# Speed difference

1. Jul 13, 2011

### Hippasos

Hi!

I was havin again one of my "weaker moments" as we were trawelling to our wacation at a freeway and numerous cars were speeding to pass our car. I begun to think about speed differences like 120 km/h compared to 60 km/h is of course 60 km/h. And 0 km/h compared to 60 km/h is also 60 km/h. But wait - 0 km/h mans you're not moving at all... What a heck - please explain.

Thanks!

2. Jul 13, 2011

### ghwellsjr

Well, if your speeds are the values that each car reads on their speedometers, that is, they are all relative to the pavement on the freeway, then everything you said is correct.

What is the problem that you feel you need to have explained?

3. Jul 13, 2011

### nitsuj

I agree, very confusing.

It's as if those highway vehicle speeds need a term. One that eludes to a reference frame.

Maybe "ground speed" would work.

4. Jul 13, 2011

### Hippasos

"What is the problem that you feel you need to have explained? "

:D

Ok.

To me it seems confusing that we are seemingly able to calculate the SPEED difference between objects that even doesn't MOVE ie. doesn't have a measurable SPEED... or it is 0.

Hope this clarified... :D

5. Jul 13, 2011

### Staff: Mentor

Yes, 60 - 0 = 60, even if there are units attached.

6. Jul 13, 2011

### ghwellsjr

Nope--it doesn't clarify.

Maybe you could give one example of two objects, state their speeds and directions and what you think their speed difference is or why you think it can't be calculated or measured.

7. Jul 13, 2011

### OnlyMe

I think both ghwellsjr and nitsuj have provided the answer as quoted below. The speedometer in a car measures speed/velocity relative to the road, which is a common rest frame of reference, for each of the cars. The road is not moving so its velocity is 0, as far as the speedometers in cars are concerned.

8. Jul 13, 2011

### Staff: Mentor

Speed is measured between two objects - any two objects. Sometimes it is zero. What's hard to understand?

9. Jul 14, 2011

### Hippasos

russ

Well it stroke me then as like with stationary objects no speed calculations would apply. As if they (stationary objects) were somehow without measure or unit thereof. Of course we mark the stationary objects' speed as zero unit but it then crossed my mind as objects - when stationary - are in such a state that wouldn't "fit" "somehow" in speed calculations...

Cheers

10. Jul 14, 2011

### ghwellsjr

The only time a speed of zero would be a problem in a calculation is when it is in the denominator of an equation. For example speed = distance / time, right? So we can say time = distance / speed, correct? Now let's use that equation to calculate how long it takes to go 1 km at a speed of 0 km/hr. Well we are trying to divide 1 by 0 which is not allowed. And, of course, in this simple example, it is obvious that you're not going to get anywhere going 0 km/hr.

11. Jul 14, 2011

### yuiop

Imagine you are driving at 60 mph and another car going in the same direction is also travelling at 60 mph and is alongside you then the speed of your car relative to the other is 0 mph so in the reference frame of the other car you are stationary. So are you rreally stationary or really travelling at 60 mph? You might think it is obvious that you are doing 60 mph because that is what your speedometer tells you, but what if the world and road where rotating in the opposite direction at 60 mph relative to the Cosmic Microwave Background then your speed would be zero relative to the background. So you can see speed is just a relative concept and depends on the reference frame that you measure it in. The 60 mph indicated by your speedometer is just the speed of your car relative to a road which may or may not be moving so you never know your true absolute speed.

12. Jul 14, 2011

### nitsuj

I imagine if you were going east in a car at 10km/h and another car is going [STRIKE]west [/STRIKE] in Reverse!! at 10km/h, that really mucks up the calculation.

13. Jul 14, 2011

### ghwellsjr

Why do you think that mucks up the calculation? Each of you sees and measures the other one to be going 20km/hr. You would see him traveling west and he would see you traveling east.

Unless by "going [STRIKE]west [/STRIKE] in Reverse" you meant he was actually facing west but traveling east, in which case each of you would see the other as stationary with respect to yourself.

14. Jul 15, 2011

### Hippasos

That's true of course, and if we could absolutely forget all about relativism - would we then know any absolutely stationary object in the universe?

15. Jul 15, 2011

### nitsuj

lol, it was me trying to be funny. If a car going 0km/h relative to a car going 60km/h was confusing for the OP, I figured if they thought of the car not going 0km/h but negative 10km/h (i.e. reverse) relative to the car travelling 60km/h it would add a whole new level of confusion, or (as I truly hoped) make the idea of traveling 0km/h more comprehensible.

16. Jul 15, 2011

### ghwellsjr

If by "relativism" you mean the Theory of Special Relativity, then, no, there would still be no other theory that can "know" of any absolutely stationary object. Prior to Einstein, scientists believed in an absolutely stationary ether, but they didn't know where it was. That's what Michelson and Morley's famous experiment was all about--to locate the absolutely stationary ether--but they could not. As a result of that experiment, several scientists developed the Lorentz Ether Theory in which it is presumed that an absolutely stationary ether exists, and it's a perfectly viable theory, even today, but since no experiment can determine where that ether is, Einstein argued that the concept of absolute rest does not belong in a theory.

So even a theory that believes in the existence of an absolutely stationary object still cannot identify one.

17. Jul 15, 2011

### ghwellsjr

Adding a whole new level of confusion seems to be what you are good at but it isn't funny to me. I still can't figure out what you are talking about.

When you talk about a car going a certain speed, you have to be clear about what that speed is relative to. That's why I said in my first post:
For example, you could have said, you're going down the freeway at 60km/h. Another car passes you at 70km/h. Relative to that other car, you are going backwards at 10 km/hr.

Or you could have said, you're going down the freeway at 60km/h. Next to you on your right is another car going 60km/h. You need to take the next offramp so you slow to 50km/h. Now you are traveling at negative 10km/h relative to the other car.

In your latest post, I don't see how you could have come up with a negative 10km/h with only a bunch of cars traveling at 60km/h. And in your previous post #12, you only set up a confusing scenario with no conclusion.

18. Jul 15, 2011

### nitsuj

I could have said it many different ways, thanks for the point! [STRIKE]Oh and to throw your comment back at you, being confused seems to be what you are good at.[/STRIKE]

Last edited: Jul 15, 2011