Imagine a semi truck was traveling at exactly 90 miles an hour with its back door open. Then imagine a major-league baseball pitcher standing in the trailer looking towards the open back. If he threw a fastball at exactly 90 miles an hour, what would happen? Assuming there was no spin on the ball, could it just bounce to stop in the middle of the road without rolling an inch in either direction?
[This message has been edited by Synthesis (edited 11-13-2013).]
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09:44 AM
PFF
System Bot
dratts Member
Posts: 8373 From: Coeur d' alene Idaho USA Registered: Apr 2001
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Pratical issues where the truck speed at 90 mph is assumed to be constant since the truck provides its own power. A throw by a pitcher at 90 mph would not be constant since the power is only delivered before the release of the pitch and is then slowed by air resistance. Also most road are crowned for drainage, if the ball were to land on the road it would roll to the side due to the crown.
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10:37 AM
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
Assuming there was no spin on the ball, could it just bounce to stop in the middle of the road without rolling an inch in either direction?
Yes. (Also assumes a flat road surface and no aerodynamic effects on the ball.)
Consider this: Pick a spot on the tread of one of the truck's tires. Even at 90 mph, that spot will be perfectly stationary (moving exactly 0 mph with respect to the road) once per revolution, and half a revolution later, when the spot is the farthest from the road, it will be moving forward at ~180 mph.
[This message has been edited by Marvin McInnis (edited 11-13-2013).]
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12:27 PM
Formula88 Member
Posts: 53788 From: Raleigh NC Registered: Jan 2001
Yes. (Also assumes a flat road surface and no aerodynamic effects on the ball.)
Consider this: Pick a spot on the tread of one of the truck's tires. Even at 90 mph, that spot will be perfectly stationary (moving exactly 0 mph with respect to the road) once per revolution, and half a revolution later, when the spot is the farthest from the road, it will be moving forward at ~180 mph.
Ok, this is an interesting one. Start adding vector and angular velocities and I get a headache.
[This message has been edited by Formula88 (edited 11-13-2013).]
In a perfect world, the ball would drop straight down after it leaves the pitcher's hand. But in the real world, there are lots of variables that can affect that result. That's why it took the Mythbusters so long to get that perfect shot.
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01:41 PM
Patrick's Dad Member
Posts: 5154 From: Weymouth MA USA Registered: Feb 2000
Regarding the baseball problem; the truck should be going about 70MPH, then have a knuckleballer throw the ball. The inherent idea behind the knuckler is that the ball doesn't spin, so, when pitched off a mound, the air takes it where it will.
As to the helicopter on the turntable, I would hope the blades rotate faster than 33-1/3RPM. Or even 78RPM.
Regarding the baseball problem; the truck should be going about 70MPH, then have a knuckleballer throw the ball. The inherent idea behind the knuckler is that the ball doesn't spin, so, when pitched off a mound, the air takes it where it will.
As to the helicopter on the turntable, I would hope the blades rotate faster than 33-1/3RPM. Or even 78RPM.
Blade root or blade tip? RPM or velocity? There is a big difference between velocity at the blade tip and rpm.
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09:31 PM
Patrick's Dad Member
Posts: 5154 From: Weymouth MA USA Registered: Feb 2000
That helo closely resembles a Sikorsky CH53 or the civilian version S-65 --I don't know exactly what version it's supposed to be , but if it's a 53E model then the rotor hub (the center part the blades attach to) is physically limited by gear reduction to only turn about 179 rpm @ MAX full power throttle settings, and it has a 79 foot blade circle. It uses the same transmission, rotor hub and blades as the CH53D I flew on. They just added some extensiions to the transmission output hub to make the blade circle wider and give more lift for the E models--as well as adding a 3rd engine. If it is a CH53 D, then the max full power rotor hub RPM is 185 with (IIRC) a 73 foot blade tip circle.
Not as fast RPM as you would have guessed huh? Lots of gearboxes and then the main reduction gear in the transmission. Don't remember what the max output rpm of the T-64 GE turbines were before reduction, but I rememer whatthe little Solar T62T gas turbine rpm was before it's gear reduction (it drove the auxillary generator--an APU) . That little sucker turned 61,000 rpms.
(someone else can do the math to figure the blade tip velocities)
[This message has been edited by maryjane (edited 11-13-2013).]
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10:55 PM
Patrick's Dad Member
Posts: 5154 From: Weymouth MA USA Registered: Feb 2000
The turntable is a Numark TT-1600, a belt drive turntable, which offers nominal speeds of 33RPM and 45RPM. Pitch control allows for rotational speed variance up to 10%. The platter is 12" in diameter, so the circumference would be 37.7".
You're right - I would never have guessed that a helicopter blade moves so slowly (rotationally speaking). At 73', the rotational circumference is 248.186'. Doing that 179 times a minute means that the tip of the blade is moving at 8.414 miles per minute, or 504.833MPH. By comparison, the edge of the turntable platter is moving about 21.2MPH (at +10% pitch).
They look like a blur when turning but we used to track them with a strobe that we set on the ground out at the tip ring area--worked sort of like a big ignition timing light.
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12:09 AM
Patrick's Dad Member
Posts: 5154 From: Weymouth MA USA Registered: Feb 2000
They look like a blur when turning but we used to track them with a strobe that we set on the ground out at the tip ring area--worked sort of like a big ignition timing light.
Many turntables had strobes as well. The black and silver pattern would appear stationary when the platter speed was what it was supposed to be.
The turntable is a Numark TT-1600, a belt drive turntable, which offers nominal speeds of 33RPM and 45RPM. Pitch control allows for rotational speed variance up to 10%. The platter is 12" in diameter, so the circumference would be 37.7".
You're right - I would never have guessed that a helicopter blade moves so slowly (rotationally speaking). At 73', the rotational circumference is 248.186'. Doing that 179 times a minute means that the tip of the blade is moving at 8.414 miles per minute, or 504.833MPH. By comparison, the edge of the turntable platter is moving about 21.2MPH (at +10% pitch).
505 mph is well below the limiting factor of the speed of sound, but, the speed or velocity of the turntable has to be considered as well. Still, I beleive it will take off, because even tho relative to the ground, blade rotation has been negated, the air itself is still stationary, and that (blade airfoil reaction with air) is what provides lift. Could get into all kinds of things like centrifugal forces on the pilot, ground effect, and retreating blade effects, but we won't go there.
[This message has been edited by maryjane (edited 11-14-2013).]
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12:55 PM
zetabird Member
Posts: 1303 From: nappanee, IN Registered: Dec 2006
Originally posted by cliffw: What if he was facing the direction of the trucks travel. It's doin' 90 and he throws the ball forward at 90. Does the ball float stationary ?
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Originally posted by Formula88: No. It moves forward at 180mph.
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07:36 AM
rice.1 Member
Posts: 329 From: Bloomfield, IN Registered: Jan 2013
