Vehicle moving at a certain speed, onto a conveyor moving at same speed....
- Thread starter snedie
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Are you the same person that believes two cars that hit head on at 30mph have a combined impact speed of 60mph?
This. Sending the vehicle at 30 using cruise control/person inside maintaining the speed, would mean it would try and keep that up going down the conveyor. Without it, it would go so far with its momentum, then once stopped, travel back with the conveyor. Or if the car is dropped onto it at 30, then the conveyor is totally ******.
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Assuming the speed of the wheels and conveyor won't change and if there is indefinite friction between the wheels and conveyor then the car would stop instantly.
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Well nobody mentioned the plane was moving forward, thats the assumption that sends this argument in circles. If for whatever reason the plane can move regardless of the belt speed then its possible to take off. Its a trick question, no fair 
If you assume equal masses and entirely elastic collision (not ideal but the principle is the same), then yes.
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I have one sandwich which I cut in half, leaving me with two sandwiches. Where did the other sandwich come from? 
I have always struggled with this plane one.
If the plane is still and the conveyor is still, the plan thrusts up, it will move forward and take off, but then a car would equally move forward.
However, if, as the wheels of the plane start rotate, the conveyor moves in the opposite direction at a matched speed to the wheels, the plane will not move forward so air cannot move over it's wings
If the plane is still and the conveyor is still, the plan thrusts up, it will move forward and take off, but then a car would equally move forward.
However, if, as the wheels of the plane start rotate, the conveyor moves in the opposite direction at a matched speed to the wheels, the plane will not move forward so air cannot move over it's wings
The plane is not driven by its wheels, so the speed of the conveyor is irrelevant.
Consider it this way... you have a shopping trolley on a conveyor belt, and it has a handle protuding from the side, to an area not on the conveyor belt. If you stand behind that handle and push, the trolley will move forward at whatever speed you push it, regardless of the speed of the conveyor. The only effect the conveyor will have is make the wheels spin faster.
This is the same as the airplane on a conveyor, except the force is supplied by the engines rather than a handle.
This is the same as the airplane on a conveyor, except the force is supplied by the engines rather than a handle.
I doubt the car would get it's driven wheels on the belt would it?
Unless you hit the belt at exactly 30 while accelerating then you will be cruising at a constant 30 with the engine running with constant revs with a specific amount of throttle.
Surely then when you hit the belt with the driven wheels engine braking would kick in and you be pushed back off the belt? Unless you could absolutely nail the throttle and match the revs as you hit the belt?
Unless you hit the belt at exactly 30 while accelerating then you will be cruising at a constant 30 with the engine running with constant revs with a specific amount of throttle.
Surely then when you hit the belt with the driven wheels engine braking would kick in and you be pushed back off the belt? Unless you could absolutely nail the throttle and match the revs as you hit the belt?
I think it would depend if the wheels are allowed to spin faster than 30mph.
As it hits the conveyor belt, it's speed to a third party will be 30mph in one direction. The moment it touches the conveyor belt, ignoring air resistance the speed differential to the conveyor belt moving at 30mph in the opposite direction (to the third party) means that the wheels should start off at 60mph.
If you don't allow the wheels to spin faster than 30mph then you need an instantaneous brake applied (how you'd do that I don't know, would have to be incredibly strong) which never lets speed of the wheels increase to above 30mph.
In this scenario then the moment it hits the conveyor belt it would become stationary to a third party.
However if all that happens is that you take your foot off the throttle, then the engine will rev up to 60mph and then decelerate (engine braking) until the wheels reach 30mph at which point you put your foot on the throttle and maintain 30mph.
As it hits the conveyor belt, it's speed to a third party will be 30mph in one direction. The moment it touches the conveyor belt, ignoring air resistance the speed differential to the conveyor belt moving at 30mph in the opposite direction (to the third party) means that the wheels should start off at 60mph.
If you don't allow the wheels to spin faster than 30mph then you need an instantaneous brake applied (how you'd do that I don't know, would have to be incredibly strong) which never lets speed of the wheels increase to above 30mph.
In this scenario then the moment it hits the conveyor belt it would become stationary to a third party.
However if all that happens is that you take your foot off the throttle, then the engine will rev up to 60mph and then decelerate (engine braking) until the wheels reach 30mph at which point you put your foot on the throttle and maintain 30mph.
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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.
Yes but in this case the engine doesn't face the problem of being sent into overspeed. As they said that inertia means it is very difficult to accelerate the car instantly an additional 30mph as it makes contact with the HGV and so the wheels decelerate very quickly.
In that video, to an external observer the car is travelling at 30mph pre and post. Kinetic Energy is exactly the same. It requires next to no work to slow the circular velocity of the wheels.
In the case of the conveyor belt the kinetic energy has to be cut back in order for the wheels to continue spinning at 30mph. As such there must be some kind of braking force and this would have to act over a period of time to bring the vehicle from a positive velocity (KE>0) to that of zero.
That would not be as instant as going from a wheelspeed of 30mph -> zero as in the HGV test. It requires significantly more work.
Easy way to think about it is to simply forget that the car has drive. What happens if I throw a car without brakes onto a conveyor belt at 30mph? It will go forward for a bit before being sent backwards. You then add 30mph of drive to the wheels and so the final state you know must be a matching of the car wheelspeed and conveyor belt speed. What happens in between?
For an object to have momentum it must have velocity. So you should be careful about your use of terminology.
If the jet engines have a positive force, then the object must move forward unless there is an opposing force. The wheels are free to spin (designed with minimal friction) and so the conveyor belt provides next to no force on the plane. Therefore the plane must still move forward.
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muon said:
Yep, basically what he said. If you're interested in this kind of thing there are good online courses in kinetics/mechanics in these online university things that are popping up.