Physics final today!! Challenging problem! Edit: SOLVED!

[yhelothar]

I'm trying to solve this problem where it asks me to find the value of "g" on a planet.
It gives me the following.

1. I'm on a cliff that's 200m high.
2. I take out a pendulum that's .2m long, and it swings 35 times in 31.4s

How the heck do I figure this out if I don't know the angle it swings at? The answer would be greatly affected depending on the angle of the swing.

EDITx2: OK nm, I still can't figure out this one. Even with waves, I still need the angle of the swing to solve it.

 

[Suspicious-Teach8788]

ok screw you and your mechanics

i would do this any day over my quantum mechanics..

FVCK THE TIME INDEPENDENT SCHROEDINGER EQ.

 

[yhelothar]

Originally posted by: DLeRium
ok screw you and your mechanics

i would do this any day over my quantum mechanics..

FVCK THE TIME INDEPENDENT SCHROEDINGER EQ.

I have a final on that tomorrow
This physics stuff is more tricky IMO.

 

[MrColin]

Seems like you have to state some assumptions about the angle or the mass.

 

[yhelothar]

Originally posted by: MrColin
Seems like you have to state some assumptions about the angle or the mass.

Are you sure?
My prof. often makes mistakes on the sample exams.. but on the other hand, he often gives tricky questions too. So I'm not sure if it's a mistake or a tricky quesiton.

 

[yhelothar]

ANYONE?!?!?!
PLEEASSSEEEEE!!
I need the value of g for the rest of the questions on the test!!!!

 

[skim milk]

gee golly gosh I hate physics

 

[Vinfinite]

dude you shoulda so gotten a study group together...waited till now to look at your sample test?

 

[yhelothar]

Forget it.. I figured out the first one. I can solve it by using wave motion instead of energy.
angular acceleration = -A(2(pi)f)^2cos(2(pi)f)t
Frequency would be 35/31.4s = 1.11Hz
t = 0.9s

 

[yhelothar]

Originally posted by: Vinfinite
dude you shoulda so gotten a study group together...waited till now to look at your sample test?

should've could've would've..
I thought it was going to be fairly easy. Finals are generally pretty easy.. as professors usually put general questions on there. But this professor puts very tricky questions.

 

[Vinfinite]

Originally posted by: virtualgames0

Originally posted by: Vinfinite
dude you shoulda so gotten a study group together...waited till now to look at your sample test?

should've could've would've..
I thought it was going to be fairly easy. Finals are generally pretty easy.. as professors usually put general questions on there. But this professor puts very tricky questions.

generally true

good luck anyways

I personally hate physics

 

[kitkat22]

I betcha somewhere in all the equations the mass will cancel out for the second question. I'll see what I can do.

 

[Papagayo]

Always choose D. (All of the above)?

 

[sao123]

Originally posted by: cscpianoman
I betcha somewhere in all the equations the mass will cancel out for the second question. I'll see what I can do.

or maybe the answer should be in S / Kg
Like...

10 seconds per kilogram.

 

[sao123]

BTW... my physics final (way way back when) was a multiple choice final. That was so friggin awesome.

 

[yhelothar]

Originally posted by: cscpianoman
I betcha somewhere in all the equations the mass will cancel out for the second question. I'll see what I can do.

Thanks. I hope you're right, but I'm just not seeing it.
I try to solve time with the equation v²=at and I get t=v²/a, but then I don't have "a".
F = ma. I have force, but I don't have mass to solve for acceleration.

 

[yhelothar]

Anyways, forget number two..
In the next problem, it tells you that your mass is 165kg, so I'm just going to use that for the mass.

 

[blueshoe]

Originally posted by: virtualgames0
I'm trying to solve this problem where it asks me to find the value of "g" on a planet.
It gives me the following.

1. I'm on a cliff that's 200m high.
2. I take out a pendulum that's .2m long, and it swings 35 times in 31.4s

How the heck do I figure this out if I don't know the angle it swings at? The answer would be greatly affected depending on the angle of the swing.

EDITx2: OK nm, I still can't figure out this one. Even with waves, I still need the angle of the swing to solve it.

You can find "g", the acceleration due to gravity on the cliff using the T= 2pi*sqrt(g/L), where T is the period of the pendulum and L is the length of the pendulum. Solve for g.

T= 31.4/35, assuming that one "swing" is the time it takes for the pendulum to reach the position at which it started.

I get g= .004077 m/s^2 which seems kinda small...hmm

Well anyway, that gives you g on top of the cliff.

g=(GM)/(R+h)^2

where
G is the gravitational constant 6.67300 × 10^-11
M is the mass of the planet
R is the radius of the planet
h is the height above the surface of the planet

And because you aren't given the mass or the radius of the planet, I don't know where to go from there. But you do know it will be very slightly larger than the calculated g.

I had my physics final yesterday. And it sucked.

EDIT: I said the period wrong..it should the reciprocal of that
EDIT: fixed
EDIT: ok, as mentioned later on, my equation was wrong, should be sqrt(L/g), not sqrt(g/L)

 

[DaShen]

Originally posted by: virtualgames0
I'm trying to solve this problem where it asks me to find the value of "g" on a planet.
It gives me the following.

1. I'm on a cliff that's 200m high.
2. I take out a pendulum that's .2m long, and it swings 35 times in 31.4s

How the heck do I figure this out if I don't know the angle it swings at? The answer would be greatly affected depending on the angle of the swing.

EDITx2: OK nm, I still can't figure out this one. Even with waves, I still need the angle of the swing to solve it.

Actually, no it shouldn't. Because of gravity the swing time will be the same whether it is angled high or low. You are given enough information, but it has been about 3-4 years since I did that stuff so I am really rusty.

**EDIT**
Hey OP, why do you think clocks with a pendulum ticker are still so accurate? Because the swing time is derived from the length of the pendulum and the force of gravity, not the angle of the swing. Of course air resistance and friction do slow the pendulum down, the accuracy is still quite good.

 

[blueshoe]

Originally posted by: DaShen

Originally posted by: virtualgames0
I'm trying to solve this problem where it asks me to find the value of "g" on a planet.
It gives me the following.

1. I'm on a cliff that's 200m high.
2. I take out a pendulum that's .2m long, and it swings 35 times in 31.4s

How the heck do I figure this out if I don't know the angle it swings at? The answer would be greatly affected depending on the angle of the swing.

EDITx2: OK nm, I still can't figure out this one. Even with waves, I still need the angle of the swing to solve it.

Actually, no it shouldn't. Because of gravity the swing time will be the same whether it is angled high or low. You are given enough information, but it has been about 3-4 years since I did that stuff so I am really rusty.

Correct, angle of swing is not needed.

 

[DaShen]

Hey OP, I believe blueshoe answered your question in the post above my first post. Just use the acceleration due to gravity since that is the only force acting on the pendulum, if you factor out air resistance.

 

[Trikat]

I absolutely hate physics.
Physics 2102 I believe was insane and almost the whole class failed.
The 1st exam was F over F over F. Lucky for me I withdrew before the 1st exam knowing I didn't know crap.
A horrible teacher (The absolute worst I had) + hard physics = nearly impossible.

Goodluck on your final.

 

[DaShen]

Originally posted by: Trikat
I absolutely hate physics.
Physics 2102 I believe was insane and almost the whole class failed.
The 1st exam was F over F over F. Lucky for me I withdrew before the 1st exam knowing I didn't know crap.
A horrible teacher (The absolute worst I had) + hard physics = nearly impossible.

Goodluck on your final.

Mechanical Physics is actually quite easy. All you do is recognize the factors and plug them into known mechanical physics equations.

Electrical physics is a little more tricky, but the same rules apply. (I actually had a little trouble with electrical)

It is when you get to Quantum that things get all crazy. A lot of it has to deal with probabilities of things happening because in the quantum universe, things are never definite, and everything is possible in the quantum universe. (I didn't even bother taking anything on this, not in my major, but I think I would have enjoyed learning a little)

 

[yhelothar]

Originally posted by: blueshoe

Originally posted by: virtualgames0
I'm trying to solve this problem where it asks me to find the value of "g" on a planet.
It gives me the following.

1. I'm on a cliff that's 200m high.
2. I take out a pendulum that's .2m long, and it swings 35 times in 31.4s

How the heck do I figure this out if I don't know the angle it swings at? The answer would be greatly affected depending on the angle of the swing.

EDITx2: OK nm, I still can't figure out this one. Even with waves, I still need the angle of the swing to solve it.

You can find "g", the acceleration due to gravity on the cliff using the T= 2pi*sqrt(g/L), where T is the period of the pendulum and L is the length of the pendulum. Solve for g.

T= 31.4/35, assuming that one "swing" is the time it takes for the pendulum to reach the position at which it started.

I get g= .004077 m/s^2 which seems kinda small...hmm

Well anyway, that gives you g on top of the cliff.

g=(GM)/(R+h)^2

where
G is the gravitational constant 6.67300 × 10^-11
M is the mass of the planet
R is the radius of the planet
h is the height above the surface of the planet

And because you aren't given the mass or the radius of the planet, I don't know where to go from there. But you do know it will be very slightly larger than the calculated g.

I had my physics final yesterday. And it sucked.

EDIT: I said the period wrong..it should the reciprocal of that
EDIT: fixed

Thanks for the help, but that number does not make any sense.
There's no way a pendulum would make one full swing if gravity was such a small number.
I'm also confused about the definition of one swing. One swing would mean the pendulum travelling one direction. One period means the the pendulum returns to its original position, meaning it takes another direction back. So that would mean one period is two swings right?
Anyways, I googled the definition you provided, and I found this weird site that had a definition, and I tried to work it out, and I got a pretty reasonable number.
T=2Pi/w ; w=sqrt(g/L)
By using that, I got 6.35m/s²

 

[yhelothar]

Any ideas?