# Help with this problem on non-uniform circular motion?

1. Sep 21, 2009

### tjohn101

A car at the Indianapolis-500 accelerates uniformly from the pit area, going from rest to 285 km/h in a semicircular arc with a radius of 194 m.

Determine the tangential acceleration of the car when it is halfway through the turn, assuming constant tangential acceleration.
m/s2

Determine the radial acceleration of the car at this time.
m/s2

If the curve were flat, what would the coefficient of static friction have to be between the tires and the roadbed to provide this acceleration with no slipping or skidding?

Last edited: Sep 21, 2009
2. Sep 21, 2009

### kuruman

3. Sep 21, 2009

### tjohn101

If I had an idea about how to approach the problem and knew all of the equations, do you really think I would be posting on here? I just need someone to tell me how to get the problem going. Not be an unhelpful forum cop.

4. Sep 21, 2009

### tjohn101

One more question:

What will a spring scale read for the weight of a 66 kg woman in an elevator that moves as follows?

upward with acceleration of 0.31g
and
downward with acceleration 0.31g

I would post the equations as the forum cop up there would like me to, but I don't even know what .31g is. I've never learned it and the book is about as helpful as a stick. If you could explain this ".31g" stuff to me I should be able to do this on my own.

Thanks

5. Sep 21, 2009

### jambaugh

Still you should use the format, if you don't know the relevant equations say so.... But if you can't look up equations in your text then you ought to get a job flipping burgers instead of waisting your time in school. Don't get hostile with people here who are trying to help. I've half a mind to just give you the answer so you'll learn nothing!

If you don't have a clue as to how to start such a problem it means you are too soon trying to solve problems and need to go back and read your text (including working through given examples) so you understand basic concepts. You can't put a puzzle together with your eyes closed. You have to see the pattern. There is no cut and dried formula for every problem. Understand the concepts and how they fit together with the equations and then practice with different problems to pin down what you've learned.

I'll get you started with this hint:

For the first part of the problem tangential acceleration means acceleration in the direction of motion. So that part of the problem is no different than a straight line acceleration problem. I.e. drag racer. If you can't do that type of problem you need to go back and review your linear kinematics before tackling circular motion.

6. Sep 21, 2009

### willem2

I don't think this was the answer to the question of this thread, wich was about a car on a semicircular track

7. Sep 21, 2009

### jambaugh

What textbook are you using that is so unhelpful?

Look at the units of acceleration (meters per second per second). Clearly there are no grams in it so g can be grams. You've heard the expression "pulling gees" in terms of jet pilots haven't you? What is a "gee"? No matter how bad your textbook it should have an index. Have you tried using it yet? Look up g in the index. [edit: If you don't find g then look up acceleration.]

8. Sep 21, 2009

### tjohn101

Look. I'm already pissed off about this homework enough as it is. I don't consider calling me out on my mistakes after I just joined "helping". And for you, I have a 4.0 college GPA and a 97 in this class. I'm stuck on this ONE chapter because my teacher skipped a day of class and decided to teach it in 20 minutes the next day due to the fact that we had a test. She never even went over this .31g thing, and the book does not mention it a single time. So before you go telling me I should be flipping burgers, I suggest you get to know me and my book a bit better.

9. Sep 21, 2009

### tjohn101

Yeah I'm sorry. I have too many tabs going at once. My mistake.

10. Sep 21, 2009

### jambaugh

Quid pro quo, before you go sniping at "forum police" you should calm down and think about what we have to deal with in terms of students wanting answers without think.

11. Sep 21, 2009

### tjohn101

g=gravity. What does "upward with acceleration of 0.31g" MEAN? Is gravity increasing? That's all I need to know, and that's all I honestly care about right now.

12. Sep 21, 2009

### jambaugh

g is a unit of gravitational acceleration. It is an amount of acceleration equal to that of gravity. You ask a question, I'm happy to answer.

[EDIT: BTW, I am serious about wanting to know the particular text you are using. Could you please tell me?]

13. Sep 21, 2009

### tjohn101

Giancoli Physics 6th Edition. The question comes from the book, but the book doesn't explain one bit how to do it or what .31g means. Plus, My teacher hates Giancoli's book and teaches us her way. So everything we read in there is either completely different or in a different form than what we were lectured on.

I'm still confused. Is .31 adding on to 9.8? What's happening there?
By the way, there were 5 parts to this question. I got the first three in a matter of seconds, but the .31g threw me off and I only have three tries to answer the question. I've used two.

14. Sep 21, 2009

### jambaugh

When two numbers are written together it is the convention that you take their product.
0.5x = 3 when x =6.

As far as the textbook is concerned. I sympathize. When you get a chance go to your library and check out a physics textbook written before 1980. I'm looking right now for online public domain physics texts and will post any I find that are decent.

15. Sep 21, 2009

### tjohn101

Okay. What do I do to the .31g to make it useful with the 66kg? If I can get a grasp on that I will be fine.

16. Sep 21, 2009

### jambaugh

Start with the fact that gravity is a uniform acceleration. Gravity on Earth's surface is indistinguishable from the elevator being in free space and being accelerated upward (say by rockets) at 9.8m/s/s.

Now back on Earth if the elevator is falling at 9.8m/s/s=1g one is in free-fall and should feel no weight (until we get to the basement). So accelerating downward takes away from the experienced gravitational force. If it isn't accelerating downward as much as 1g then you feel some weight but not as much as if it were standing still (or moving with uniform speed).

17. Sep 21, 2009

### tjohn101

I got the upward as 847.308. But I still have yet to get the downward. What's the difference?

EDIT: Nevermind I got it. I could still use some help with the two problems in my other thread, though.