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Ummm i didnt know where to post this so if this is'nt allowed, then remove it.

I was wondering if there were any people that have some good physics experience. I am taking AP Physics B right now and i am completely stuck on one of my problems…

Here it is:

A car accelerates uniformly from rest and reaches a speed of 19.3 m/s in 13.2 seconds. The diameter of the tire is 69.8cm. Find the revolutions the tire makes during this motion, assuming no slipping. Answer in units of rev/s.

I approached this problem using a direct example from class, where I solved for the angular speed 'w', known as omega, by taking the average velocity (19.3/2) and dividing that by the radius (.349m) . That answer is in rad/s , so i then converted that into rev/s by dividing that by (2*pi). That answer put me at 8.801 rev/s. It was wrong…

I was wondering if anyone could help me out and see if I went wrong anywhere, or if I should use a completely different approach altogether…

BTW my homework is submitted online and I only have 5 more chances to answer it correctly…

Thanks in advance! :D

d = 1/2 (Vf+Vi) * t

d = 127.38 meters

Circumference = pi*diameter = 2.19 m

127.38/2.19 = 58.16 revolutions

58.16 rev / 13.2 sec = 4.4 rev/sec

That is, assuming the top formula is correct.

http://www.students.yorku.ca/~thejacob/acceleration.html

Looks like you had an extra 2 somewhere…

aha that is the first way i did it , but didnt divide so i thought it was way wrong…

thanks so much :D

EDIT: Meh it was wrong somehow…im completely positive the way you explained it is the correct way though…

ugh AP Physics is so gay sometimes, this could be an error in the answer system though

ANY OTHER THOUGHTS ON THE PROBLEM?

EDIT2:

AHHH im so stupid!!! the problem asked for the revolutions, not revs/sec BAH I CANT READ FFS

the answer was the 58.089

thanks tara again :)

Woah. Woah. Woah.


Awesome maths!

whoa!

im in AP physics and we are doing the SAME EXACT THINGS

we just learned about Revs and rotational kinematics.

how freaky…

Lol I'm in AP Physics also, we just learned about that stuff… weird. But today we made some really good Ice Cream using liquid nitrogen to actually make it… really cool!

From: R0b0t1
Woah. Woah. Woah.


Awesome maths!

hah totally knew that you would post for this thread :D

@legendary and v3l : are you guys seniors? Thats cool that we are learning the same stuff, i may be posting my questions more often here lol :p

Another AP Physics guy here :D

Were on centripetal force now. Just finished angular rotation.

I think theres a way to prove that there isn't centripetal force (Thats the once where its like… You spin the water and it doesn't fall out, Right?). The theory has to do with it being gravity or something like that.

hmm never heard about that, but how could it be true when you could design a space station with artificial gravity using the principles of centripetal forces?

All you have to do is get an very large cylindric station and you spin it at a certain velocity, and you could create artificial gravity.

Im in physics too this year lol, if you need any more help just call me

From: gerauchert
hmm never heard about that, but how could it be true when you could design a space station with artificial gravity using the principles of centripetal forces?

All you have to do is get an very large cylindric station and you spin it at a certain velocity, and you could create artificial gravity.

They are actually discussing what type of space ship should be made to travel to Mars. For centripetal forces to act, it doesn't have to be cylindric, it could be just a spinning rod and the living/work stations on one end or both.

The reason they want gravity in space is because it is one of the two requirements that needs to be solve for deep space travel. Gravity is required to stop the deterioration of the body muscles and health. Second thing required is a electro magnetic field around the space ship similar to the one around Earth but needs to be 10 times stronger. Now that sounds like a lot but the Earths magnetic barrier is actually very weak, its just really thick so it does the job. Why a magnetic field? Its to stop the high velocity particles and radioactive materials from entering the space ship.

And now I'm just babbling :sasmokin:

But yeah, try chemistry with all the crazy equations for predicting the outcome of mixtures and the amount of energy it creates. Good fun. ;)

My mole is bigger then your mole! (chemistry thing)

From: Narcle


But yeah, try chemistry with all the crazy equations for predicting the outcome of mixtures and the amount of energy it creates. Good fun. ;)

My mole is bigger then your mole! (chemistry thing)

Hah yeah i took AP Chem last year and it SUXED :(

that class kicked my ass so hard im not gonna lie, i had like at best a C+ for one of the quarters…still managed a 3 on the exam though :D

Lol and our classroom pass was a stuffed animal mole :p

Thank God I'm done with Physics..Although it wasn't too bad for me, we had online homework and I manged to get a teacher's solution manual (worked out problems, opposed to just the answers) and it really helped me to understand it all.

From: daforeigner
Thank God I'm done with Physics..Although it wasn't too bad for me, we had online homework and I manged to get a teacher's solution manual (worked out problems, opposed to just the answers) and it really helped me to understand it all.

For our online homework, we have a built in forum where our class can discuss the problems together. So once someone figures out how to do one of them they will post the equations and steps as they are the same problems with diff numbers. :p