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Author Topic:   Physics Question (Sort Of)
GMx
Super Geek

Posts: 115
From: Somewhere inside the TARDIS
Registered: Jul 2002

posted August 08, 2002 07:21     Click Here to See the Profile for GMx   Click Here to Email GMx     Edit/Delete Message   Reply w/Quote
I was watching the Clint Eastwood/Dirty Harry movie The Dead Pool last night and there was a scene in it that is still bugging the hell out of me. The killer had used a radio controlled car to blow up someone's car as he was backing out of his driveway. He had the controller attached to his dashboard to control the car which was filled with C-4, then detonated it as the toy car slipped under the victim's car. No problem with that, the victim's car was moving slowly as he backed out of the driveway. Later, the killer uses the same kind of radio controlled car to try to kill Dirty Harry. He sees the car, catching on to the fact that a toy car had been used to blow up the last victim, because he found a wheel from the car at the scene. Now, here's what's bugging me-Dirty Harry screeches off, and they begin a high speed chase through the streets of San Francisco. The radio control car is able to keep up with Dirty Harry although he is flying through the streets. the killer is following in his car, controlling the toy. What I want to know-is this possible? I mean if the killer's car is able to keep up with Dirty Harry and the radio signal is still transmitting in front of the car, can the radio controlled car also keep up at the same speed? What would be the formula for that? Try to keep it simple, because I've only had basic Physics, and I didn't do too well in them anyway. Would it have something to do with the velocity of the car that had the controller in it?

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"Well, aren't you going to say it's bigger on the inside than it is on the outside? Everybody else does." The Doctor (No.3)

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Danapoppa
Geek Apprentice

Posts: 45
From: Tokyo
Registered: Jul 2002

posted August 08, 2002 07:29     Click Here to See the Profile for Danapoppa   Click Here to Email Danapoppa     Edit/Delete Message   Reply w/Quote
I vaguely remember seeing this movie, and the short answer to your question is, I dunno.

I'm wondering instead about the size of the wheels on the remote controlled car, and the speed with which that little motor would have to be turning over. Are there remote controlled cars that can move that fast?

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GMx
Super Geek

Posts: 115
From: Somewhere inside the TARDIS
Registered: Jul 2002

posted August 08, 2002 07:37     Click Here to See the Profile for GMx   Click Here to Email GMx     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Danapoppa:

I'm wondering instead about the size of the wheels on the remote controlled car, and the speed with which that little motor would have to be turning over. Are there remote controlled cars that can move that fast?



The tires seemed to be maybe about an inch or an inch and a half in diameter. Don't know how big the engine was. I don't know how he could have fit a big one in there and still have room for the C-4. It was a Corvette, though.

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"Well, aren't you going to say it's bigger on the inside than it is on the outside? Everybody else does." The Doctor (No.3)

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spungo
Highlie

Posts: 697
From: Hell's toilet
Registered: Jan 2002

posted August 08, 2002 07:40     Click Here to See the Profile for spungo     Edit/Delete Message   Reply w/Quote
Yeah, this is an engineering question - I have seen remote controlled cars do up to 100mph... pretty hard to find, though.

As for the signal from the controller - it wouldn't make much difference about it's motion - a little bit of a Doppler shift in frequency... but not a great deal... it would only be a problem if the distance between them is near the edge of it's range.

EDIT: in fact, if they are both moving at the same speed in the same direction, the effective range is increased as there is no Doppler shift (as ordinarily, the controller is stationary and the car is moving). This is only true if the tuning circuit in the controlled car is exactly matched to the peak frequency of the controller signal.

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"But you've got everything now
and what a terrible mess I've made of my life."

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GMx
Super Geek

Posts: 115
From: Somewhere inside the TARDIS
Registered: Jul 2002

posted August 08, 2002 07:54     Click Here to See the Profile for GMx   Click Here to Email GMx     Edit/Delete Message   Reply w/Quote
So, I guess it was possible. Thanks. I just wondered because you know how Hollywood sometimes doesn't really think these things out.

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"Well, aren't you going to say it's bigger on the inside than it is on the outside? Everybody else does." The Doctor (No.3)

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snupy
Highlie

Posts: 746
From: Chicago
Registered: Mar 2002

posted August 08, 2002 08:57     Click Here to See the Profile for snupy   Click Here to Email snupy     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by spungo:
Yeah, this is an engineering question - I have seen remote controlled cars do up to 100mph... pretty hard to find, though.

As for the signal from the controller - it wouldn't make much difference about it's motion - a little bit of a Doppler shift in frequency... but not a great deal... it would only be a problem if the distance between them is near the edge of it's range.

EDIT: in fact, if they are both moving at the same speed in the same direction, the effective range is increased as there is no Doppler shift (as ordinarily, the controller is stationary and the car is moving). This is only true if the tuning circuit in the controlled car is exactly matched to the peak frequency of the controller signal.



Please, spungo, have some pity on me-you know how I get when you talk like that. I have to remember to bring a change of underwear to work

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spungo
Highlie

Posts: 697
From: Hell's toilet
Registered: Jan 2002

posted August 08, 2002 10:06     Click Here to See the Profile for spungo     Edit/Delete Message   Reply w/Quote
Oh, Lord. The flames of Hades await us both.

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"But you've got everything now
and what a terrible mess I've made of my life."

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Tau Zero
BlabberMouth, the Next Generation.

Posts: 1762
From:
Registered: Jan 2000

posted August 08, 2002 10:47     Click Here to See the Profile for Tau Zero     Edit/Delete Message   Reply w/Quote
The short answer:  In theory, it could be done.  In practice, it would be so difficult as to be implausible.  (But then, Hollywood is where they dreamed up effects such as people being hit by arrows flying up in the air and through walls, as in The Scorpion King.  Hollywood and reality intersect, but only by accident or lack of budget. )

Doppler shift isn't an issue.  Even at pursuit speeds, the frequency shift would be measured in hertz.  (50 meters per second divided by, say, 4 meters per cycle = 12.5 cycles per second.  You'd need some mighty specialized gear to notice.)

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snupy
Highlie

Posts: 746
From: Chicago
Registered: Mar 2002

posted August 09, 2002 04:02     Click Here to See the Profile for snupy   Click Here to Email snupy     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by spungo:
Oh, Lord. The flames of Hades await us both.


You, me, and Zorro.Hmmm....

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Zwilnik
Assimilated

Posts: 426
From: London, UK
Registered: Dec 2000

posted August 09, 2002 05:58     Click Here to See the Profile for Zwilnik     Edit/Delete Message   Reply w/Quote
on the speed issue, it could have been a petrol engined R/C car (although it looked more of an electric motor job, we can put that down to movie magic), which would be more sensible than an electric motor as it would be one less electrical signal that might accidentally set off the C-4 as you're starting it up. It would also give the assasin better range.
The only downside is, petrol engines are pretty noisy, even the really nice 4 stroke ones, so it wouldn't exactly have been stealthy.

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GMx
Super Geek

Posts: 115
From: Somewhere inside the TARDIS
Registered: Jul 2002

posted August 09, 2002 06:42     Click Here to See the Profile for GMx   Click Here to Email GMx     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Zwilnik:

The only downside is, petrol engines are pretty noisy, even the really nice 4 stroke ones, so it wouldn't exactly have been stealthy.

It sounded like it was an electric. But it was pretty fast. In one part of the scene, the cars rears back on its back wheels and peels out!

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The probability of someone watching you is proportional to the stupidity of your action.

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mcgyver27
Single Celled Newbie

Posts: 1
From: Kahului, HI USA
Registered: Aug 2002

posted August 09, 2002 11:05     Click Here to See the Profile for mcgyver27     Edit/Delete Message   Reply w/Quote
Hi everyone,
Unmodified, electric R/C cars (1/10 scale) can go up to 45mph. Unmodified, gas R/C cars can go up to 60mph. I have seen custom built electric R/C cars that have gone up to 110mph. Check out RC Car Action (http://www.rccaraction.com/articles/need_speed3.asp/). I don't recall the exact issue. The only problem with the electric cars is that stock cars only have about 5min battery life. If it is modified the life can drop to 3min. It would have to be a very short chase

The one main problem is the power of the Radio control transmitter-most non-licensed ones have a 100-150 foot range so the pursuit car needs to be close to the R/C car.

One of the better chase scenes I've seen is the freeway chase from _Runaway_ with Tom Seleck, (I think that's the title) with rocket powered RC cars.

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Steen
BlabberMouth, the Next Generation.

Posts: 1430
From: Maryville, TN, USA
Registered: Jan 2000

posted August 09, 2002 22:23     Click Here to See the Profile for Steen   Click Here to Email Steen     Edit/Delete Message   Reply w/Quote
mcgyver27 wrote:
The one main problem is the power of the Radio control transmitter-most non-licensed ones have a 100-150 foot range so the pursuit car needs to be close to the R/C car.

Not to defend the movie, since I haven't seen it and I find it difficult to believe someone could pilot both their car and the remote control car at the same time in a chase sequence regardless of technology, I don't think the transmitter would be a problem. After all, if you're going to start sticking C4 in the car blow people up with it, I don't think you're going to pay much attention to FCC limitations on broadcast power.

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ZorroTheFox
BlabberMouth, the Next Generation.

Posts: 1859
From: Milton, WA, USA
Registered: Oct 2001

posted August 11, 2002 21:03     Click Here to See the Profile for ZorroTheFox   Click Here to Email ZorroTheFox     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by snupy:
You, me, and Zorro.Hmmm....

I think the first one in should reserve the seats, maybe we can get a group discount >;o) ...........Z

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