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Metla
19-04-2010, 07:25 PM
300kg mass, traveling at 100km/h, if it hit an object what would its weight register at?

GameJunkie
19-04-2010, 07:34 PM
depends on the mass of the object its hitting doesnt it?

300x277.77m/s = 83331 (momentum = mass x velocity) i think

its been a while since college:stare:

Renmoo
19-04-2010, 08:19 PM
Ah, you got only the first half, GameJunkie

Impulse = Change in Momentum / time

Let's say the object bounced off the target and resumes its motion at a 180 degree angle at a speed of 10km/h:

Momentum towards = 83331
Momentum away = 8333.1

Change in momentum = 82497.9 kg m/s

To determine impulse:
Let's say it takes one second for the momentum to change its vector:
Impulse = Change in momentum / time
= (82497.9 kg m / s) / 1 s
= 82497.9 kg m s^(-2)
= 82497.9 Newton (http://www.daviddarling.info/encyclopedia/N/newton_unit.html)
= ....... (I'm stuck)

Metla
19-04-2010, 08:24 PM
I'm what?:waughh:

Sweep
19-04-2010, 08:24 PM
I thought Metla actually wanted the Kinetic energy.

Metla
19-04-2010, 08:35 PM
Impact force.

From what I can gather the attached pic is the formula, means nothing to me though.

pcuser42
19-04-2010, 09:11 PM
Force = change in impulse divided by change in time. ([velocity final - velocity initial] over [time final - time initial])

So the force is what you're after. (Weight is not equal to mass.)

Let's see... Assuming it stops, (360 - 0) / (tf - ti) {100km/h = 360 metres per second}

We still need to know how quickly it stopped.

gary67
19-04-2010, 09:27 PM
Impact force.

From what I can gather the attached pic is the formula, means nothing to me though.

Looks more like a music score sheet

pcuser42
19-04-2010, 09:32 PM
Looks nothing like sheet music :lol:

zqwerty
19-04-2010, 09:32 PM
F = ma where a = v/t The well-known Force equals Mass times Acceleration
therefore
Ft = mv
re-ordering
F = mv/t
thus as t (time of impact) tends to 0 (zero) F tends to infinity, hence before you drop your precious iPhone onto the concrete pavement wrap it in a spongy cover to increase the time of impact away from zero and thus decrease the impact (F).

Metla
19-04-2010, 09:34 PM
Force = change in impulse divided by change in time. ([velocity final - velocity initial] over [time final - time initial])

So the force is what you're after. (Weight is not equal to mass.)

Let's see... Assuming it stops, (360 - 0) / (tf - ti) {100km/h = 360 metres per second}

We still need to know how quickly it stopped.

Motorcycle into police car.

I'd assume motorcycle and rider deflected off and traveled an unknown distance (bike would have been destroyed so it wouldn't be a huge distance), The police car was spun.

The question was just to ascertain if the bike traveling at road legal speeds would hit with enough force to shunt a car....

pcuser42
19-04-2010, 09:38 PM
I'll assume half a second to stop, but a precise value will help. :)

So carrying on, 360 / (0.5 - 0)
=360/0.5
=720N
=72kg

Hope I got it right :p

gary67
19-04-2010, 09:42 PM
Well I have seen a racehorse kick a car in the side and shunt one end of it sideways about 45°

19-04-2010, 10:38 PM
I'll assume half a second to stop, but a precise value will help. :)

So carrying on, 360 / (0.5 - 0)
=360/0.5
=720N
=72kg

Hope I got it right :p

You're a factor of 30 out: you forgot the mass & converted to m/s wrong.

100kph = 36m/s, so

As Jamuz said before, Force = (change in momentum)/time, so if we keep assuming that the bike just comes to a complete stop in 0.5s, then

F = ((36*300) - (0*300))/0.5
=21600N, or roughly equivalent to a weight of 2200kg

... the car would definitely move, I'd say ;)

pine-o-cleen
19-04-2010, 10:53 PM
I thought the title said 'quick' physics question?

johcar
19-04-2010, 11:46 PM
Don't forget to factor into the equation the inertia (in the direction the bike hit it) the car would have had at the time of impact. Plus the friction of the car tyres on the road. But since the car was doing a u-turn, and therefore moving, albeit at a lowish speed, the tyre friction would have been less than if it were stationary.

And the fact the I would assume the rider (at approximately 100kg), would have been separated from his bike (the other 200kg) shortly after impact. Whereas the bike would have continued shedding momentum on the side of the police car for several more moments.

So as pine-o-cleen so rightly pointed out, a "quick physics question?"!!!

I don't think there's an easy answer to this one. I wonder if Mythbusters could recreate it for us?

tweak'e
19-04-2010, 11:47 PM
Motorcycle into police car.

I'd assume motorcycle and rider deflected off and traveled an unknown distance (bike would have been destroyed so it wouldn't be a huge distance), The police car was spun.

The question was just to ascertain if the bike traveling at road legal speeds would hit with enough force to shunt a car....

just watched it on the news.
it dosn't take a lot to shunt a car sideways. hitting at the rear by the looks of it, bit of leverage effect, its effectively only pushing on half a car.
even tho it looses a fair bit of force as the rider comes off, even a 50km/h impact would shunt a car.

tho it looks like a @#\$ of a place to do a U turn :mad:

Cicero
20-04-2010, 08:15 AM
No one was present,so let the conjecture begin.......

Gobe1
20-04-2010, 08:46 AM
No one was present,so let the conjecture begin.......

Yes there was, 1 police officer, and one dead dude (got that from bill and ted :))

pcuser42
20-04-2010, 08:50 AM
you forgot the mass & converted to m/s wrong.

100kph = 36m/s

Oops, multiplied by 3.6 instead of divided :blush:

Still, 100km/h = 27.78m/s.

Cicero
20-04-2010, 09:00 AM
Yes there was, 1 police officer, and one dead dude (got that from bill and ted :))

That may have been a tricky one ,one was talking of witness's.

SP8's
20-04-2010, 11:16 AM
Lets get back to basics ..... 1 + 1 = ?

prefect
20-04-2010, 12:12 PM
Well I have seen a racehorse kick a car in the side and shunt one end of it sideways about 45°
What sort of car was it, a Cadillac or Heinkel Bubble car?

pcuser42
20-04-2010, 04:54 PM
Lets get back to basics ..... 1 + 1 = ?

Window :D

Sweep
20-04-2010, 05:21 PM
Lets get back to basics ..... 1 + 1 = ?

For what value of 1? :p

SP8's
20-04-2010, 08:09 PM
pcuser ..... wrong! correct answer is DOOR

sweep ..... value of 1 = X

rebels181
20-04-2010, 10:57 PM
What speed would the cop have to do to catch the ute had the the bike not hit his car? I would have thought about 170 -180 kph

Whenu
20-04-2010, 11:15 PM
Bit rusty but maybe this

KE =1/2mv^
=300*100^/2
=300*10000/2
=3000000/2
=1500000 Nm

R2x1
20-04-2010, 11:36 PM
300kg mass, traveling at 100km/h, if it hit an object what would its weight register at?
Well, ~ I guess it's weight would be still be 300 Kg

Metla
21-04-2010, 12:02 AM
Well, ~ I guess it's weight would be still be 300 Kg

So if you drove a 300kg weight into a set of scales bolted to the point of impact on the car at 100km/h it would register a reading 300kg?

Wait, I think I just stumbled onto a way for Mythbusters to test it.....

Metla
21-04-2010, 12:02 AM
That said, It wasn't a trick question, Just a poorly presented one.

Whenu
21-04-2010, 12:21 AM
Option B

109350 Joules
109.35 KiloJoules

http://www.csgnetwork.com/kineticenergycalc.html

pcuser42
21-04-2010, 08:31 AM
Well, ~ I guess it's weight would be still be 300 Kg

It's mass would still be 300kg, but it's weight would increase with the acceleration (if it was stationary, it's weight is 3000N - weight and mass are NOT the same thing).

R2x1
21-04-2010, 11:06 AM
Weight is the same as momentum? We are progressing in this millenium ;)
(I am presuming that nobody left the earth while the accident was under way, and gravitational anomalies were minimal.)

zqwerty
21-04-2010, 06:12 PM
In outer space you have Mass but no weight.

Sweep
21-04-2010, 06:17 PM
True. But how many motor cycles collide with police cars there?

pcuser42
21-04-2010, 06:42 PM
I had a think about the original question, and I think Metla is after momentum, not weight force/impulse.

The formula for momentum is mass times velocity. Momentum is conserved during a collision, hence if a moving motorbike hits a stationary car, if the motorbike stops (possible, but unlikely due to more physics laws :groan:), the car will be shunted back (but not as fast). Let's take the original values for the motorbike:

Velocity = 100km/h = 27.78m/s
Mass = 300kg

Momentum before the collision is 8334kgm/s, hence it must remain 8334kgm/s. Assuming the motorbike stops, the car (say 2000kg) will be shunted back at a velocity of x = 8334 / 2000 = 4.2m/s.

I know you didn't want to know how fast, but it's just to prove it will move back. :D

johcar
21-04-2010, 07:30 PM
In outer space you have Mass but no weight.

Unless you're not Catholic - then you have no Mass and no weight to get into heaven...

:devil

WalOne
21-04-2010, 07:55 PM
Unless you're not Catholic - then you have no Mass and no weight to get into heaven...

:devil

:lol:

Burnzee
21-04-2010, 08:42 PM
Hi Guys

Enough maths here to make your head spin but one simple truth has been overlooked.

Speed has never killed anyone, suddenly becoming stationary... That's what gets you. Game Junkies Quote. :devil :eek:

BURNZEE

Greven
21-04-2010, 10:28 PM
If the neighbours could hear the motorcyclist revving the hell out of his engine, he was almost certainly travelling at a speed far greater than the limit. Guy probably still would have died if he had obeyed the limit though - bloody stupid place to do a U turn.

Jester
21-04-2010, 10:31 PM
Given the extensive calculations, is this related to the plane on a treadmill question?

prefect
21-04-2010, 10:57 PM
People are quoting the stopping in the clear distance in front of you law, but imo it cant apply if someone pulls out in front of you.
I guess the federales will work out his speed at impact if he was hooning over 100 kph he would be at blame for the crash. My reasoning if he had not been speeding the fed could have pulled the u turn and the bike wouldnt have been in the area at the time.
I aint no saint I used to wind my Commando up to over 110mph no probs and my Z900 bored out to 1300cc used to check its tune by doing 200kph. Although the speedos may have been a bit optimistic.
When I was teenager in Mot I rode my Norton at 100mph with pillion thru a 10 mph zone at the Kina reserve.
I dont care about this guy for speeding if indeed he was its just a fact of life riding bikes on the open road. But dont heap blame on the fed he is probably at an all time low now.

Metla
22-04-2010, 12:05 AM
I don't know if he was speeding or not, But I do know the silly old coot should never had his claims broadcast on TV.

Not only is it preposterous that he can claim to gauge the speed of a motorcycle he never saw and knows nothing about (like exhaust note) but in case no one noticed he also claimed the back of the police car was ripped off, which is clearly not the case.

As for the cop, he should be at an all time low, A man died due to his actions.

Metla
22-04-2010, 12:13 AM
I guess the federales will work out his speed at impact if he was hooning over 100 kph he would be at blame for the crash. My reasoning if he had not been speeding the fed could have pulled the u turn and the bike wouldn't have been in the area at the time.

If you were doing 120Km/h down the motorway and you were killed by someone throwing a concrete block off an over bridge, would you accept it was your fault because you were on that piece of road?

To take it to its next logical step, Would you be at fault for leaving home when you did rather then waiting another few minutes?

The courier driver shot dead by the police, should he have had his travel time examined to see if he may have been speeding before arriving at the scene where the police killed him?

The speed of the motorcyclist may have contributed to the severity of the injuries but was in no way responsible for the actions of the police officer.

The Police officer should thank god it wasn't a stock truck coming over that blind crest or a family in a car.

Gobe1
22-04-2010, 10:23 AM
Not only is it preposterous that he can claim to gauge the speed of a motorcycle he never saw and knows nothing about (like exhaust note) but in case no one noticed he also claimed the back of the police car was ripped off, which is clearly not the case.

I dont know about that Metla. we get a driver down our road in the early hours of the morning occasionally, you can definatley tell by the sound as he goes past and wakes the whole neighbourhood up he is doing well over 50kph. It also has that distinctive nissan turbo sound. Probably a Laurel or Cefiro. Once awake i just wait for the crunch as he misses the corner at the end. If that ever happens i will get out of bed in my pyjamas and run down to laugh.
You are correct in what speed they are actually doing is impossible

Gobe1
22-04-2010, 10:25 AM

If you were doing 120Km/h down the motorway and you were killed by someone throwing a concrete block off an over bridge, would you accept it was your fault because you were on that piece of road?

To take it to its next logical step, Would you be at fault for leaving home when you did rather then waiting another few minutes?

The courier driver shot dead by the police, should he have had his travel time examined to see if he may have been speeding before arriving at the scene where the police killed him?

The speed of the motorcyclist may have contributed to the severity of the injuries but was in no way responsible for the actions of the police officer.

The Police officer should thank god it wasn't a stock truck coming over that blind crest or a family in a car.

Also if he was travelling at 100 he would not have been in that same time or place, what if the sky was made of candy floss, you could argue this forever.
Stock truck, interesting concept lol

prefect
22-04-2010, 10:37 AM
I have never been known for rational thinking. Maybe the rest of the world is mad and I am the only sane person?
Yeah I know fat chance.
Just trying to point out that when speeding on a bike it has consequences specifically death.

Metla
22-04-2010, 11:06 AM
Speeding on a bike causes death?

How long have you been dead for?

prefect
22-04-2010, 01:36 PM
What about speeding on bike can cause death. I have lost 2 mates in bike accidents although one was pissed and speeding.

Scouse
22-04-2010, 01:41 PM
Hi Metla.
re: 300kg mass, traveling at 100km/h, if it hit an object what would its weight register at?

Would the formula change if the 300kg mass was following (possibly attracted by) a 2 tonne mass travelling in the same direction and slightly ahead of it?

Metla
22-04-2010, 03:12 PM
What about speeding on bike can cause death. I have lost 2 mates in bike accidents although one was pissed and speeding.

:illogical

Thats akin to saying crossing the road can cause death.

Utterly meaningless, The act itself doesn't kill people, doing it stupidly or beyond your capabilities is another matter entirely.

Metla
22-04-2010, 03:15 PM
Hi Metla.
re: 300kg mass, traveling at 100km/h, if it hit an object what would its weight register at?

Would the formula change if the 300kg mass was following (possibly attracted by) a 2 tonne mass travelling in the same direction and slightly ahead of it?

If you inserted 10 trucks into the equation would it make any difference?

The truck has no bearing on the course of events. If the police officer has such poor judgement that he would cause a death of a person over a potential ticket then it was only a matter of time.

pcuser42
22-04-2010, 03:23 PM
Given the extensive calculations, is this related to the plane on a treadmill question?

No, it's just that no one knows how to do it :p

SP8's
22-04-2010, 04:42 PM
At last count, this quick (yeah right) physics question has been on for 4 days, been viewed by 1,029 and received 53 replies ... of which only 10% have given a "reasonable" answer or information in an attempt to solve the original problem.

Priceless !!

pcuser42
22-04-2010, 04:43 PM
That's physicists for ya :p

shermo
23-04-2010, 01:35 PM

However, if we assume the motorbike rider was princess diana then: http://physics.info/acceleration/ this has the useful number of 70-100g acceleration experienced in her fatal crash. Of course Diana was probably going faster than 100kph.

Another useful number for force I found was fatal limits of 15-45g depending on orientation. Since this guy died, it was probably > 15. If you can find a more useful acceleration number use that instead. I didn't google for long.

A 300kg motorbike decelerating at 30g produces a force (this is the 'weight' you asked for) of 88kN.

Now, the second part of your question: could this 'spin' a police car? Let's assume that the bike hit the car side on in line with an axle. The coefficient of a tire on concrete is 1.7, and the weight of the car over that axle is about 7kN, so it would need a force of about 12kN to move it.

88>12 so yes, although this is highly dependant on the g force assumed in the collision.

shermo
23-04-2010, 01:43 PM
That answer sucks, so here's a better way to approach it.

Assume inelastic collision (note that we know this is NOT the case, but it makes things calculable). Also, assume that since the bike hit the car off its centre of mass we can halve the mass of the car (also NOT true, but in the absence of knowing exactly where the bike hit the car it's a good guess)

Momentum before collision = momentum after collision.

Mbc = 300*28 = 8400kgms
Mac = 1000*x = 8400kgms (half a 1500kg car)
velocity after collision = 8400/1000 = 30.24kph.

Let's bypass all the impulse, coefficient of friction stuff. If you're driving at 30.24kph and lock your tires, do your tires skid? Probably

So this is consistent with the answer above. Yes, if the bike was doing 100k it could spin a police car.