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post #1 of (permalink) Old 10-07-2002, 07:54 PM Thread Starter
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OT - Random Physics Question

So as I pull out of work and leave the pea gravel for pavement, I undergo the attack of small rocks assualting the rockers and fenders of my Jeep. That isn't a big deal, what is fun is the one or two pebbles that hold on to my tire until I'm going 70mph and then let go with a massive SMACK!

So what I was wondering is how fast is that little peice of compressed former biomass going when it leaves my wheel at 70mph.

Here's the (made up) stats:
--- 3.73 gearing
--- ~30 inch tires
--- Taching around 2500 rpm

What else is needed? I don't know, so just add to it with whatever seems good to you as I didn't supply precise figures anyway!

Fritz
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post #2 of (permalink) Old 10-07-2002, 08:14 PM
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Re: OT - Random Physics Question

Wouldn't it be going 70 mph? I could be wrong but that answer seems to be the simplest one. I don't see any reason to incorporate your engine speed and gearing into it. Your tires are turning at 70 MPH so whatever comes off will be going the same speed. It'll actually slow a little bit as it goes through the air before it hits your rockers, but not enough to count in this formula. I guess if we knew the wind speed and direction and the coefficient of friction of the particles we could figure out how much they slowed, but I don't really care about any of that.
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post #3 of (permalink) Old 10-07-2002, 08:20 PM
 
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Re: OT - Random Physics Question

Well, I think that 70mph isn't right...but I don't know the right answer...lol...It seems that it would have to be going faster than that to leave the tire that is going 70....Unless I suppose the rock went backwards...hmmm I don't think I know what I'm talking about here
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post #4 of (permalink) Old 10-07-2002, 08:25 PM
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Re: OT - Random Physics Question

</font><blockquote><font class="small">In reply to:</font><hr />
Wouldn't it be going 70 mph?

[/ QUOTE ]Yep. Read the centripetal force discussions for further understanding.
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post #5 of (permalink) Old 10-07-2002, 08:43 PM
 
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Re: OT - Random Physics Question

I agree with the 70mph answer. It was caught in the tread, the tread travels at the same speed as the road / vehicle. As it was trapped in the tread then let go, it's speed can't exceed that of the tread, no further force to accellerate it. Now if one is so unfortunate as to be standing by the roadside with a jeep travelling towards you, and a stone flies off the top of the tread forwards, couldn't the stone hit you at 140 mph?
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post #6 of (permalink) Old 10-07-2002, 08:44 PM Thread Starter
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Re: OT - Random Physics Question

Here's some fun.

A helicopter's speed is limited by the fact that as the blades rotate around the hub, one side will always be traveling away from the direction of travel, effectively slowing down while the other blade is accellerating into the direction of travel. When this latter blade hits the speed of sound, it causes dangerous stress loadings. The hub though is nowhere near the speed of sound.

Assuming the above and that the Jeep (not the edge of the tire) is traveling forward at 70mph, then what is the speed of the rock leaving the tire moving into the direction of travel?

Fritz

By the way, I'm going to go back and review the past post as I don't remember what it dealt with, but I think teh topic was about whether an object leaves staight out or tangentally. A different question. [img]images/graemlins/cool.gif[/img]
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post #7 of (permalink) Old 10-07-2002, 08:58 PM
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Re: OT - Random Physics Question

</font><blockquote><font class="small">In reply to:</font><hr />
…and a stone flies off the top of the tread forwards, couldn't the stone hit you at 140 mph?

[/ QUOTE ]Yep again, assuming no loss from resistance of the air. Relative to the Jeep the wheel is attached to the speed is 70 mph. Relative to a stationary object it would depend on the angle when it left the tire. If it were at -90, bottom, when it left it would just lay down on the pavement, still 70 mph with respect to the Jeep but zero with respect to the pavement.
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post #8 of (permalink) Old 10-07-2002, 09:17 PM Thread Starter
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Re: OT - Random Physics Question

Still not buying it.

In one full rotation, the tire will go from a dead stop to full speed.

Now the full speed is the speed it needs to overtake the Jeep. If said Jeep has 20 inch tires, then the point "a" on the tire wil have to cover the circumfernece of the tire minus on half in a set period of time. Now the same Jeep at the same speed has a tire diameter of 44 inches, then the same point "a" will have to travel further to make the half rotation in the same set period of time.

mph is distance travled per period of time, so the point "a" on the second tire is having to travel further to reach the same spot in time thereby moving faster than the point "a" on the smaller tire. Is this not the reason gears work?

So the rock on the larger tire leaves at a higher speed than on a smaller tire. That is why I reason a need for tire diameter for this question. To say that the rock is going 70mph may be correct at only <u>one point</u> in the tires rotation. At any other time, the speed differencial (relative to itself) is always changing.

I have stated my argument, please respond as I love livly debate.

Oh yeah, I reread the BB post and it does have a similiar slant, but in the entire post, there isn't mentioned the difference made by tire hieght.

Fritz
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post #9 of (permalink) Old 10-07-2002, 09:23 PM
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Re: OT - Random Physics Question

</font><blockquote><font class="small">In reply to:</font><hr />
When this latter blade hits the speed of sound, it causes dangerous stress loadings.

[/ QUOTE ]Don't know how dangerous the stress loading is, depends on what it's made of, but it loses it's lift or push in the case of a prop. I believe there's a name for it which eludes me now.

If the 'copter is flat, the blade cannot influence forward motion of the craft when it's going forward. For much forward acceleration, they kick up the tail. Now I can't remember the pitch terms, one is cyclic, what's the other?

Never mind, it's collective. Remembered as I hit the continue key.
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post #10 of (permalink) Old 10-07-2002, 09:35 PM Thread Starter
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Re: OT - Random Physics Question

Don't you hate those lat minute thoughts when you want to hit the "stop" button which will only duplicate your mistake! [img]images/graemlins/wink.gif[/img]

Anyway, as I was replaying this scenario with my wife's lipstick container (it was there [img]images/graemlins/crazy.gif[/img]) I realized that the problem has been that we have been using models of movement that all rely on stationary hubs. If the hub of the rotational model is moving at a constant velocity, then any point on the edge of the rotational model will aways be changing speed relative to all points except the center of the hub which will alway perceive the rotational model as constant.

Try moving a round object along the table and mark one spot. The spot will be at a dead stop at the bottom the table surface (traction) after which it begins to accelerate to overtake the hub which is traveling in a linear direction at a constant speed (70mph). At the top of its arc, it is traveling at the same speed as the hub after which it begins its decceleration back to zero.

This was a problem on steam engines. The weight of the running gear coming to rest and then accerating back to speed put massive loads on the wheels, bearings, and the track. A problem that was never fully cured despite amazing advances in variable counterweights that tapered along the wheels edge.

Yes, a helicopter does tilt forward to "pull" itself through the air, but it doesn't go completely vertical and the hub is still traveling forward while the blades are still arcing from stop to full speed and back.

Fritz
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