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# Friction

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1. Friction
Hi,

I'm a little confused about friction in the motion of roller coasters. The wheels move backwards, and exert a force on the track. The track exerts an equal and opposite force on the wheels (i.e. friction). This causes the car to move. However, all websites I have looked at say that the friction is not wanted as it wastes energy, slowing the car down. But surely it is wanted, as it makes the car move?

Also, when the forces acting on a roller coaster car are considered, they only seem to look at the weight and the normal contact force. I don't understand why they don't look at friction. Can anyone help?
2. because the wheels are round and its moving so when u model the coDter u model it not as a box, but like a ball, and a ball rolls so you dont really consider friction. If the wheels were square say, then you would consider the friction. I hope i'm not talking utter rubbish

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3. Re: Friction
(Original post by rock_climber86)
because the wheels are round and its moving so when u model the coDter u model it not as a box, but like a ball, and a ball rolls so you dont really consider friction. If the wheels were square say, then you would consider the friction. I hope i'm not talking utter rubbish

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I think I know what you mean, thanks very much
4. Re: Friction
(Original post by rock_climber86)
because the wheels are round and its moving so when u model the coDter u model it not as a box, but like a ball, and a ball rolls so you dont really consider friction. If the wheels were square say, then you would consider the friction. I hope i'm not talking utter rubbish

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Why do lots of people talk about friction in roller coaters like it is a bad thing, as if friction were not there, there would be no traction and the wheels would just spin?
5. Re: Friction
I'm really not sure about the question being asked... Can you quote the exact passage that you're referring to?

The wheels move backwards? I can't quite see why they would
Exerts a force on the track? Yep, because there is weight, there will be a force on the tracks
Track does exert an equal and opposite force - This is the reaction
I don't see why the reaction is responsible for causing motion in the roller coaster though. Friction opposes motion, it doesn't encourage motion.

Indeed, friction is not wanted. It will slow down on the tracks. That's why they use circular wheels instead of square ones - They can roll easily with minimal friction.

In mechanics, which you'll have to say if you've covered or not in maths, F=μR (Friction is equal to the coefficient of friction multiplied by the reaction). So a rougher surface creates more friction. When you're working out motions, you want to consider the sum of all the forces acting on a system. So for a roller coaster there is weight (mg), there is a reaction, there is force forwards (either by the chain lift, or because it's going downhill) and there's air resistance (usually negligible in physics experiments) and friction. Friction opposes motion, so less friction is better for the travel of the roller coaster.

I'm not entirely sure about traction though, I've never really covered it and can't say for certain what it actually is. I would've thought that it just be the normal reaction between the track and the wheels.
6. Re: Friction
(Original post by SillyEddy)
I'm really not sure about the question being asked... Can you quote the exact passage that you're referring to?

The wheels move backwards? I can't quite see why they would
Exerts a force on the track? Yep, because there is weight, there will be a force on the tracks
Track does exert an equal and opposite force - This is the reaction
I don't see why the reaction is responsible for causing motion in the roller coaster though. Friction opposes motion, it doesn't encourage motion.

Indeed, friction is not wanted. It will slow down on the tracks. That's why they use circular wheels instead of square ones - They can roll easily with minimal friction.

In mechanics, which you'll have to say if you've covered or not in maths, F=μR (Friction is equal to the coefficient of friction multiplied by the reaction). So a rougher surface creates more friction. When you're working out motions, you want to consider the sum of all the forces acting on a system. So for a roller coaster there is weight (mg), there is a reaction, there is force forwards (either by the chain lift, or because it's going downhill) and there's air resistance (usually negligible in physics experiments) and friction. Friction opposes motion, so less friction is better for the travel of the roller coaster.

I'm not entirely sure about traction though, I've never really covered it and can't say for certain what it actually is. I would've thought that it just be the normal reaction between the track and the wheels.
What I mean is that when a wheel turns, it exerts a force on the track. The track exerts a force on the wheel (friction) which makes the car move forwards. If there was no friction (e.g. on ice) the wheel would just spin. Therefore, friction is needed to move.
7. Re: Friction
I think this is becoming quite a deep question.

Well with roller coasters, there isn't the same driving force as with a car. With a car, you do want that traction (which is why you buy grippy tyres to get a good purchase on the road) or the wheels would spin. But even so, the beneficial friction to move forwards will be countered by the negative friction causing the car to slow down. That's one of the reasons why a road bike has really thin wheels (small contact patch) compared to a mountain bike with thick wheels (big contact patch) to get the best grip.

The roller coaster uses the wheels to stay on track, and the force it has to move is given through gravity (generally)... If it runs out of speed, then it doesn't have any motors of its own to move along. It will want a low friction because it wants to glide over the tracks. Yeah, the wheels will still spin as it's not a perfect system, but if there is too much friction then the wheels will wear quickly and the ride will be too slow.
8. Re: Friction
(Original post by chroli1)
Why do lots of people talk about friction in roller coaters like it is a bad thing, as if friction were not there, there would be no traction and the wheels would just spin?
maybe what they're talking about is the friction between where the axle of the wheels connect to the carriage? The friction there would cause the coaster to slow down, heat up, waste energy. Indeed there needs to be friction for the wheel to move on the track. But also note that a lot of coasters is down to momentum - chain pulls carriage to top of the coaster ride, then gravity brings it down, so if you think of it like this, friction even between the wheels and the carriage would not be important Move onto the next question - this question sucks

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Last updated: July 6, 2012
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