A question of speed (Not the drug).

Bipolar Joe

Let's say you're driving along at fifty kilometres an hour. You point one those speed guns out the window, and someone travelling in the opposite direction at the same speed goes by. Does the speed gun read fifty or one hundred kilometres?

Seedy Arling

Yes.

kneemos

Fifty boss.

Schism

I thought those speed guns had to be stationary to get a correct reading.

Sergeant

0

kneemos

Schism wrote: »

I thought those speed guns had to be stationary to get a correct reading.

It's the theory he's wondering about.I don't think the speed of the car would matter compared to the speed of the laser.

Reamer Fanny

About 3 fiddy

seamus

Depends on the type of gun being used.

Standard doppler-based guns will register a double reading.

Laser-based guns in theory shouldn't be affected, but I don't actually know how they work, so they may be.

Peregrine

100kmh

Look up about 'the Doppler Effect'. It's how radar guns work, it's also why you hear a higher frequency siren when you're in front of an ambulance or other ER vehicles, even the sound from a train.

The definition of the Doppler Effect is:

The apparent change in frequency of a wave due to relative motion between the wave source and the observer

When you are stationary, the velocity of a car moving 50kmh relative to you is 50kmh.

But if you are moving 50kmh in one direction and another is moving 50kmh towards you, the velocity of the other car relative to you is 100kmh.

I think..

Here's the Doppler Effect..explained by um..Dr. Sheldon Cooper :pac:

Bipolar Joe

Nimrod 7 wrote: »

100kmh

Look about 'the Doppler Effect'. It's how radar guns work, it's also why you hear a higher frequency siren when you're in front of an ambulance or something.

The definition of the Doppler Effect is:

The apparent change in frequency of a wave due to relative motion between the wave source and the observer

When you are stationary, the velocity of a car moving 50kmh relative to you is 50kmh.

But if you are moving 50kmh in one direct and another is moving 50kmh towards you, the velocity of the other car relative to you is 100kmh.

I think..

Very clever, didn't think about it in terms of the doppler effect. That makes sense.

kneemos

What's the difference between dobbler radar and the radar aircraft and ships use?

Wossack

both use radar - just the speed guns are specifically interested in the freq change due the doppler effect, and use that to judge speed (they've a much smaller scope/scan arc too, course)

martinedwards

a hand held radar gun will give the total converging speed because it doesn't know that it's moving.

ie 100 kph

I THINK that the incar ones that police forces use are tuned to the speedometer (or maybe GPS as it's more accurate) to do the sums and subtract the gun's speed.

OldGoat

25kph.


Just thought I'd throw that in to mess wid yer head.

Knifey Spoony

The Doppler effect is the reason why the speed gun won't give the correct answer. As the car with the gun is moving , there will be a shift in the frequency emitted by the gun and then a further shift when the emitted frequency is then reflected back off the approaching car. So, the gun should give a higher speed than there actually is.

If we are taking in terms of velocities, then we are taking about relative velocities and in that case the speed is 100 kph. This is because velocity is a vector and since both cars are traveling in opposite directions, the velocity of the approaching car will be negative relative to the speed of the car i.e. relative velocity = 50 kph - 50 kph = 0 kph.
EDIT: Relative velocity = 50kph - (-50kph) = 100 kph

Or I could be wrong and just taking outta my hole.

Peregrine

Knifey Spoony wrote: »

The Doppler effect is the reason why the speed gun won't give the correct answer. As the car with the gun is moving , there will be a shift in the frequency emitted by the gun and then a further shift when the emitted frequency is then reflected back off the approaching car. So, the gun should give a higher speed than there actually is.

If we are taking in terms of velocities, then we are taking about relative velocities and in that case the speed is 0 kph. This is because velocity is a vector and since both cars are traveling in opposite directions, the velocity of the approaching car will be negative relative to the speed of the car i.e. relative velocity = 50 kph - 50 kph = 0 kph.

Or I could be wrong and just taking outta my hole.

I did ponder over that for a few minutes.

Now my understanding of relative velocity is quite basic but to get the relative velocity you take away the velocity of car A from car B. That would give us zero but because like you said the second car has a negative velocity to the first one so wouldn't it be

50kph - (-50kph) which would equal to 100kph?


Maybe I'm talking out of my arse too :pac:

PinkLemonade

The relative velocity is 100, dunno if you need to bring the doppler effect into it, I don't know much about speed guns.

jimgoose

The speed is 100km/h, whether measured with a Doppler radar, a laser or a photon cannon.

Wossack

Knifey Spoony wrote: »

The Doppler effect is the reason why the speed gun won't give the correct answer. As the car with the gun is moving , there will be a shift in the frequency emitted by the gun and then a further shift when the emitted frequency is then reflected back off the approaching car. So, the gun should give a higher speed than there actually is.

If we are taking in terms of velocities, then we are taking about relative velocities and in that case the speed is 0 kph. This is because velocity is a vector and since both cars are traveling in opposite directions, the velocity of the approaching car will be negative relative to the speed of the car i.e. relative velocity = 50 kph - 50 kph = 0 kph.

Or I could be wrong and just taking outta my hole.

nah

if they were both travelling in the same direction - relative velocity would be 0
if one is stationary (gun car, or target car, doesnt matter) - velocity would be 50
in this case both in opposite directions - velocities combined, 100


doppler would have no effect on the beam as it leaves the gun initially (bit in bold above) - the gun car is not changing in speed during the beams 'shot'

Guill

If you are driving at 50kph and hit a parked car you are involved in a 50kph crash.
If you are travelling at 50kph and hit a car travelling towards you at 50kph that's a 100kph crash.

Your speed is relative.

yeppydeppy

ok, can we get some proper physics nerds in here 'cause now I'm confused? I thought all velocity was relative depending on direction (as already mentioned) so it would be 50 - 50 = 0?

Peregrine

yeppydeppy wrote: »

ok, can we get some proper physics nerds in here 'cause now I'm confused? I thought all velocity was relative depending on direction (as already mentioned) so it would be 50 - 50 = 0?

Yes but relative velocity is calculated by taking away the two velocities.

So it's 50 take away -50, which is 50 + 50 = 100

Guill

yeppydeppy wrote: »

ok, can we get some proper physics nerds in here 'cause now I'm confused? I thought all velocity was relative depending on direction (as already mentioned) so it would be 50 - 50 = 0?

50-50 if you were both going in the same direction. Think about it if you were running towards someone and they were running towards you at the same speed would the speed = 0????

IM0

well it all depends if there are walls in front of the cars or not and how far away they are and how fast the cars are travelling

IM0

and in before general theory of relativity

antodeco

My thinking would be that if you measured a car driving towards you, the speeed would be different, as the distance travelled by moving towards an object at speed, decreases by the time it has been sent out. So basically, at point of "shooting the beam" the first point will be at 50kph, but when it bounces back, both cars are now closer, so the return distance is shorter. Would that not mean 75kph, as the speed of both needs to be divided by two, to compensated for the shorter return distance of the beam?

Wossack

speed of light we're talking about here - not me throwing a frisbee at some dude and asking them to write down their speed on it and pass it back

Peregrine

This is really bothering me now :mad:

Knifey Spoony

Nimrod 7 wrote: »

I did ponder over that for a few minutes.

Now my understanding of relative velocity is quite basic but to get the relative velocity you take away the velocity of car A from car B. That would give us zero but because like you said the second car has a negative velocity to the first one so wouldn't it be

50kph - (-50kph) which would equal to 100kph?


Maybe I'm talking out of my arse too :pac:

Got my basic maths arse-ways. You're right, it should be 100 kph, forgot the negative sign.

Knifey Spoony

Wossack wrote: »

doppler would have no effect on the beam as it leaves the gun initially (bit in bold above) - the gun car is not changing in speed during the beams 'shot'

But the Doppler effect has nothing to do with changing speed, just the velocities of the source and observer. So since the gun has a velocity at the time it is used there should be a Doppler shift on the signal that leaves the gun.

pa990

Bipolar Joe wrote: »

Let's say you're driving along at fifty kilometres an hour. You point one those speed guns out the window, and someone travelling in the opposite direction at the same speed goes by. Does the speed gun read fifty or one hundred kilometres?

approaching it reads 50k/hr
receding it reads -50k/hr