# Questions on speed of an object from different reference points.

1. Feb 27, 2012

### forumsuser

This thought experiment involves two people named Person-A and Person-B.

Person-A fires a gun from a car which is traveling at 60mph. Imagine that the bullet travels at a constant speed of 300mph for some time, assuming that the air friction is ignored.

Person-B is stationary observing the whole "shooting from the car" action.

From the point of view of Person-B, the bullet is traveling at 360mph, and from Person-A it is traveling at 300mph.

Is the above assumption correct?

What mechanism is behind above situation?

2. Feb 27, 2012

### Drotzer

I don't quite understand what you are getting at. But an objects speed is that speed regardless of where you are viewing it from and regardless of what the person viewing it, thinks.

3. Feb 27, 2012

### forumsuser

Oh I see, so are you saying that the bullet should never ever be 360mph? Even to Person-B?

4. Feb 27, 2012

### tiny-tim

welcome to pf!

hi forumsuser! welcome to pf!
your question is not clear …

do you mean that the bullet on this occasion travels at 300mph … in which case that presumably means relative to the ground, and Drotzer is correct

or did you intend to write that the bullet always travels at 300mph when the gun is stationary?

5. Feb 27, 2012

### luckis11

The question is answered by the definition of speed which is:
(increase of distance between two objects)/(time passed)

in every 1 sec that passes:
(increase of distance between car and bullet)=300metres
(increase of distance between ground and bullet)=360 metres
(increase of distance between car and ground)=60metres

I guess what you really want to ask is what speed of the bullet is the one that has more meaning, as it seems to have no meaning the bullet's speed relative to the centre of the Moon? The 360 has the meaning that its crushing power if it meets a wall on the ground, depends on the 360, if it has that speed at the time of the crush. Βut if it has that speed at the time of the crush which happened e.g. 2 secs after the explosion, then it didn't have 300 and 360 at the time of the explosion. And if it has 300 and 360 at the time of the explosion, then after 2 secs its speed is e.g. 300-30 from the car (and 360-30 from the ground) when it is fired from the car, and 300-10 from the ground when it is fired from the ground, because it has greater deccelararion because it meets higher air resistence when it is fired from the car. Also, if the crushing happens very soon after the explosion, then the shock wave of the explosion is near and plays its role. So, if you are getting confused whether the speed 300 or the 360 is the more "real", then you must take in account all such details.

Last edited: Feb 28, 2012
6. Feb 27, 2012

### DaveC426913

The fact is, the difference between their observations of the bullet's speed is going to be 60mph.

What's not clear is who (A or B) measures it travelling at 300mph.

So, your possible answers are:

A: 300mph B: 360mph
A: 240mph B: 300mph

7. Feb 27, 2012

### forumsuser

Could the following explanation be a more clear description of the scenario?

This thought experiment involves two people named Person-A and Person-B.

Person-B on the ground first fires a gun and measures the speed of the bullet to be 300mph, then he gives the same gun to Person-A.

Person-A fires the gun inside a stadium-sized automobile as it is traveling at 60mph relative to the ground and measures the speed of the bullet to be 300mph (this is relative to the automobile).

Person-A fires a second bullet out the window towards the direction at which the automobile is moving. Person-B on the ground is stationary observing the whole situation.

What is the speed of the bullet from the point of view of Person-B? how about Person-A?

I answered: From the point of view of Person-B, the bullet is traveling at 360mph, and from Person-A it is traveling at 300mph.

Well, as you can see from luckis11's equations:

and from DaveC426913's possible answers:

Person-B knows that when he fires the gun from the ground the speed of the bullet is 300mph, but apparently when he views the same gun being fired by Person-A, the bullet is traveling at 360mph. So, to respond to Drotzer's statement, it doesn't seem like that the bullet's speed is not always the same regardless of where the person is viewing it from.

But, maybe I can say that the bullet's speed is always 300mph as long as you are the one who is shooting it and measuring the speed.

Am I looking at this whole thing the correct way?

Last edited: Feb 27, 2012
8. Feb 28, 2012

### tiny-tim

hi forumsuser!

(just got up :zzz:)
admirably clear!
all correct

(btw, this is the 1D version of the general rule for combining 3D relative velocities … you combine them as vectors, eg VBG = VBT + VTG

this is the Galilean group of transformations … in special relativity, of course, we have to add relative velocities using the Lorentz group of transformations )

9. Feb 28, 2012

### forumsuser

The following message is a note to myself:

There's an error on my statement... I meant to say (omitted the word "doesn't"):

... it seems like that the bullet's speed is - not - always the same regardless of where the person is viewing it from. ...

10. Feb 28, 2012

### forumsuser

Thank you tiny-tim! : )
I've been trying to understand this concept for some time and now it's more clear.
I certainly will dig deeper and look into transformations and special relativity.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook