Foo - Physics :D

Hello guys! I am stuck(once again)...

I have been trying the below problem for some time now and I cant seem to make any progress...

If anyone can help--I'd greatly appreciate it!!

Question: a) Prove that when an object starts from rest and rotates aboutr a fixed axis with constant angular acceleration, the radial acceleration of a point in the object is directly proportional to its angular displacement.
b) Through what angle has the object turned at the instant when the resultant acceleratoin of a point makes an angle of 36.9* with the radial direction
Answer: a)proove, b) .666radians


This is what I tried so far...

a) Az=dWz/dt
theta-thetao=1/2(Wz+Wzo)T
theta-0=1/2(Wz+0)T
theta=1/2(Wz)T
Wz=2theta/T

Arad=W^2R
Arad=(4theta^2R)/T^2

Arad/4theta^2=R/T^2

directly proportional to me ;D
but, trying to utilize this in B...

b)No idea...I guess the angle corresponds to the other angle which it should be identical as 36.9* and when you multiply 36.9*(pi/180) = .654rad

Close but that is as far as I could get

Below is a diagram that I drew to help visualize what is going on

Thanks guys!!!

Anyone ? :"D

Thanks !!!

I wish i had the time. Its usually extremely easy to find the solution if you know the answer

Sorry for a), I used Arad/4theta^2=R/T^2
Arad/theta^2=4R/T^2

"b) .666radians"

That's the problem...

1/3 of a radian?

1/3 pi ? don't mind me.

"b) .666radians"

That's the problem...



??? lol :D

I did a google search on physics problem "angular displacement" and came up with:

http://www.sparknotes.com/physics/rotationalmotion/rotationalkinetics/problems2.rhtml

Problem 2.1:
A jet engine, starting from rest, is accelerated at a rate of 5 rad/s2. After 15 seconds, what is the angular velocity of the engine? What is the total angular displacement over this period of time?



We are able to solve this problem using our basic kinematic equations. Firstly, the final angular velocity is calculated through the equation:


ωf = ωo + αt

Since ωo = 0, α = 5 and t = 15,


ωf = 0 + 5(15) = 75 rad/s

The second quantity we are asked for is the total angular displacement:



φ - φo = ωot + (1/2)αt2
= 0(15) + (1/2)(5)(152) = 563 rad

In part b), you need to be careful when you find the total acceleration. It is easiest to write it as a vector which is the sum of two components, a radial part and a tangential part. You can think about the radial part as being due to the instantaneous angular velocity; the extra tangential part is present because there is angular acceleration.

When you know the acceleration vector you can do some elementary trigonometry to figure out the angles.

Good luck.

^^^ was thinking the same thing, it would be difficult to solve for the accelation not in radial and tangential directions. Will try and work it out but your little attachment is not working.

eww.. just get away from this crap while ya can. all they do is make it harder and harder and more confusing and by the time they're done with you all you will be is this husk full of equations and numbers and you won't understand a damn thing about anything... it's evil!!

run!