PM
format_quote Originally Posted by
Ameeratul Layl
i have a mock for Physics on thursday. pleasssssssse make dua.
:sl:
all the best with it sis! ameen.
:w:
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