Physics question.

Colin62 said:
I can't believe that something as elementary as this needs to be explained to anyone older than about four years old...
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Boggles the mind, some of the responses are are even worse than the original question.
 
Colin62 said:
I can't believe that something as elementary as this needs to be explained to anyone older than about four years old...
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*ahem*

120km/h in my car = 120km/h in any car/truck/bakkie

we are arguing about this concept, that if a car and a truck can both travel at 120km/h in the same direction, the car will be faster than the truck due to the fact the the car is lighter... and again due to the fact that the aerodynamics of the car are better than those of the truck. keep in mind that the speed is constant 120km/h...

we both agree that more power will be needed to make sure the truck is at 120km/h and less power will be needed to make sure the car is at 120km/h.

i say that the car will be faster due to the weight for one... he say the will be the same

please help...
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http://mybroadband.co.za/vb/showthread.php/182279-120km-h-in-my-car-120km-h-in-any-car-truck-bakkie
 
Sinbad said:
you won't believe how many people can't get their heads around things like this.
I remember once a massive argument about placing a model plane on a treadmill that moved backwards at the same speed as the plane moved forward, would it be able to take off - and people were adamant that it wouldn't.
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Throwing a spanner in the works here, but if the plane is stationary in space (forward movement cancelled by treadmill movement) it won't be able to take off, as there is no airflow over the wings.

As to the car, if it is loose on the dyno (as for a brake test at roadworthy, not a dyno but similar) the rocket will push the car off if the force is greater than the gravitational force keeping the wheels between the rollers. This has already been posted. If the car is tied down, why is there a thread about it?
 
furpile said:
Throwing a spanner in the works here, but if the plane is stationary in space (forward movement cancelled by treadmill movement) it won't be able to take off, as there is no airflow over the wings.
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You fail. Why will the treadmill hold the plane stable? It's not driven by its wheels.
 
furpile said:
Throwing a spanner in the works here, but if the plane is stationary in space (forward movement cancelled by treadmill movement) it won't be able to take off, as there is no airflow over the wings.
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Why would the plane remain stationary? It's driven by the prop/jets, the wheels just hold it off the ground... The wheels will spin twice as fast as if it was on a static runway yes but it'll still take off, in fact Mythbusters proved this:

[video=youtube;YORCk1BN7QY]https://www.youtube.com/watch?v=YORCk1BN7QY[/video]
 
ScottulusMaximus said:
Why would the plane remain stationary? It's driven by the prop/jets, the wheels just hold it off the ground... The wheels will spin twice as fast as if it was on a static runway yes but it'll still take off, in fact Mythbusters proved this:

[video=youtube;YORCk1BN7QY]https://www.youtube.com/watch?v=YORCk1BN7QY[/video]
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Wheels spinning on an aircraft has nothing to do with taking off. The only thing that makes a plane take off is airflow over the wings, which is usually done by moving the aircraft foward thru the air.
Find the right type of aircraft with a prop that blows the air back over it's own wings and it can lift-off without moving forwards.
 
Pox said:
Wheels spinning on an aircraft has nothing to do with taking off. The only thing that makes a plane take off is airflow over the wings, which is usually done by moving the aircraft foward thru the air.
Find the right type of aircraft with a prop that blows the air back over it's own wings and it can lift-off without moving forwards.
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Yeah it's called a helicopter ;)
 
Sinbad said:
Yeah it's called a helicopter ;)
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Nope. A helicopter still moves its 'wings' (in this case the rotor blades) thru the air, it doesnt move the air over the wings.
Small single-engine prop-driven planes would work, a lot of the air it pushes backwards goes over part of the wings.
Something like a stunt-plane would do it. Small and very light.
 
Pox said:
Nope. A helicopter still moves its 'wings' (in this case the rotor blades) thru the air, it doesnt move the air over the wings.
Small single-engine prop-driven planes would work, a lot of the air it pushes backwards goes over part of the wings.
Something like a stunt-plane would do it. Small and very light.
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It's all relative to your point of view ;) If you're sitting on the blade, the air's moving over you
 
evilstebunny said:
If the absolute size of the rocket is larger than the square of the hypotenuse of the dyno, and the relative weight of the car is less than half the length of the force exerted by gravity on the circumference of the combination of the two objects times pi, then say goodbye to both your car and the dyno.. (even tho the wheels still won't turn).
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But does this take quantum effects into account?
 
Rickster said:
On land speed record car, the rockets arnt connected to a drive train.
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Yes but the car is held back with straps.

A dyno measures power at the wheels....there fore the wheeels need to be self-propelled to measure anything.
 
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Swa said:
The rocket would transfer energy to the car making it move. Because the wheels are not frictionless they would turn regardless of being on the road or a dyno. But because the rocket transfers energy to the car directly and not to the wheels it would also propel the car forward. If this force is greater than that of gravity the car won't continue to fall back into the pit of the dyno and will eventually go off it.
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Yes the car would move...but off the dyno and into the wall in front of it.

The wheels won't move because of the rocket directly, but purely by virtue of moving out of place and going forward.
 
SauRoNZA said:
Yes but the car isn't held back with straps.

A dyno measures power at the wheels....there fore the wheeels need to be self-propelled to measure anything.
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Of course the car is strapped down or it'll just drive off the rollers...

cl550_dyno6-1.jpg

Turn the engine off and strap a rocket to the roof and either the straps will break and it'll interview the wall or the rocket will pick up the whole dyno into the wall. If the car is not strapped down it'll be pushed off the dyno(yes the wheels will turn if still in contact with the dyno) and once again say hello to the wall
 
Sinbad said:
You fail. Why will the treadmill hold the plane stable? It's not driven by its wheels.
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Read your post again:
a model plane on a treadmill that moved backwards at the same speed as the plane moved forward,
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That means the plane only applies enough thrust to stay in one place, thus no airflow over the wings (apart from propwash) and thus no lift to take off. Where is the fail? The wheels have nothing to do with it. If the plane's engine is strong enough to propel it forward faster than the treadmill is moving back, then it will accelerate and will be able to take off.
 
ScottulusMaximus said:
Of course the car is strapped down or it'll just drive off the rollers...

View attachment 302048

Turn the engine off and strap a rocket to the roof and either the straps will break and it'll interview the wall or the rocket will pick up the whole dyno into the wall. If the car is not strapped down it'll be pushed off the dyno(yes the wheels will turn if still in contact with the dyno) and once again say hello to the wall
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Sorry that was meant to read IS, not isn't.

My bad.

****

Fixed now.
 
ScottulusMaximus said:
Why would the plane remain stationary? It's driven by the prop/jets, the wheels just hold it off the ground... The wheels will spin twice as fast as if it was on a static runway yes but it'll still take off, in fact Mythbusters proved this:

[video=youtube;YORCk1BN7QY]https://www.youtube.com/watch?v=YORCk1BN7QY[/video]
Click to expand...

I am not saying the plane would remain stationary, that was the original question.
a model plane on a treadmill that moved backwards at the same speed as the plane moved forward
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In this video the plane uses more thrust and thus accelerates, as you can see with the traffic cones. Then obviously it can take off. But using just enough thrust to remain in one place it won't take off.
 
If the vehicle is tied to the dyno. No change in speed. The cables holding the car will counter the force of the rocket. So no additional acceleration. If the vehicle is not tied to the dyno, there will be a force exerted. One of two things can happen:

1. If the rocket's force is sudden, there is a big possibility that the force will overcome both the friction between the dyno and wheel of the car and propel the vehicle forward off the dyno into the great yonder. Assuming that the rocket is a real rocket (like the Rocket Assisted Take-off rockets).
2. If the rocket's force is applied slowly, the force will apply indirectly to the wheel. It is simple that the dyno will absorb the force to forward motion that would've occurred due to the application of the force of the rocket. It will increase until the force is higher that the friction forces and then 1 will apply.

Disclaimer: It is Friday. Rough day. I last saw dynamics textbook last century. I might be wrong
 
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