# Lost with exercise about circular motion

1. May 1, 2014

### physicos

Hi everyone, my physics final is coming in 3 days , and I really need to have an answer to this exercise , but I'm stuck ! I don't even understand the problem statement HELP !

A typical fastball is thrwon at approximately 90 mph and with a spin rate of 113 rpm (I don't understand what it is).If the distance between the pitcher's point of release and the catcher's glove is 60.5 ft, how many full turns does the ball make between release and catch ? Neglect any effect of gravity or air resistance on the ball's flight

ANY GRAPH OR EXPLAINING WOULD BE PRECIOUS !!

2. May 1, 2014

### SteamKing

Staff Emeritus
It might be more helpful if you would explain specifically what you don't understand.

Do you know what a ball is?

Have you ever thrown a ball?

Do you understand the units in the problem? (what do 'mph', 'ft', and 'rpm' mean?)

You can ask all sorts of questions, but tell us the ones you really are having the most trouble with.

3. May 1, 2014

### Staff: Mentor

Draw a diagram.

Determine the time the ball takes to cover the distance between pitcher and catcher.

As the ball travels it also rotates. The rotation rate is 113 RPM, which is roughly twice per second.

4. May 1, 2014

### physicos

Well , I have already thrown a ball (thanks god) , but I come from a french institution , and we didn't use 'mph', 'ft', and 'rpm' or else , it was more : m/s , m , rad/s !
In France , we don't play baseball , so I don"t really know what pitcher and catcher are located or what they are supposed to do !
A diagram and some explanation would help ! So far , I undestrood that vi = 90 mph and w = 113 rpm and I = 2/5 mr² and the ball goes a distance d= 60.5 ft , but how can I relate this to number of turns that I consider as frequency ??

5. May 1, 2014

### SteamKing

Staff Emeritus
Surely, in France, they must know what a revolution is! I can't imagine that a typical Frenchman would say that the earth turns on its axis 2π radians every 24 heures.

Like most simple physics problems, understanding the units is key to solving the problem. I understand you may not be familiar with feet, miles, etc., but you are using a computer to communicate with PF, so you should be able to find unit conversions for feet to meters, miles to kilometres, etc. The question isn't asking you about the rules for playing baseball; it's just using two people playing pitch and catch as an illustration.

6. May 2, 2014

### physicos

I've been looking for a link between what we learn in France in US and got an explanation , still,it is not helping me relate what they are asking for and what we have got as informtion

7. May 2, 2014

### haruspex

Can you now convert 90 mph to m/s, 113 rpm (revolutions per minute) to rad/s, and 60.5 ft to metres?
How long is the ball in the air?

8. May 2, 2014

### physicos

well vi= 40.5 m/s and ωi= 11.83 rad/s and d= 18.15m !

I'm thinking about using ωf²-ωi²= 2*α*Δθ

and Δθ would be the number of turns ,i have ωi , how about ωf ? should I use ωf = vi/d ??

9. May 2, 2014

### SteamKing

Staff Emeritus
The spin rate is constant, so I don't think your formula has much applicability to this problem.

It's a simple problem. The ball is travelling at a constant speed (90 miles/hour). It's rotating at a constant rate (113 turns/minute). The ball is thrown a certain distance (60.5 feet). You have a problem of related rates and unit conversions, in finding the number of turns the ball makes while it is in flight. You are over thinking about what to do to solve it.

10. May 2, 2014

### physicos

so , thinking about t = v*d = 730.2 s= 12.17 min ! and w= Δθ/t so Δθ=w*t= 1375,21turns ??

11. May 2, 2014

### haruspex

I don't think so!

12. May 2, 2014

### SteamKing

Staff Emeritus
That's one slooooooowwwww pitch.

In that amount of time, you could brew a pot of coffee, drink the coffee, and wash the pot.

13. May 2, 2014

### Staff: Mentor

During the French Revolution an edict was issued re-defining Pi to be precisely 3 in order to simplify mathematics for the masses. While the edict was subsequently rescinded, it led to a host of conceptual difficulties, some of which It is said endure to this day.

14. May 2, 2014

### haruspex

Do you have a reference for that? The only clear example I can find of such a move was in Indiana in 1897 http://en.wikipedia.org/wiki/Indiana_Pi_Bill#Legislative_history.

15. May 3, 2014

### SteamKing

Staff Emeritus
Yeah, and 5 euros ought to get you a decent cuppa Joe in any bistro in Paris. The OP's difficulties with this problem seem to stem from something more basic than what some crazy Frenchmen did 200 years ago w.r.t. defining pi. I don't know if it's a language difficulty, the fact that the French never had a good baseball team, or what, but it's something basic. I mean, t = v*d? That's got nothing to do with the value of pi. At this point, the OP would be better off telling his school he came down with a sudden case of Ebola, because it doesn't look like he is ready to take this physics final exam.

16. May 3, 2014

### physicos

Well, I needed help in physics , not history.... !!!

17. May 3, 2014

### SteamKing

Staff Emeritus
Good. Let's start with t = v*d. Where did that come from?

You realize that d = distance = length units and v = velocity = length units / unit time

So t = time = v*d = length units * length units / time units = (length units)$^{2}$/time

Thus t = v*d cannot be right. Try again.

18. May 3, 2014

### TheAustrian

Well, that explains why the dropout rate in first year tends to be ~50%

With mechanics questions you can always check your own answers. Always ask yourself the question: Is this realistic? If I throw a ball, is it really going to rotate around its axis 1000 times??

19. May 4, 2014

### physicos

OHH ! I just realized !!! SHAME ON ME !!! I'M REALLY SORRY ! I'm more stressed than ever , and I have so much to handle ! I'm really sorry !
v=d/t so t= d/v ! It is evident that I'm aware of this ! just question of units would make me know it ! I'm so sorry agaain !
so t= 0.45s=7.5 *10^-3 min so Δθ=w*t=5.30 turns (I changed unit of w to rad/min when I used time in minutes and rad/s to double check with time in seconds don't worry)

20. May 4, 2014

### physicos

TheAustrian, the dropout rate won't concern me , I was just stressed and overwhelmed with load of studies ! Watch me having an A in my physics ! I'm not dumb ! I just didn't understand what exercise asked and as you can see I had "basic" formulas , so no need to "monter sur tes grands chevaux " as would French say !!