Any Dynamics experts out there?

[Phillip Thorne]

So the wealth of knowledge here is amazing and seeing as how most of my classmates are probably a little intoxicated watching something called the super bowl, i'll throw this out here and see if anyone else knows what I'm doing wrong...

an elevator w/ a mass of 500 kg, has a counterweight of 150 kg and the motors lifting force is 5kN

what is the velocity of the elevator after 3 secs

so i have the tension from the cw and the tension from the motor pulling up w/ the weight of the elevator..sooo

F=ma

Tcw+Tm-We=mea


150kg*9.81m/s^2 + 5kN -500kg*9.81m/s^2=500kg*a

a=3.133m/s^2

from constant acceleration...

V=Vi + at
V=0 + 3.133m/s^2*3s
V=9.4m/s but I'm supposed to have 7.32m/s

anyone have any ideas??

Thanks!

[Phillip Thorne]

ok...so just for those who were really wanting to know the answer...i figured it out!

you have to include the mass from the CW too

150kg*9.81m/s^2 + 5kN -500kg*9.81m/s^2=650kg*a (not 500)

a=2.41m/s^2

V=0 + 2.41m/s^2*3s
V=7.23m/s^2

now i'm off to solve the last two problems....

~Phillip

[Dustin Nickerson]

I have no clue but here is what I found.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

[Carson Saunders]

This is exactly why I decided that engineering wasn't for me. After completely bombing dynamics, I switched majors. Congrats on figuring it out! You are a smarter man than I!

[Ron Wessels]

Phillip, you are correct that the mass in the acceleration term must be 650kg, not 500kg. You probably got confused with the description of the motor's "lifting force". A motor doesn't have a "lifting force", since the motor has no idea if it is "lifting", "dragging", or whatever. In the system you describe, the motor is applying force to accelerate the total inertial mass of all the things it is moving. The mass that is accelerating is both the elevator and the counterweight.

I take it this is some sort of study problem? In real life, you have to take into account the (very non-trivial) mass of the attachment cables as well. Additionally, a motor would not specify "force". It would specify "torque", which is translated into force by the specifics of the pulley system employed.

[ Oh, and Carson, I decided that engineering _was_ for me. :) ]

[Russ Filtz]

PLus friction in the pulleys/sildes, air resistance of the elevator in the shaft, etc, etc. Most times these minor "losses" can be ignored and a factor of safety employed, or just round the motor/pulley up to the next size to be safe!

Most engineers, unless you work for NASA or other demanding precision industry, don't need to be that accurate and a lot of the times the calculations have already been done for you when you follow the proper codes!

[Phillip Thorne]

the last sentence in the problem stated to ignore the mass of the cable and of the pullies, i get slightly frustrated with problems like this b/c they do not include all the variables I will have to consider in "the real world"

[Ron Wessels]

By the way Phillip, you can look at the problem a slightly different way that might make things a lot more intuitive for you.

Perform a simple force balance.

For the elevator:
To accelerate the elevator up with acceleration a, the force required is
Fe = Me * (g + a)

For the counterweight:
Since the counterweight is accelerating downwards with gravity, the net force that the counterweight provides is
Fc = Mc * (g - a)

And, of course, the motor provides a force of Fm.

So, balancing forces:
Fe = Fm + Fc
Me * (g + a) = Fm + Mc * (g - a)
(Me + Mc) * a = Fm - (Me - Mc) * g
a = (Fm - (Me - Mc) * g) / (Me + Mc)

solving

a = (5000 - (500 - 150) * 9.81) / (500 + 150) = 2.41 m/s/s

Then, as before,

v = v0 + at = 2.41 * 3 = 7.23 m/s

I've omitted units except for the final answer for clarity.

[Phillip Thorne]

that's the great thing about mathematics..there are soo many different ways in which to solve problems! Thanks for the alternative method! I might have to use it sometime. How good are ya w/ thermodynamics? Entropy and enthalpy compeltly confuse me, the idea of measuring something I cannot see or touch baffles my mind.

[Ron Wessels]

It's been a while. But measuring something that cannot be seen or touched happens all the time. How about wind speed? Can't see or touch wind. Temperature? What about the caloric content of food like ice cream being more fattening than celery?

Have a look at the Wikipedia article Introduction to entropy (http://en.wikipedia.org/wiki/Introduction_to_entropy). There's also the Wikipedia Enthalpy (http://en.wikipedia.org/wiki/Enthalpy) article.

[Russ Filtz]

If I remember correctly, entropy is more of a measure of "dis-orderliness" and enthalpy can be thought of as more of a capacitance measure of heat, like an electrical capacitor, how much heat can be stored.

enthalpy - cast iron skillets hold heat better than aluminum

entropy - things like molecules, air, etc. get more "active" and spread apart the more energy/heat you apply.

[Phillip Thorne]

It's been a while. But measuring something that cannot be seen or touched happens all the time. How about wind speed? Can't see or touch wind. Temperature? What about the caloric content of food like ice cream being more fattening than celery?


I can feel the wind, sense a temperature change, you can burn food to "see" it's caloric content. It's hard to visually test entropy and enthalpy, atleast hard enough that the professors here just tell us to understand how to calculate it, but to not worry about trying to "see" it.

We had our first test yesterday, the instructor used old test problems on the test. Gotta love him making old test material available to study with.

Part of my struggle with thermo is that it's a bunch of concepts that have not been used in any of our classes yet.

Interesting example I'll have to dig up about the efficiency of the engines in our car, I'll post it later as I must get ready to head to work, then to 6 hours comprised of engineering econ, dynamics and electrical circuits.

Good day to all!

[Bryan Somers]

There was once upon a time in a land far far away I would have been able to show you real quick how to solve that problem but like I said that was years ago. glad you solved it though. As far as thermodynamics go I still love a more loose defininition of the second law Thats crap happens and it gets worse