Vehicle moving at a certain speed, onto a conveyor moving at same speed....

DoneADougalOnSofa · 2 Dec 2013 at 02:21

Django x2 said:
So, we know that if a plane had no wheels, it would still take off. The wheels just make it easier to get forward momentum. But if that forward momentum was matched in the opposite direction, the plane would never move forward. It's wheels would just be spinning.

The plane will move forwards! The engines push against the air to give forward thrust. The wheels are free to rotate as fast as they like, and as long as the pilot doesn't apply the wheel brakes, it would be be a normal takeoff. You can move the conveyor at 200mph and the plane will still move forwards and take off as if from a static surface.
Elixir's shopping trolley post above is a good analogy.

Can we please put this subject to bed now?

DoneADougalOnSofa · 2 Dec 2013 at 02:44

To the original threadpoint (think I just invented a word), we need a small remote-control car and a friendly branch of Tesco... and preferably Dara O'Briain.

Srengam · 2 Dec 2013 at 02:54

Fuzz said:
If the two cars are both doing 30mph towards each other, that statement is correct.

Sorry,

if cars are identical, and all other variables are removed, each car hits with an impact exactly the same as hitting a stationary and immovable object at 30 mph.

DoneADougalOnSofa · 2 Dec 2013 at 03:12

No they don't

Tefal · 2 Dec 2013 at 03:25

Django x2 said:
So, we know that if a plane had no wheels, it would still take off. The wheels just make it easier to get forward momentum. But if that forward momentum was matched in the opposite direction, the plane would never move forward. It's wheels would just be spinning.

the only way you'd match the forward momentum is by spinning the wheels so fast the bearings welded to the axels and they locked up providing friction.

jet engines with tens of thousands of pounds of thrust will happily drag a plane along the ground with the undercarriage up and the belly dragging on the ground.

Tefal · 2 Dec 2013 at 03:27

dougal1331 said:
No they don't

they do as each car absorbs the same energy. for it to be the same as 1 car crashing at 60 one car would have to be indestructible and imoveable or both cars traveling at 60.

you can test this with some clay and a big swinging hammer like myth busters.

Elixir · 2 Dec 2013 at 13:58

Srengam said:
Sorry,

if cars are identical, and all other variables are removed, each car hits with an impact exactly the same as hitting a stationary and immovable object at 30 mph.

But there is twice as much energy involved in one system than the other. It's not at all relevant to the problem posted above.

Fuzz · 2 Dec 2013 at 18:07

Not wishing to move too far away from the OP's original question but...

Are you trying to tell me the damage to my car would be identical if I crashed into a parked car head on compared to one travelling toward me at 30 mph.

Err NO.
There would be twice the energy to absorb.

Atlas23 · 2 Dec 2013 at 18:58

So much unnecessary arguing. Can people not use Google nowadays?

http://scienceblogs.com/gregladen/2012/10/01/mythbusters-on-head-on-collisions/

In reference to two cars both going 50mph, head on collision with each other:

So it is like a 100 mph collision, shared evenly by two cars (so each gets 50 mph worth!)

estebanrey · 2 Dec 2013 at 19:10

Tefal said:
they do as each car absorbs the same energy. for it to be the same as 1 car crashing at 60 one car would have to be indestructible and imoveable or both cars traveling at 60.

you can test this with some clay and a big swinging hammer like myth busters.

OK so if I held my fist out and you ran into it at 10 mph you are saying that would hurt as much as running at me at 10mph with me throwing a punch at 10mph in your direction?????

Really?

lord filbuster · 2 Dec 2013 at 19:39

Fuzz said:
Not wishing to move too far away from the OP's original question but...

Are you trying to tell me the damage to my car would be identical if I crashed into a parked car head on compared to one travelling toward me at 30 mph.

Err NO.
There would be twice the energy to absorb.

Yep, if you crashed into a wall at 30mph or another car travelling at 30mph towards you, the results would be identical. Whilst there is twice as much energy to absorb, there is also twice the mass, assuming the cars are the same. Each car will absorb half the energy of the crash, hence the same result as one car into a wall.

Two cars colliding whilst travelling towards each other at 30mph (closing speed 60mph) is not the same at one car crashing at 60 into a wall/parked car.

Energy is equal to mass times velocity squared. Double the velocity you quadruple the energy. 60mph into a parked car would cause much more damage.

muon · 2 Dec 2013 at 19:39

estebanrey said:
OK so if I held my fist out and you ran into it at 10 mph you are saying that would hurt as much as running at me at 10mph with me throwing a punch at 10mph in your direction?????

Really?

Think about a cue ball hitting another ball with inelastic collisions.

If the object ball is still then the momentum and energy is transferred. The cue ball decelerates and comes to a stop because a force acts on the cue ball. The object ball accelerates and moves because of the equal and opposite force.

If that ball is coming towards you and you hit it with exactly the same velocity then what happens? Both balls must come to a stop (or bounce back, but we have said these collisions will be inelastic, as most car collisions are) due to conservation of momentum. So how much force has acted on the cue ball?

We know it takes a certain amount of force to bring it to a stop as we saw in the first scenario. It has come to a stop and so the change in momentum, change in kinetic energy must be the same.

edit:

I bet I turn out to be wrong. I believe the above makes sense from what I remember.

Atlas23 · 2 Dec 2013 at 20:20

lord filbuster said:
Yep, if you crashed into a wall at 30mph or another car travelling at 30mph towards you, the results would be identical. Whilst there is twice as much energy to absorb, there is also twice the mass, assuming the cars are the same. Each car will absorb half the energy of the crash, hence the same result as one car into a wall.

Two cars colliding whilst travelling towards each other at 30mph (closing speed 60mph) is not the same at one car crashing at 60 into a wall/parked car.

Energy is equal to mass times velocity squared. Double the velocity you quadruple the energy. 60mph into a parked car would cause much more damage.

/thread

Fuzz · 2 Dec 2013 at 20:52

lord filbuster said:
Yep, if you crashed into a wall at 30mph or another car travelling at 30mph towards you, the results would be identical.

Two cars colliding whilst travelling towards each other at 30mph (closing speed 60mph) is not the same at one car crashing at 60 into a wall/parked car.

Energy is equal to mass times velocity squared. Double the velocity you quadruple the energy. 60mph into a parked car would cause much more damage.

don't add in walls that don't move that confuses it further.
The orginal question to which I replied was

"Are you the same person that believes two cars that hit head on at 30mph have a combined impact speed of 60mph"

Newton’s 3rd Law
The force exerted by car 1 onto car 2 is equal in magnitude and opposite in direction to the force exerted by car 2 onto car 1.

Each car has kinetic energy K directly before the collision. At the end of the collision, both cars are at rest, and the total kinetic energy of the system is 0.

In one example, there is only one car moving, so the energy released during the collision is K. In the second example however, there are two cars moving, so the total energy released during the collision is 2K. So the crash where both cars are heading toward each other is clearly more energetic than the single car crashing into a parked one.

How can you argue with that? :confused:

Mercenary Keyboard Warrior · 2 Dec 2013 at 21:45

Mythbusters covered the head on as well..

http://www.youtube.com/watch?v=r8E5dUnLmh4

Django x2 · 2 Dec 2013 at 21:53

one last thing from me.

If the plane is stationary on a conveyor belt that starts moving backwards, the plane moves backwards, correct?

If the plane then starts its engines, it begins to move forwards, correct? At what point, if all the time it's travelling backwards, does it start to move forwards?

muon · 2 Dec 2013 at 23:30

Django x2 said:
one last thing from me.

If the plane is stationary on a conveyor belt that starts moving backwards, the plane moves backwards, correct?

If the plane then starts its engines, it begins to move forwards, correct? At what point, if all the time it's travelling backwards, does it start to move forwards?

Depends how fast the conveyor belt is moving. Friction is different to other types of forces. There is static friction and kinetic friction.

If the conveyor belt is moving slow enough then static friction will apply a force enough to move the plane back with the conveyor belt.

If however the conveyor belt is moving fast enough then the wheels will start spinning. Kinetic friction will mean that the plane will move backwards but not at the speed of the conveyor belt.

If the plane has enough thrust then it will be almost certainly be in the second state.

This is the same as on a runway. The plane needs to apply enough thrust to overcome static friction. It can overcome it there, and will do the same on a conveyor belt.

panyan · 2 Dec 2013 at 23:36

tom_e said:
Sensible answers are not allowed.

this disclaimer should be in all GD threads

Fuzz · 2 Dec 2013 at 23:48

Mercenary Keyboard Warrior said:
Mythbusters covered the head on as well..

http://www.youtube.com/watch?v=r8E5dUnLmh4

Yes and it makes perfect sense.
That's why I wanted to keep the wall out of the equation.
Both cars have crumple zones that absorb energy..

If you smack one car at 30mph into a wall that has ZERO crumple zone, ALL the energy goes into that singular car on deceleration. So you'll have x amount worth of damage to the front of the car.
I resist the urge to say "30mph worth of damage" there as it will not be the same as replacing the wall for another car.

Smack one car at 30mph into another stationary car that IS NOT an immovable object like the wall and you will damage both cars but not to the extent of the one car at 30mph into the wall scenario because some of the energy of that deceleration got imparted into the second car.
Smack two cars together at 30mph each and each will have 30mph's worth of damage to each. but twice the damage overall exactly as per the video

SO, if you are going to crash into something at 30mph, I suggest you pick the stationary car as both other scenarios (smacking into a wall or smacking into another 30 mph car head on) will impart more damage into your car.

Tefal · 2 Dec 2013 at 23:49

estebanrey said:
OK so if I held my fist out and you ran into it at 10 mph you are saying that would hurt as much as running at me at 10mph with me throwing a punch at 10mph in your direction?????

Really?

ok put it this way.

your car A has 30 units of energy, it crashes into a solid indestructable wall, it recies 30 units of energy worth of damage.

you now have two cars A and B each traveling towards each other each with 30 units of energy, so there are 60 units you can distribute.

please distribute these units of energy to show that car A should receive the equivalent of 60 units of energy worth of damage.