Questions on speed of an object from different reference points.

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Discussion Overview

The discussion revolves around the speed of a bullet fired from a moving car, as observed from different reference points. It explores the implications of relative motion and how speed is perceived by different observers, including theoretical considerations and definitions of speed.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Person-A fires a gun from a car traveling at 60mph, and the bullet is assumed to travel at 300mph. Person-B, who is stationary, observes the bullet traveling at 360mph.
  • Some participants argue that the bullet's speed is constant at 300mph regardless of the observer's position, while others suggest that it can be perceived differently based on the reference frame.
  • One participant proposes that the speed of the bullet has different meanings depending on the context, such as its impact speed when hitting a wall.
  • There is a discussion about the definitions of speed and how they relate to the measurements taken by different observers.
  • Participants mention the Galilean transformations and the differences in velocity addition in classical mechanics versus special relativity.

Areas of Agreement / Disagreement

Participants express differing views on whether the bullet's speed is always the same regardless of the observer. Some agree that the bullet's speed can be perceived differently, while others maintain that it is a constant value depending on the reference frame.

Contextual Notes

There are unresolved assumptions regarding the definitions of speed and the conditions under which measurements are taken. The discussion also touches on the implications of air resistance and the timing of measurements relative to the bullet's motion.

Who May Find This Useful

This discussion may be of interest to those studying physics, particularly in the areas of relative motion, reference frames, and the principles of classical mechanics and special relativity.

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

hi forumsuser! welcome to pf! :smile:
forumsuser said:
Imagine that the bullet travels at a constant speed of 300mph for some time …

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? :confused:
 
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 resistance 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.
 
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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 traveling at 300mph.

So, your possible answers are:

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

by Drotzer
But an objects speed is that speed regardless of where you are viewing it from and regardless of what the person viewing it, thinks.

by tiny-tim
... in which case that presumably means relative to the ground, and Drotzer is correct ...

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

(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

and from DaveC426913's possible answers:

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

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?
 
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hi forumsuser! :smile:

(just got up :zzz:)
forumsuser said:
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.

admirably clear! :biggrin:
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.
…
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.

all correct :smile:

(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 :wink:)
 
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
tiny-tim said:
hi forumsuser! :smile:

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

(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 :wink:)


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.
 

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