# Car on a treadmill **URL:** \ **Category:** Factual Questions\ **Created:** [September 6, 2008, 8:39pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384 "2008-09-06T20:39:50Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 7, 2008, 3:14am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/41 "2008-09-07T03:14:28Z")
> [@Zhing](#): > > In **my** scenario this doesn’t happen! The track and car are still to an outside observer, or are both moving in opposite directions at equal speed relative to an observer. That’s the way the treadmill is set up! In that case, the car would move forward at speed _x_. The treadmill will be moving backward at speed _x_. The wheels would be moving over the surface of the treadmill at a speed of _x \* 2_. So if the car is moving forward at 50kph and the treadmill is moving backward at 50kph, then the outside observer will see the track moving backward at 50 kph, the car moving forward at 50kph, and the speedometer in the car will be reading 100kph. Where’s the problem? ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [September 7, 2008, 3:27am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/42 "2008-09-07T03:27:02Z")
> [@Zhing](#): > > If the car doesn’t move, the rollers can’t either. The premise is “The car moves in one direction, while the conveyor moves in the opposite direction”. It’s the same thing. The car moves in one direction, the roller moves in the other. The roller is like a tiny conveyor belt. This is basic physics, you need to understand this before you go any further. The motion of the car is coupled to the wheels, and the motion of the wheels is coupled to the treadmill (or roller). This is the key difference between the airplane scenario and a car scenario… the motion of an airplane is almost completely uncoupled from that of the wheels. If the treadmill or roller exactly opposes the motion of the car, then by definition the car cannot move. A roller in this case is exactly the same as the treadmill. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [September 7, 2008, 3:27am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/43 "2008-09-07T03:27:38Z")
> [@Zhing](#): > > Yes, the track moves. Nevertheless? ---
**Author:** ![Zhing](https://avatars.discourse-cdn.com/v4/letter/z/9d8465/32.png) [@Zhing](https://boards.straightdope.com/u/Zhing)\ **Post date:** [September 7, 2008, 3:47am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/44 "2008-09-07T03:47:51Z")
> [@Johnny\_L.A](#): > > Where’s the problem? What the fuck! I didn’t know the answer to what would happen SO I ASKED! That WAS the fucking problem! It finally sunk in what I was asking after I clarified at least five times and now you’re going to act like a genius by giving the answer AFTER SOMEONE ALREADY GAVE IT and I acknowledged that I was satisfied with the answer? Well goody for you! ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 7, 2008, 3:57am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/45 "2008-09-07T03:57:32Z")
> [@Zhing](#): > > What the fuck! I didn’t know the answer to what would happen SO I ASKED! That WAS the fucking problem! It finally sunk in what I was asking after I clarified at least five times and now you’re going to act like a genius by giving the answer AFTER SOMEONE ALREADY GAVE IT and I acknowledged that I was satisfied with the answer? Well goody for you! You know, they make decaffeinated coffees that taste just as good as regular coffee. ---
**Author:** ![Jman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jman/32/4416_2.png) [@Jman](https://boards.straightdope.com/u/Jman)\ **Post date:** [September 7, 2008, 4:02am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/46 "2008-09-07T04:02:35Z")
We all seem to have gotten off on the wrong foot. I honestly think half the issues in this thread were simple miscommunications between the OP and the responders…we didn’t quite understand the question in full, and it led to a buildup of stuff. I hope we haven’t put you off the SDMB…we’re usually much more cordial…I apologize for the confusion. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 7, 2008, 4:28am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/47 "2008-09-07T04:28:15Z")
> [@Zhing](#): > > If I’m understanding you correctly, the car stays still to an outside observer and if the rollers were a treadmill belt, it would be moving to an outside observer.. That’s not the scenario I’ve described multiple times. Try reading posts #14 and #18. No this is incorrect. The car in fact did not move (except a bit from side to side) the wheels were going 100 MPH. The fact that we did not hit the brick wall that was 10’ in front of us is proof enough for me that there was no forward motion. If we were to substitute a treadmill belt for the set of rollers there would be no difference. Assuming that the rollers match the speed of the wheels, the car does not move. That is exactly what I was describing. > [@](#): > > Chronos gave me an explanation I’m satisfied with and I believe it’s correct. Maybe you guys can argue with him. **Chronos** is correct, you however keep restating his position incorrectly (see above) I think what you are trying to ask is: _If I put a car on a treadmill where the belt is moving at 30 MPH, and I accelerate the car to and indicated 60MPH what will the speed of the car to an outside observer? A) 0 MPH **B) 30 MPH forward** C) 30 MPH backward D) Speed of light squared_ 60MPH forward - 30 MPH backward = 30 MPH forward. ---
**Author:** ![Zhing](https://avatars.discourse-cdn.com/v4/letter/z/9d8465/32.png) [@Zhing](https://boards.straightdope.com/u/Zhing)\ **Post date:** [September 7, 2008, 4:48am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/48 "2008-09-07T04:48:48Z")
> [@Jman](#): > > I hope we haven’t put you off the SDMB…we’re usually much more cordial…I apologize for the confusion. Thanks, Jman. I appreciate that. > [@Rick](#): > > No this is incorrect. The car in fact did not move That’s what I said, and you just quoted me saying it: “the car stays still to an outside observer”. In my scenario, the track and car are still to an outside observer, or are both moving in opposite directions at equal speed relative to an observer. That’s the way the treadmill is set up. > [@Rick](#): > > **Chronos** is correct, you however keep restating his position incorrectly (see above) I don’t see how I restated his position incorrectly. > [@Rick](#): > > I think what you are trying to ask is: > _If I put a car on a treadmill where the belt is moving at 30 MPH, and I accelerate the car to and indicated 60MPH what will the speed of the car to an outside observer? > A) 0 MPH > **B) 30 MPH forward** > C) 30 MPH backward > D) Speed of light squared_ > > 60MPH forward - 30 MPH backward = 30 MPH forward. Close. The treadmill is setup so to an outside observer the belt will seem to go the same speed as a car on it in the opposite direction. The car and belt are both not moving at all at first. Then, the driver steps on the gas. What happens? I’m satisfied with the answer given way back in post #14. Thanks to all those who contributed and I apologize for any confusion I caused. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 7, 2008, 4:54am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/49 "2008-09-07T04:54:22Z")
**Johnny** ’s right. Switch to decaf ---
**Author:** ![Zhing](https://avatars.discourse-cdn.com/v4/letter/z/9d8465/32.png) [@Zhing](https://boards.straightdope.com/u/Zhing)\ **Post date:** [September 7, 2008, 5:11am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/50 "2008-09-07T05:11:00Z")
> [@Rick](#): > > **Johnny** ’s right. Switch to decaf Can you explain that comment please? I replied to your post calmly and cordially. You said I was incorrect about something and that the car didn’t move. I stated that that is what I had said (that the car didn’t move), implying that I wasn’t incorrect. I was not sarcastic or dickish in any way; I just tried to help you be more clear on my statement. You said I incorrectly restated Chronos’ position. I stated that I didn’t see how I did that. Actually, I don’t think I ever restated it at all, but regardless, I just mentioned that I didn’t think I did in a matter of fact way. You wrote about what you thought I was trying to ask, and instead of me taking a defensive position as some might when someone implies they tried to ask something and didn’t succeed (I clarified my question several times), I calmly restated my question. Why do I need decaf? Maybe you can tell me what I did wrong in that post to you so I can better understand what was inappropriate for further reference on this board. Thanks. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [September 7, 2008, 5:19am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/51 "2008-09-07T05:19:45Z")
Okay, I know you’re satisfied, but I’m playing a game with myself now, so don’t take it personally. 🙂 You said in the OP that the treadmill moves due to the speed of the CAR, but not the WHEELS. The way I’m seeing this is that the treadmill would keep stuttering on and off (at least until the car fell off from minute movements forward). 1.Car floors it, moves a little bit forward at (say) 60MPH 2. Treadmill reacts, goes 60MPH the other way 3. Car is not moving (even if wheels are), therefore the treadmill “compensates” by slowing to 0MPH again 4. Car goes 60MPH again 5. See 2 6. See 3. 7. Etc ---
**Author:** ![usedtobe](https://avatars.discourse-cdn.com/v4/letter/u/ec9cab/32.png) [@usedtobe](https://boards.straightdope.com/u/usedtobe)\ **Post date:** [September 7, 2008, 5:44am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/52 "2008-09-07T05:44:39Z")
> [@Q.E.D](#): > > That’s not at all equivalent to the situation posited in the OP: the front wheels are completely constrained. The car’s not going anywhere even if the roller seize up at full power. **YOU** be the one standing infront of the car when the rollers sieze, m’k? The problem here seems to center on the treadmill - some seem to think the readmill is powered somehow, and said power must show up somewhere (hence the idea that the speedometer will go bezerk). What happens if I’m operating noitice I did not say “DRIVING” he car. I shift to neitral, apply brake. The brakes perform as intended, and prevent the rotation of the wheels. Is the treadmill moving? Can it possibly move while the wheels are NOT turning? If the treadmill can do nothing but negate the propulsion of the turning wheels, then it is nothing more than a roller - take the two rollers in the pics, move them 30’ apart, and stretch a belt over them. The speedometer shows the distance the drive wheel (2 pi r and all that) would travel if it could. If the treadmilll can be powered independent of the wheel rotation, the car either turns its wheels to counteract the motion of the treadmill or it gets thrown off the treadmill. In no case will the (properlly functioning) speedometer indicate anything more than (2 pi r \* rpm) \* 60. p.s. the car on the roller is not indicating 2x its nominal speed (2 pi r, et. al) while on the rollers. Oh, hell, why not: what does the tach read in all this nonsense? Damn, I loved the plane on a treadmill discussion. :rolleyes: Sometime I fear for the republic when speculating on the voting power of some of my fellow citizens ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [September 7, 2008, 7:25am UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/53 "2008-09-07T07:25:32Z")
Key question: Is the car started? (That’s all I’ve got. I saw this thread and felt like the other Russian sub commander in hunt for red october when he says “Damn, these orders are 8 hours old”, except for me it was “Damn, this treadmill thread is 24 hours old, I’m too late”) ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [September 7, 2008, 2:21pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/54 "2008-09-07T14:21:59Z")
There’s a communication problem in this thread, and it’s not because of **Zhing**. The problem is that **Jman** , **Johnny L.A.** , **Rick** , **usedtobe** , **Jragon** , **Cosmic Relief** and possibly others are bringing their preconceptions of what **Zhing** is asking to the thread. The preconception is that they assume **Zhing** is referring to the speed of the car relative to the surface of the treadmill. He clarified way back in post 13 that he is talking about the speed of the car relative to the ground. With so many people making the same wrong assumption, it’s not surprising that **Zhing** is a little exasperated. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [September 7, 2008, 2:38pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/55 "2008-09-07T14:38:40Z")
Yeah, I think a few people leapt to conclusions about what **Zhing** was trying to ask. It’s worth pointing out that **Chronos** ’s answer: > [@](#): > > So the car’s speedometer reads 120, the car’s going 60 one way (relative to the ground), and the belt’s going 60 the other way. is exactly what happens with an airplane, too, except that their is no speedometer on the instrument panel to show how fast the wheels are turning. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 7, 2008, 3:14pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/56 "2008-09-07T15:14:09Z")
> [@ZenBeam](#): > > There’s a communication problem in this thread, and it’s not because of **Zhing**. The problem is that **Jman** , **Johnny L.A.** , **Rick** , **usedtobe** , **Jragon** , **Cosmic Relief** and possibly others are bringing their preconceptions of what **Zhing** is asking to the thread. The preconception is that they assume **Zhing** is referring to the speed of the car relative to the surface of the treadmill. He clarified way back in post 13 that he is talking about the speed of the car relative to the ground. > > With so many people making the same wrong assumption, it’s not surprising that **Zhing** is a little exasperated. I reposted exactly what **Chronos** said in post #28 (except I halved the speeds) and **Zhing** told me I was wrong. So he agrees with Chronos and disagrees with me.:dubious: I stand by my answer, he should consider decaf. ---
**Author:** ![Zhing](https://avatars.discourse-cdn.com/v4/letter/z/9d8465/32.png) [@Zhing](https://boards.straightdope.com/u/Zhing)\ **Post date:** [September 7, 2008, 3:52pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/57 "2008-09-07T15:52:46Z")
> [@Rick](#): > > I reposted exactly what **Chronos** said in post #28 (except I halved the speeds) and **Zhing** told me I was wrong. I told you that was close to the question I asked. You told me what you think I was trying to ask and I repeated what I in fact did ask. In your scenario, you “put a car on a treadmill where the belt is moving at 30 MPH”. All I did was clarify that in my scenario, both the treadmill and car start off at a standstill. I wanted to know what would happen if someone stepped on the gas. It was a minor clarification of my original question, but I wasn’t exactly calling you “wrong” about anything. I merely wanted to clarify that what I was trying to ask was exactly what I did ask. What about this makes it seem to you that I need decaf? I asked you nicely to explain what was so offensive or hyper or whatever is wrong with it that you would make that comment and instead of explaining it to me, you go ahead and say it again. I’m not sure what your problem is, but I wasn’t rude, nor did I act in any other manner that would indicate I’m hopped up on caffeine. So he agrees with Chronos and disagrees with me.:dubious: I stand by my answer, he should consider decaf. [/QUOTE] ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [September 7, 2008, 3:57pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/58 "2008-09-07T15:57:11Z")
> [@Rick](#): > > I reposted exactly what **Chronos** said in post #28 (except I halved the speeds) and **Zhing** told me I was wrong. So he agrees with Chronos and disagrees with me.:dubious: > I stand by my answer, he should consider decaf. I agree with **ZenBeam** , I almost posted the same thing but decided it was done. As I read the OP I was thinking “this is going to be fun”, then I saw that **Zhing** clearly pointed out it was the speed to the car, not the wheels, so most of the confusion was gone. Based on the wording, we know the conveyor is powered (otherwise it’s speed can’t be calibrated independent of the car effect on it). The only thing left is whether the statements refer to motion relative to the ground or not and relative to the ground is the only logical position to take based on his/her wording, but **Zhing** clarified all of this also. I read post #14 and thought, “it’s done”, then I read posters going in all kinds of different directions yet **Zhing** attempts to be calm and redirect to post #14 multiple times (I just went back and counted, 9 times he/she referred back to either post number 14 or **Chronos** explanation) I don’t think the issue is with **Zhing’s** coffee intake. ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [September 7, 2008, 4:05pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/59 "2008-09-07T16:05:26Z")
> [@Rick](#): > > I reposted exactly what **Chronos** said in post #28 (except I halved the speeds) and **Zhing** told me I was wrong. So he agrees with Chronos and disagrees with me.:dubious: > I stand by my answer, he should consider decaf. Was there forward motion of the car relative to the ground? Was there a powered conveyor that was able to dynamically adjust it’s speed (relative to the ground) according to the speed of the car relative to the ground? Here’s part of your post: > [@Rick](#): > > not a dyno but free spinning rollers Your post #28, while interesting and informative was simply not the same as the OP. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 7, 2008, 4:43pm UTC](https://boards.straightdope.com/t/car-on-a-treadmill/462384/60 "2008-09-07T16:43:52Z")
> [@RaftPeople](#): > > Was there forward motion of the car relative to the ground? > Was there a powered conveyor that was able to dynamically adjust it’s speed (relative to the ground) according to the speed of the car relative to the ground? > > Here’s part of your post: > Your post #28, while interesting and informative was simply not the same as the OP. My bad not #28 I restated **Chronos** ’s solution in post #47. Damn lack of coffee in the AM. :smack: [Previous page](https://boards.straightdope.com/t/car-on-a-treadmill/462384.md?page=2) [Next page](https://boards.straightdope.com/t/car-on-a-treadmill/462384.md?page=4)