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Rupert Young
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If a car is being driven with a constant force (the angle of the pedal is kept constant) and it goes around a curve, will the velocity change? If so, is this due to centripetal force?
Ummm, because there is a sideways force?Rupert Young said:Don't you mean that the velocity changes because you are going around a curve?
Could you clarify why it is changing, given that the forward force is the same? Why does going around a curve exert a counter force?
jbriggs444 said:Ummm, because there is a sideways force?
CWatters said:Velocity has components speed and direction. Going around a corner means the direction is changing so the velocity is changing. The speed need not be changing.
Forum rules discourage us just giving people the answers because they learn more when they figure things out for themselves. Have a think about the forces that act on a car. Have you tried to push one sideways? Why would that be hard?
Because the road is slippery?Why do you have to slow down in wintery conditions?
The idea that "velocity" and "speed" are different is drilled into one's head constantly in first year physics. As others have said, "speed" is how fast you are going without regard to direction. By contrast, "velocity" takes direction into account. 50 miles per hour forward is a different "velocity" than 40 miles per hour forward and 30 miles per hour leftward even though both are exactly the same "speed".Rupert Young said:Ok, I thought velocity and speed were the same thing. So, if a car is being driven with a constant force and it goes around a curve, does that mean the speed also remains constant?
Because the road is slippery?
What does this mean in terms of forces?Rupert Young said:Because the road is slippery?
Well, I'm not a physicist, and I'm hoping this forum is open to those physicsally challenged!jbriggs444 said:The idea that "velocity" and "speed" are different is drilled into one's head constantly in first year physics.
As others have said, "speed" is how fast you are going without regard to direction. By contrast, "velocity" takes direction into account. 50 miles per hour forward is a different "velocity" than 40 miles per hour forward and 30 miles per hour leftward even though both are exactly the same "speed".
In an ideal car -- perfect wheels with no rolling resistance and no sideways slip, and no air resistance, there would be no reason to put your foot on the accelerator at all. You could coast at constant speed forever. Since your car requires force to maintain a constant speed, we need to consider what makes your car less than ideal.
So.. how about you tell us? Why does your car slow down if you do not keep your foot on the gas?
A.T. said:What does this mean in terms of forces?
CWatters said:So what is the car relying on to turn a corner?
Yes, that's Pythagoras in action.Rupert Young said:So, if you add a leftward force (wind, say) of 30 mph to a car that was going 40 mph it would then be going 50 mph? (computed by Pythagoras I guess)
jbriggs444 said:Yes, that's Pythagoras in action.
No, as I said, the relationship between force and speed is not direct. The relationship between wind speed, then force, then car speed is even less direct. And if you do not turn the steering wheel, a lateral force will (ideally) have zero effect on the vehicle.Rupert Young said:Just to clarify, are you answering yes to the question as I'd thought the answer would be no. Are you saying that if the forward speed of a car is 40 mph, which is then exposed to a leftward force (wind, say) of 30 mph, the car would then speed up (over time) to go at a forward speed of 50 mph?
If you turn the steering wheel, the car turns. When you normalize the wheel, the car retains its current heading. The wind does not enter in.If the steering wheel was turned, into the wind, the speed and direction of the car would remain the same. Yes?
Have you thought about looking it up yourself?Rupert Young said:I feel you want me to say friction. But is that a force?
jbriggs444 said:No, as I said, the relationship between force and speed is not direct. The relationship between wind speed, then force, then car speed is even less direct.
And if you do not turn the steering wheel, a lateral force will (ideally) have zero effect on the vehicle. If you turn the steering wheel, the car turns. When you normalize the wheel, the car retains its current heading. The wind does not enter in.
A.T. said:Have you thought about looking it up yourself?
https://en.wikipedia.org/wiki/Friction
Friction is not itself a fundamental force.
The fundamental force behind friction is electromagnetic.Rupert Young said:Ah yes, as I thought,
No, I do not agree. The rightward turning of the steering wheel creates a rightward angle of the front wheels on the car which tends to cause the car to progressively turn farther and farther right. This is desirable if it maintains the desired course while the vehicle "crabs" left under the influence of a leftward wind. However, if one maintains the steering in this position (ignoring caster effects on the steering mechanism) the car will continue to turn further right than is desired. Ideally you want a one-time rightward steering input to counter a continuing leftward force.Rupert Young said:L
This doesn't seem right to me. If there is a lateral force on an object surely that object will move laterally. If it is wind then the car will be moved laterally (maybe only slightly), so it would be necessary to turn the steering wheel into the lateral force to maintain the heading. Generally we don't notice the effects of wind except when we go across a windy bridge and we have to make an abrupt correction with the steering wheel to maintain our heading.
Similarly, if a car is being driven along a straight road but one which slopes from left to right the force of gravity would draw the car to the right. However, by turning the steering wheel to the left the straight heading of the car is maintained. In essence we would be applying an upward force to compensate for the downward force of gravity. Do you not agree?
jbriggs444 said:No, I do not agree. The rightward turning of the steering wheel creates a rightward angle of the front wheels on the car which tends to cause the car to progressively turn farther and farther right. This is desirable if it maintains the desired course while the vehicle "crabs" left under the influence of a leftward wind. However, if one maintains the steering in this position (ignoring caster effects on the steering mechanism) the car will continue to turn further right than is desired. Ideally you want a one-time rightward steering input to counter a continuing leftward force.
In addition, I used the word "ideally" in my earlier posting. Ringing in non-ideal effects is not appropriate.
are you saying that it is necessary to turn the wheel to the right to compensate for a force coming from the right? If so, that makes sense and agrees with what I said.The rightward turning of the steering wheel creates a rightward angle of the front wheels on the car which tends to cause the car to progressively turn farther and farther right. This is desirable if it maintains the desired course while the vehicle "crabs" left under the influence of a leftward wind.
you appear to be saying that you need to turn the wheel to the right and than back again to "normal" to compensate, i.e. an impulse turn. Is that what you are saying? I don't think that could be correct because after the impulse (one-time rightward steering input) you would be back to square one with the lateral force once again moving the car laterally. I'd say you have to maintain a turned wheel, all the time the lateral force exists, in order to maintain the straight-ahead heading. In other words, although the wheel is turned the car does not turn, due to the opposing lateral force. Yes?However, if one maintains the steering in this position (ignoring caster effects on the steering mechanism) the car will continue to turn further right than is desired. Ideally you want a one-time rightward steering input to counter a continuing leftward force.
You don't necessarily need steering to compensate for a lateral force. If you have sufficient traction, the static friction at all the wheels can have a lateral component, which balances the lateral force.Rupert Young said:I'd say you have to maintain a turned wheel, all the time the lateral force exists, in order to maintain the straight-ahead heading. In other words, although the wheel is turned the car does not turn, due to the opposing lateral force. Yes?
Perhaps in an idealized case, but in the real world case, the car/wheels/tires will probably want to move sideways at some rate that the driver will simply compensate for.A.T. said:You don't necessarily need steering to compensate for a lateral force. If you have sufficient traction, the static friction at all the wheels can have a lateral component, which balances the lateral force.
I'm assuming sufficient traction as stated. Static friction doesn't let the wheels move where they "want".olivermsun said:Perhaps in an idealized case, but in the real world case, the car/wheels/tires will probably want to move sideways at some rate...
Even given slippage, you still don't need to "maintain a turned wheel to maintain the straight-ahead heading" as Rupert Young suggests. You can have the entire vehicle oriented slightly off it's movement direction, against the lateral force.olivermsun said:...that the driver will simply compensate for.
No, but compliance in the tires can allow the wheels/car to "walk" sideways even if the tread itself doesn't slip sideways on the pavement.A.T. said:I'm assuming sufficient traction as stated. Static friction doesn't let the wheels move where they "want".
See my post #22.Even given slippage, you still don't need to "maintain a turned wheel to maintain the straight-ahead heading" as Rupert Young suggests. You can have the entire vehicle oriented slightly off it's movement direction, against the lateral force.
A.T. said:Even given slippage, you still don't need to "maintain a turned wheel to maintain the straight-ahead heading" as Rupert Young suggests. You can have the entire vehicle oriented slightly off it's movement direction, against the lateral force.
If a car is being driven with a constant force and it goes around a curve, will the speed change? If so, is this due to centripetal force?
There are different options as post #22 describes.Rupert Young said:Ok, I think there is some agreement that it is necessary to turn the wheel to compensate for a lateral force, though some dispute about whether the car itself is redirected or just the wheels themselves are.
How ideal/realistic you want to model it? What exactly do you mean by "being driven with a constant force" ? What is the direction of that force?Rupert Young said:I'd be interested help in how this could be quantified and modeled mathematically.
olivermsun said:sophiecentaur's post #25 gives a good suggestion for how to start with a simple model of a vehicle going around a curve.
A.T. said:How ideal/realistic you want to model it? What exactly do you mean by "being driven with a constant force" ? What is the direction of that force?
Whether the speed changes or not then depends on whether that engine force is greater or less than the restive forces (rolling resistance, aerodynamic drag). The centripetal force doesn't change the speed per definition, just the direction.Rupert Young said:basically a constant force exerted by the engine. The direction is forward, relative to the car.