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Author Topic: What are the forces on a car travelling in a circle?  (Read 7133 times)

Erik Moeser

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Erik Moeser  asked the Naked Scientists:
   
Hi Chris:   A couple of years ago we talked about the amazing number of water molecules in a single drop.  I still "don't believe" the conclusion drawn!!

This day my brother Robert and I are talking about physics, and hope you can shed light on a difference of opinion we are having about "lateral" g forces.

We all know that a car traveling in a straight line down the big highway uses a certain amount of energy to offset aerodynamic drag, and mechanical and rolling friction.   If that same car at the same speed was traveling not in a straight line but in a large circle there would be additional energy needed to offset the likely increased rolling friction of the tyres, and would be "measurable" in the form of increased temperature in those tyres.

Here is the question.    Would there be even more energy needed just to offset the lateral g force generated by the circling car?  There would be "force" felt by occupants.Stuff would slide around in the car until it came to rest against something, but after that it would be "isometric".

One of us things that the lateral g force is somehow a "byproduct" and cost-free, and one of us thinks there would be a cost in terms of energy,  possibly the same as if the same g force was generated in straight line acceleration.

I hope you can shed some light on this!

Best,

Erik Moeser, Wisconsin, USA
Robert Moeser, Massachusetts, USA

What do you think?
« Last Edit: 09/07/2010 15:30:03 by _system »


 

Offline syhprum

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What are the forces on a car travelling in a circle?
« Reply #1 on: 09/07/2010 16:58:45 »
A similar problem was discussed last month concerning how much power was required to force a satellite into a 40 minute orbit around the Earth.
Little agreement was found amongst correspondents. 

http://www.thenakedscientists.com/forum/index.php?topic=31563.0
« Last Edit: 09/07/2010 17:01:48 by syhprum »
 

Offline Geezer

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What are the forces on a car travelling in a circle?
« Reply #2 on: 09/07/2010 17:26:03 »
Always a sucker for punishment, I'm going to say no - well, not once everything in the car had stopped moving around in the car and the car has reached a constant angular velocity at least.

Imagine your car is tethered to a pole by a cable and all sources of friction are miraculously removed. Once the car starts circling the pole, it will require no further energy to keep circling the pole.

In the real world, the tires are doing the same thing as the cable although they do generate friction in the process.

 

Offline tommya300

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What are the forces on a car travelling in a circle?
« Reply #3 on: 10/07/2010 02:31:37 »
Quote

If that same car at the same speed was traveling not in a straight line but in a large circle


You would need to define how large the radius of the circle if it is to large the angular velocity and coefficient of friction centrifugal/centripetal forces may be negligible.
 

Offline syhprum

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What are the forces on a car travelling in a circle?
« Reply #4 on: 10/07/2010 16:51:32 »
If the car is forced to travel in a circular path this requires a force to be generated at right angles to the direction of travel.
This force can be generated either by running on a banked track or by a cable attached to a central post in either case no power is required other than to replace additional frictional forces that might be caused.
If neither of these things are available the angle of the wheels must be set to drive the vehicle inwards towards the centre of the circle and this adsorbs power that can be easily calculated and must be supplied by the engine.
There are of course additional frictional losses that are not so easy to calculate.
« Last Edit: 10/07/2010 17:34:05 by syhprum »
 

Offline wolfekeeper

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What are the forces on a car travelling in a circle?
« Reply #5 on: 10/07/2010 20:24:45 »
The answer to this is that, in principle, a car going in a circle takes no more energy than one going in a straight line.

However, in practice, tyres give increased friction when you're cornering; the tyres scrub a little and the car tends to run wide of where the wheel hubs are pointing (the tyre has a 'slip angle' due to the distortion of the sidewall).

So in practice, you end up using more power going in a circle, but if rubber tyres were replaced with steel and the circle was a rail, the amount of power loss can be made a lot less.
 

Offline syhprum

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What are the forces on a car travelling in a circle?
« Reply #6 on: 10/07/2010 20:30:58 »
The road is flat, there is no banking, cable or rails you must use engine power to make it travel in a circle, how much ?.
 

Offline wolfekeeper

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What are the forces on a car travelling in a circle?
« Reply #7 on: 10/07/2010 22:19:03 »
The energy over and above that to go in a straight line almost entirely depends on the tyres.
 

Offline Geezer

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What are the forces on a car travelling in a circle?
« Reply #8 on: 10/07/2010 23:57:07 »
The road is flat, there is no banking, cable or rails you must use engine power to make it travel in a circle, how much ?.

Slightly more than you would need to travel in a straight line  ;D

 

Offline tommya300

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What are the forces on a car travelling in a circle?
« Reply #9 on: 11/07/2010 00:24:37 »
Quote
What are the forces on a car travelling in a circle?
.
 

Offline Geezer

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What are the forces on a car travelling in a circle?
« Reply #10 on: 11/07/2010 00:53:13 »
BTW - the OP was clear that there would be more energy dissipated in the tires during a turn. The question was:

"Would there be even more energy needed just to offset the lateral g force generated by the circling car?"
 

Offline wolfekeeper

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What are the forces on a car travelling in a circle?
« Reply #11 on: 11/07/2010 01:31:06 »
There's no other energy simply associated with the lateral g-force. Energy is force times distance (where distance is the component that is aligned with the force), ignoring the tyres, there's no movement along the line of the force, so there's no energy.

But the tyres have a slip angle, and this means that the tyres dissipate energy. The slip angle depends on the tyre construction though- a stiff sidewall gives less slip angle.
 

Offline tommya300

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What are the forces on a car travelling in a circle?
« Reply #12 on: 11/07/2010 01:52:47 »
There's no other energy simply associated with the lateral g-force. Energy is force times distance (where distance is the component that is aligned with the force), ignoring the tyres, there's no movement along the line of the force, so there's no energy.

But the tyres have a slip angle, and this means that the tyres dissipate energy. The slip angle depends on the tyre construction though- a stiff sidewall gives less slip angle.

It is called the coefficient of friction between the tire's surface and the roads surface.
It is a fractional number with no units from 0 to 1. Ideally it is 1.
It can be determined by the 2 tangent forces. Rise over the run. The ratio is the Pressing force divided by the dragging force.   
« Last Edit: 11/07/2010 01:57:24 by tommya300 »
 

Offline Geezer

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What are the forces on a car travelling in a circle?
« Reply #13 on: 11/07/2010 02:52:54 »
There's no other energy simply associated with the lateral g-force. Energy is force times distance (where distance is the component that is aligned with the force), ignoring the tyres, there's no movement along the line of the force, so there's no energy.


I think so. It's not so different from a vehicle travelling in a straight line on a level surface. If there were no losses to friction, the vehicle would keep travelling at the same speed without the input of any energy.
 

Offline wolfekeeper

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What are the forces on a car travelling in a circle?
« Reply #14 on: 11/07/2010 04:57:35 »
It is called the coefficient of friction between the tire's surface and the roads surface.
It is a fractional number with no units from 0 to 1. Ideally it is 1.
It can be determined by the 2 tangent forces. Rise over the run. The ratio is the Pressing force divided by the dragging force.   
No, tyres don't slip. (Also coefficient of friction can go above one).

The angle is due to rotation of the wheel and lateral wall distortion, the tyre is not sliding.
 

Offline tommya300

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What are the forces on a car travelling in a circle?
« Reply #15 on: 11/07/2010 05:51:04 »
It is called the coefficient of friction between the tire's surface and the roads surface.
It is a fractional number with no units from 0 to 1. Ideally it is 1.
It can be determined by the 2 tangent forces. Rise over the run. The ratio is the Pressing force divided by the dragging force.   
No, tyres don't slip. (Also coefficient of friction can go above one).

The angle is due to rotation of the wheel and lateral wall distortion, the tyre is not sliding.
.
Tires do slide, that is what makes the squeal noise when a vehicle goes into a turn to fast.
 I should of realized we might be talking of cohesive and covalent bonding, anything short of nailing the tire to the road is possible.
Which if this is the case, you are correct, that changes everything, other forces are involved, not friction.
.
That is the reason they banned super sticky compounds tires in asphalt motorcycle racing.
The rear tire would not break free to partially slide into the turns, the driver could not negotiate the turn and crashed. The tire became a virtual pivot point and the inertia flipped him and the motorcycle to the outside of the turn. Tires somewhat, need to partially slide, somewhat. 
The compound when heated from the ride would turn to an equivalent bubblegum consistency and stick to the road.

.
"Learn and understand the equation used to find the coefficient of static friction. It is a friction of rest in that it opposes any movement, and its maximum value is f(s) = u(s)N, where u(s) is the coefficient of static friction, and N is the normal force. The coefficient of static friction, therefore, is f(s)/N, a dimensionless number. A body that overcomes the static frictional force begins to move, and this force then changes to f(k)."
.
I had the form upside down.. it is the pulling force divided by the downward force of the object being moved.
« Last Edit: 11/07/2010 07:00:07 by tommya300 »
 

Offline Erik Moeser

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What are the forces on a car travelling in a circle?
« Reply #16 on: 11/07/2010 15:27:51 »
As Geezer points out the question is:

"Would there be even more energy needed just to offset the lateral g force generated by the circling car?"

All the talk of tires and friction, and motorcycles being able (or not) to negotiate a turn of given radius is interesting.  The key word in the question is "just" or maybe better put should be "only".  And "even more" gives credit to all who imply that the screeching tires are somehow involved in the need for more energy.   But it all may figure in to an answer, somehow.

There is a formula for lateral g, and I think it goes:   1.22 times radius in feet divided by the square of the lap time (one revolution) in seconds.

We know g force would "cost" if in a straight line, as in an accelerating car needing more gas (even without or beyond any friction of air or mechanical/tires).,,
 

Offline syhprum

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What are the forces on a car travelling in a circle?
« Reply #17 on: 11/07/2010 16:26:31 »
I quite agree, losses incurred in producing this acceleration (heating of tyres etc are additional).
 

Offline Geezer

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What are the forces on a car travelling in a circle?
« Reply #18 on: 11/07/2010 17:02:32 »
Just to avoid any confusion, other than friction, there is no energy input required to offset the centripetal force that makes the car travel in a circle.

It's no different from a flywheel. If you could remove all friction from a spinning flywheel, it would spin at the same same rate indefinitely without the input of any energy.
 

Offline tommya300

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What are the forces on a car travelling in a circle?
« Reply #19 on: 11/07/2010 19:19:41 »
Quote from: Erik Moeser  link=topic=32662.msg315376#msg315376 date=1278858471
As Geezer points out the question is:

"Would there be even more energy needed just to offset the lateral g force generated by the circling car?"

All the talk of tires and friction, and motorcycles being able (or not) to negotiate a turn of given radius is interesting.  The key word in the question is "just" or maybe better put should be "only".  And "even more" gives credit to all who imply that the screeching tires are somehow involved in the need for more energy.   But it all may figure in to an answer, somehow.

There is a formula for lateral g, and I think it goes:   1.22 times radius in feet divided by the square of the lap time (one revolution) in seconds.

We know g force would "cost" if in a straight line, as in an accelerating car needing more gas (even without or beyond any friction of air or mechanical/tires).,,

You are right! I was diverted from the original question! Just to make that point!
Geezer has it in control!
« Last Edit: 11/07/2010 19:47:38 by tommya300 »
 

Offline tommya300

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What are the forces on a car travelling in a circle?
« Reply #20 on: 11/07/2010 19:46:36 »
Just to avoid any confusion, other than friction, there is no energy input required to offset the centripetal force that makes the car travel in a circle.

It's no different from a flywheel. If you could remove all friction from a spinning flywheel, it would spin at the same same rate indefinitely without the input of any energy.

Can you take into account the muscle power needed to pull the stirring wheel to maintain the position of the  tires into the turn?
 

Offline syhprum

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What are the forces on a car travelling in a circle?
« Reply #21 on: 11/07/2010 19:57:30 »
Geezer

Left to its own devices the car would travel in a straight line to make it take a circular route a lateral force equal and opposite to the centripetal force must be applied there can be no argument about this !.
this force is applied by angling the tires so that some of the energy stored in the forward motion of the car is diverted into moving it sideways.
this energy must be replaced if the car is to continue on this path.
comparison with a flywheel is not relevent where the tension in the spokes provides the force to counteract the centripetal force which of course does not require any energy.
 

Offline Geezer

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What are the forces on a car travelling in a circle?
« Reply #22 on: 11/07/2010 20:25:50 »
Geezer

comparison with a flywheel is not relevent where the tension in the spokes provides the force to counteract the centripetal force which of course does not require any energy.

Syhprum

Consider the case where a cable is producing the centripetal force. If there was no other friction, the car would circle the axis without consuming any energy. It's just the same as a flywheel, albeit a bit unbalanced.

Now remove the cable and produce the centripetal force with tyres acting on a road surface instead. Assuming those tyres generate no heat (which is highly unlikely of course) the car will circle the axis without consuming any energy.

The same thing happens with a planet orbiting a star, except that now the centripetal force is produced by gravity. If it was necessary to continuously supply energy to keep the Earth in orbit round the Sun, I think the Earth would have crashed into the Sun some time ago.
 

Offline syhprum

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What are the forces on a car travelling in a circle?
« Reply #23 on: 11/07/2010 21:00:56 »
I begin to be convinced your frictionless tyres would have the same effect as a banking in counteracting the centripetal force.
When the Earth orbiting the sun would require a continuous supply of energy would be if you wished to reduce the orbital time without it getting any closer.
I have put too much weight on the comparison with the satellite orbiting the Earth in less than 84 minutes which of course requires a continuous supply of energy to counteract centripetal force but in the case of the car this can be supplied by the tyres with no consumption of energy other than frictional losses ( I go and stand in corner and don cap of ignorance )
 

Offline tommya300

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What are the forces on a car travelling in a circle?
« Reply #24 on: 11/07/2010 21:06:34 »
I begin to be convinced your frictionless tyres would have the same effect as a banking in counteracting the centripetal force.
When the Earth orbiting the sun would require a continuous supply of energy would be if you wished to reduce the orbital time without it getting any closer.
I have put too much weight on the comparison with the satellite orbiting the Earth in less than 84 minutes which of course requires a continuous supply of energy to counteract centripetal force but in the case of the car this can be supplied by the tyres with no consumption of energy other than frictional losses ( I go and stand in corner and don cap of ignorance )

Aren't these the original conditions given...

"We all know that a car traveling in a straight line down the big highway uses a certain amount of energy to offset aerodynamic drag, and mechanical and rolling friction.   If that same car at the same speed was traveling not in a straight line but in a large circle there would be additional energy needed to offset the likely increased rolling friction of the tyres, and would be "measurable" in the form of increased temperature in those tyres."
 

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What are the forces on a car travelling in a circle?
« Reply #24 on: 11/07/2010 21:06:34 »

 

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