# Homework Help: Magnitude of the resultant acceleration

1. Nov 21, 2013

### Vlipfire

1. The problem statement, all variables and given/known data

An electric turntable 0.760m in diameter is rotating about a fixed axis with an initial angular velocity of 0.250rev/s . The angular acceleration is 0.900rev/s2 .

what is the magnitude of the resultant acceleration of point on the tip of the blade at time 0.200

2. Relevant equations
none given
the angular velocity at time 0.200 is 0.43
the tangential speed at time 0.200 is 1.03 m/s

3. The attempt at a solution
I have tried doing sqrt(tangential acceleration^2 + radial acceleration^2)
2.79
2.77
1.95
2.97
2.93
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 21, 2013

### Simon Bridge

Welcome to PF;
0.200 what? (units?)

That does not mean there are no equations that are relevant though.

OK - but why did you bother to caclulate these?
How did you calculate them?
(units?)

... how come you have lots of different numbers?
How did you calculate them?
(units?)

Note: the numbers are meaningless without units.

3. Nov 21, 2013

### Vlipfire

Thanks

the units is in seconds
I believe the blade is a typo
The reason I have posted here is because this question does not make much sense I have tried all of the methods that I have been able to find around the internet and am down to my last attempt of the question. the book has no useful examples either.
These are answers from previous parts to the question and I believed they may be useful
... how come you have lots of different numbers?

4. Nov 22, 2013

### haruspex

In rev/s yes, but you will find rad/sec more useful. All the kinematic equations assume radians.
I don't get that answer. Please post your working.  you're right - didn't notice the .76 is a diameter.
How did you calculate those?
The numerical values you have tried are not of interest. What would be useful is to post the logic that led to them.

Last edited: Nov 22, 2013
5. Nov 22, 2013

### Simon Bridge

Then what is it you are supposed to be calculating the total acceleration of?

... I figured as much: the point of asking these questions is to help you understand the question better so you can figure out were you keep going wrong and do it right.

Please show us what you have tried before.

Well presumably ... but what was it about them that made you think they may be useful?
For instance: did you have some sort of strategy in mind or were you just posting the results of random calculations?

6. Nov 22, 2013

### Vlipfire

I was using the concept of the two velocities being vectors and that the resultant vector was sqrt(a^2+b^2) I was able to get tangential acceleration, and angular acceleration from the information of the first couple questions and the problem statement. I have seen many other people trying to answer variations of this question and so far no one has actually gotten the right answer. they are all close but none quite right. I no longer remember some of the ways I have tried because I have been doing this over several days. I appreciate the help.

7. Nov 22, 2013

### ehild

That is wrong. The angular velocity ω is 2pi times f(rev/s) and the angular acceleration is β=dω/dt. Multiply your value by 2pi.

That is correct.

The method is correct.

How do you get the tangential linear acceleration from the angular acceleration and radius?
How do you get the radial acceleration from the linear speed and radius?

ehild

8. Nov 22, 2013

### Vlipfire

Maybe that was my problem. I don't want to enter it because I only have one attempt left.
To find Tangential linear acceleration I used a=rω and for the radial acceleration I believe it was given.

9. Nov 22, 2013

### haruspex

'ω' is generally used for angular speed. I assume here you are using it for angular acceleration.
No, it needs to be deduced from another value you calculated. For motion in a circle, how do you find the radial acceleration?

10. Nov 22, 2013

### Simon Bridge

That is the correct concept.

In order to help properly - we really need to see your working, not just a description of the concepts you used. If you use the right concepts but the wrong method you will still get t wrong and we cannot tell you how.

But you are doing a physics course? You know some physics? So you can apply that knowledge to show us one way you may go about it.
If we cannot see how you are thinking we cannot help - you have to do your own homework.
Is there some reason you don't want to do this?

It is almost cerainly something like that - which is why you have to show us your working.

If you do a quick dimensional analysis, you'll see that cannot be the correct equation for tangential linear acceleration. It is $v_t=r\omega$ and $a_t=r\alpha$.

See how showing us what you tried leads to an answer you can use?
What formula did you use for centripetal acceleration.