My maths isn't up to this any more! Which is worse for the driver of a car driving at 30 mph?
1. Hitting a car of similar mass also travelling at 30 mph, head on
2. Hitting a vast, static concrete block, at 60 mph
My instinct says 2 (effect of the square of the velocity) apart from crumple zones etc.
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If scenario 1 is taken to be the baseline
1. Hitting a car of similar mass also travelling at 30 mph, head on
This is an impact which is very well simulated by an impact into a static concrete block at 30 mph.
One way to visulaise this equivalence is to imagine a crassh between two cars which are perfect mirror images of each other - the common surface of the imapct would form a flat surface, normal to the road, which would not have any velocity - it would be a surface fixed in space, and hence well represented by the concrete block.
Then, yes, scenario 2,
2. Hitting a vast, static concrete block, at 60 mph
will be an impact with 4 times the kinetic energy.
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All I need to know is that both are going to hurt and are not recommended. :-)
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If two objects collided in outer space with an approach speed of 60 mph, how would you know whether they were both doing 30 mph or one was stationary and the other doing 60mph?
That's the concept I find difficult to understand. There would appear to be no difference, relatively, but why the energy difference?
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To begin with, it might be helpful to list the kinetic energy of each object before and after each collision.
For easy reckoning, perhaps assume a mass of 1 kg for each object, and use 10 m/s instead of 30 mph for the vlocity.
Last edited by: Number_Cruncher on Mon 9 Apr 12 at 12:08
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>> To begin with, it might be helpful to list the kinetic energy of each object
>> before and after each collision.
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>> For easy reckoning, perhaps assume a mass of 1 kg for each object, and use
>> 10 m/s instead of 30 mph for the vlocity.
>>
>>
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OK, but velocity relative to what? It's easy with 2 cars on a road, but with 2 spaceships, how do you define which is moving?
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Does a spaceship have any much/mass out in deep space?
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>>Does a spaceship have any much/mass out in deep space?
The mass is fixed, it's the quantity of matter that is there.
There might not be much weight, as there might not be much of a gravitational field.
As an aside, I can't wait for the LHC to hint to us what mass is, or isn't - as with so much, we don't really know the truth, we just have a model that has been created by scientists following the scientific method.
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Where I was heading towards in answer to Cliff's question was that if you consider first conservation of momentum, the velocity after the impact for the 60 / 0 case isn't zero, and therefore the whole of the kinetic energy associated with 60 mph is not lost.
Last edited by: Number_Cruncher on Mon 9 Apr 12 at 14:14
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Thank you NC! I hoped you might turn up and confirm or deny my hazy recollections of 1/2M V squared!
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Quite thought provoking.
If a car was travelling and hit a stationary identical head on, whch car would be safer for passengers?
Presumably the travelling car would suffer faster deceleration but airbags should be deployed.
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>>whch car would be safer for passengers?
Newton's third law (for every action, there's an equal and opposite reaction) tells us that the accelerations felt would be the same in each car.
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>> Newton's third law (for every action, there's an equal and opposite reaction) tells us that
>> the accelerations felt would be the same in each car.
Thankyou, so presumably the stationary car's passengers would be worse off if the airbags didn't deploy....not sure exactly how complicated the decision to fire airbags is, would they go off if the car was propelled backwards?
Still can't see the stationary car having equal accleration as the moving cars deceleration though, crumple areas will have soaked some of the speed and force therefore acceleration potential out of impact, wouldn't it?
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One of the things which bothers me ( lots of things bother me like net curtains in top flats for example ) is how do astronauts avoid satellites etc when they're zooming around through the cosmos at unbelievable speeds? I mean, swerving round one doesn't really seem like a reliable option at 18.000 mph or whatever. What's more, and to return to the topic of the thread, what on earth ( or off it ) effect would such a collision speed have on a spacecraft?
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>>lots of things bother me like net curtains in top flats for example
Telescopes!
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People who throw litter out of cars or throw litter anywhere.Tailgaters my pet hate.Please and thank you helps.Net curtains which are mucky.>:)
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>>would they go off if the car was propelled backwards?
Yes.
One of the ways that manufacturers reduce their development costs is to use so called hy-ge testing where a body is punted backwards down a track by a big gun which replicates the crash pulse. In this way, they can repeat the testing to optimise airbag firing time / seat belt pre-tension to reduce the risk of the full scale crash tests giving embarassing results.
>>though, crumple areas will have soaked some of the speed and force therefore acceleration potential out of impact, wouldn't it?
Any deformation of crumple zones must take some force. This same force is decelerating the moving car, AND accelerating the stationary car. At all times during the imapct, each car feels the same force, and therefore experiences the same acceleration.
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>> At all times during the impact, each car feels the same force, and therefore experiences the same acceleration.
Our physics teacher demonstrated this to us many years ago with a football and a pupil's foot, both covered in foil and each connected to a timer. Most of the class wrongly refused to accept that during the kick, the ball was only accelerating whilst the boot was in contact with it.
My question to NC: If the forces are the same, why does a moving car sustain more damage when colliding with a stationary one? I've seen that happen twice myself, both times the moving car came off worse by far.
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>>Most of the class wrongly refused to accept that during the kick, the ball was only accelerating whilst the boot was in contact with it.
I'm in agreement with most of the class. Why would it continue to accelerate (positively, not negative deceleration) once the external force had been removed?
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>> >>Most of the class wrongly refused to accept that during the kick, the ball was
>> only accelerating whilst the boot was in contact with it.
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>> I'm in agreement with most of the class. Why would it continue to accelerate (positively,
>> not negative deceleration) once the external force had been removed?
Rephrasing Dave's post, "the ball was only accelerating whilst the boot was in contact with it, but they (wrongly) refused to accept this".
Last edited by: Focus on Mon 9 Apr 12 at 20:18
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>> Rephrasing Dave's post, "the ball was only accelerating whilst the boot was in contact with it,
>> but they (wrongly) refused to accept this".
Ta F, I've had a long day :)
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>> Ta F, I've had a long day :)
Yeah, I've been DIY-ing too :)
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>> My instinct says 2 (effect of the square of the velocity) apart from crumple zones
>> etc.
>>
I am not sure I have understood NC's reply at post no.2.
if you treat the cars as immovable objects (i.e. no crumple zones, and in effect just the same as lumps of concrete), then there is no difference whether the driver has an impact as in case 1 or as in case 2.
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In the head on, the point where the cars are in contact wouldn't move - neither car will push the other one backwards. So to car A, car B might as well be a concrete block. This applies whether the cars have crumple zones or not.
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I think there is! In the head on case car A is decelerating from 30mph to stationary; in the case of car A hitting a concrete block at the combined speed (60 mph) it is going twice as fast and having to dissipate or absorb 4 times the energy. BTW I wish I hadn't asked this my brain hurts again! I would be shocked if NC was in error and I understand his explanation.
Last edited by: Meldrew on Mon 9 Apr 12 at 17:52
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Apology Mellers I was comparing 30+30 head on with 30 into a concrete block. Should have been clearer.
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No problem Oh Wise One! I have off days 4 times a week!
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Me too.
The energy to be dissipated in the two car example is the speed of each car times the mass of each car. A linear relationship, but messy mathematically if the two cars are of different mass, or differ in speed. And in the case of the one car hitting an immobile block collision, the speed is now times two and the mass is the same as one car. So actually that makes the block impact have less energy to dissipate in total but I bet the car that hit the block looks worse!
My head hurts...
F=ma, no squared terms here
Last edited by: Slidingpillar on Mon 9 Apr 12 at 17:52
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The square of the speed SP. twice as fast = 4 times the energy. Cocodamol isn't working I quit!
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Spotted before you posted but was doing something.
I'll go and lie down in a darkened room...
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from www.gekandassociates.com/index.htm
THE PHYSICS OF A COLLISION
www.gekandassociates.com/Physics%20of%20a%20Collision.htm
(I there is a typo one-third of the way down the page - 80.33 should be 80.66)
also see
ca.autoblog.com/2010/11/21/road-myths-physics-of-a-head-on-collision/
Road Myths: Physics Of A Head-On Collision
Last edited by: John H on Mon 9 Apr 12 at 21:10
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If I am riding on my photon at the speed of light towards Jupiter, and crash into a photon fired from Jupiter at the speed of light, why is the approach velocity only the speed of light, rather than 2*c?
;-)
Last edited by: Lygonos on Mon 9 Apr 12 at 21:45
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What happens to the sound waves created by a tuning fork if put in a vacuum? will it vibrate longer than in atmosphere?
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There aren't any (other than those contained in the metal), and yes, a little longer.
Last edited by: Lygonos on Mon 9 Apr 12 at 22:01
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>> and yes, a little longer.
Won't it be a lot longer?
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Dunno - much of the kinetic energy loss is from the metal heating up due to it's vibration as opposed to that lost pushing air.
Maybe it's out there on youchoob somewhere....
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>> Dunno - much of the kinetic energy loss is from the metal heating up due
>> to it's vibration as opposed to that lost pushing air.
Fair enough - I thought it would be the other way round, but I'm standing corrected :)
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Yes, the radiation of the sound waves represents a loss of energy from the tuning fork - so, preventing that loss of energy by putting the tuning fork in a vacuum prevents that loss of energy from the vibating system.
The loss factor of a metal is typically in the region of 0.1% - and this represents how the vibration energy is converted to heat - the loss factor is much larger if there are joints or welds.
Torsional pendulums or suspended mirror systems are sometimes used in gravitational experiments, and these are usually held within a high vacuum environment. These devices are built to have a very low loss factor, and typically take a few weeks for the residual installation vibrations to die away before the experiment can begin.
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The examples given in the Mythbusters link still confuses matters by using a concrete block rather than a stationary car. To be absolutely clear, what happens:
1) 2 cars hit head on, each doing 30
2) 1 car doing 60 hits a stationary car
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>>what happens:
1) 2 cars hit head on, each doing 30
2) 1 car doing 60 hits a stationary car
Assuming that the cars don't bounce off each other like snooker balls, but remain attached, in terms of energy, crash 1 and 2 are remarkably similar.
In scenario 2, the cars will both be travelling at 30 mph immediately after the impact, so, each car will have changed its velocity by 30mph - which is exactly the same as the case in scenario 1.
It is the residual velocity after the impact which makes scenario 2 differ from the concrete block case.
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>> It is the residual velocity after the impact which makes scenario 2 differ from the
>> concrete block case.
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Thanks NC.
I'd overlooked the transfer of energy and hence velocity to the stationary car. What I'd been getting at was how you would define moving and stationary in a scenario where only relative velocity had any meaning.
I now see that it doesn't matter.
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What are suspended mirror systems? How do these experiments benefit us in normal use.
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More about suspended mirrors here;
www.ligo.caltech.edu/advLIGO/scripts/subsystems_sus.shtml
These experiments are designed to help us understand how gravity works. There are gound based experiments, LIGO, Advanced LIGO, VIRGO, and others, and a space based experiment which is being planned, LISA.
It's fundamental physics, in the same way as the large hadron collider is looking at particle physics, these gravitational experiments are extending our understanding of gravity.
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Thanks lots to read and take in what's left of the grey mass.
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>>Thanks lots to read and take in what's left of the grey mass.
Yes, not the usual links from a motoring forum!
One happy aspect of this is that UK universities are contributing strongly to these experiments in a similar way to how we have contributed to the LHC.
One of the longer term aims of the space based experiment, LISA, is to form a space based "observatory" for gravitational radiation** - there is the possibility of able to "see" events like black holes orbiting each other.
** No-one has ever measured gravitational radiation, or gravitational waves. They were predicted by Einstein as part of General Relativity, and one aim of the experiments is to detect the radiation.
Phrased another way, we don't really know how gravity travels, and it has never been measured.
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Regardless of which is worse, I'll try equally to avoid colliding head-on with an approaching car and an enormous immovable stationary concrete block!
;-)
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Energy/Collision Question! - Number_Cruncher
