1. May 1, 2010

### pat666

1. The problem statement, all variables and given/known data
4. An electric turntable 0.750 m in diameter is rotating about a fixed axis with an initial angular velocity of 0.250 rev/s and a constant angular acceleration of 0.900 rev/s².
a) Compute the angular velocity of the turntable after 0.200s my answer:1.35rev/s
b) Through how many revolutions has the turntable spun in this time interval? my answer:0.01
c) What is the tangential speed of a point on the rim of the turntable at t = 0.200 s? my answer: 0.51
d) What is the magnitude of the resultant acceleration of a point on the rim at t = 0.200 s?
my answer: 6.9 rev/s^s really unsure of this one.
2. Relevant equations

3. The attempt at a solution
for d:
I found the angular acceleration (alpha) to be 5.5 then using the relationship alpha =a/r to find the translational acceleration. From there i used Pythagoras' theorem to solve for the resultant. I know ive been asking for a lot of checking of my work lately, just because i am not great with the topics were doing this term. I do always have a good attempt at the problem before asking for help. thanks in advance.

2. May 1, 2010

### Squeezebox

Angular acceleration is constant. How did you end up with the 5.5 value?

3. May 1, 2010

### pat666

There's one problem i think that i found the angular velocity at 0.2s, do you know if the rest is right?

4. May 1, 2010

### pat666

sorry i got it by doing alpha=(w0-w)t, why is that wrong or is my tangential acceleration meant to be 0.9?

5. May 2, 2010

### Squeezebox

B is right, the rest are wrong.

6. May 2, 2010

### pat666

Last edited by a moderator: Apr 25, 2017
7. May 2, 2010

### Squeezebox

I don't think you're calculating them correctly because they agree with the values I got which do not agree with your values.

8. May 2, 2010

### pat666

Im an idiot is the problem, my physics is correct i think but my maths is not, is your answer for a) 0.43rev/s?, for c) 0.16125m/s and could you give me a hand for d please. thanks

9. May 2, 2010

### ehild

pat666, always fill in the template and show your full work. It is easier to check what you did wrong and what is only computational mistake.
What is the equation you used to get the angular acceleration in rad/s^2 from 0.900 rev/s^2?

Just a hint: Trust in yourself and in your Physics textbook and teacher. Do not believe anything that you find on the Net.

ehild

Last edited: May 2, 2010
10. May 2, 2010

### pat666

Hey ehild, how do i get an equation to display properly here, i know its hard to read and ive seen people post equations that look like equations but i cant get it to work: Im an idiot is the problem, my physics is correct i think but my maths is not, is your answer for a) 0.43rev/s?, for c) 0.16125m/s and could you give me a hand for d please. just repeated my last post since we posted at exactly the same time.

11. May 2, 2010

### Squeezebox

Yea those are the answers I got as well.

Resultant vector depends on the tangential and centripetal accelerations.

12. May 2, 2010

### pat666

centripetal acceleration would be 0.9?????????? and tangential would be 0.9*r?

13. May 2, 2010

### Squeezebox

No, angular acceleration is 0.9 rad/s2.

ac=vt2/r

at=$$\alpha$$r

14. May 2, 2010

### ehild

People use tex to write the equations in nice form, see the thread Introductory Physics Formulary. But you can simply use symbols +,-,*,/ ^, and write out Greek letters and functions, it will do either.

As for angular speed, the SI unit is rad/s, and rev/s is rather called rotational speed. The relation between tangential speed and angular speed, v=r*w or the relation between tangential acceleration and angular acceleration a = r*alpha is valid only in the rad/s, rad/ s^2 units.

ehild

15. May 2, 2010

### pat666

Hey, did you get a_c=0.0693 and a_t=0.3375 and the resultant = 0.34???

16. May 2, 2010

### ehild

Look at the original post: angular acceleration is 0.9 rev/s^2.

ehild

17. May 2, 2010

### ehild

Write out the units.

ehild

18. May 2, 2010

### pat666

OK, ill also tell you what i did. I have angular acceleration as 0.9rev/s^2 so i stuck that in a_t=alpha*r and got 0.3375m/s^2,,,,,,,, angular acceleration is alpha isnt it? For the centripetal acceleration i did a_c=〖0.16〗^2/0.375 from that i got a_c=0.07rev/s^2 that is v_t^2/r.... sorry about the units im drawing a blank on them. I thought that tangential acceleration had units of m/s^2 and centripetal acceleration had units of rev/s^2, but reading back over what you have said i take it that that is not right?

19. May 2, 2010

### pat666

Does the acceleration have to be in rad/s if it does i would just times it by 2pi wouldnt i??

20. May 2, 2010

### ehild

at= alpha * r is valid if you measure the angular speed in rad/s. rad has de dimension of "1": the angle in radians is the arc length divided by the radius. So you have to convert angular velocity to rad/s unit before you calculate tangential velocity.
In your first post in this thread you said the angular acceleration was 5.5 (without units) Well, it is 0.9 (rev/s) * 2 pi (rad/rev) = 5.65 rad/s^2.

Your present values for the components of linear acceleration are wrong.

ehild