# The Naked Scientists Forum

### Author Topic: speed of a car  (Read 4282 times)

#### paul.fr

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##### speed of a car
« on: 15/05/2007 09:25:20 »
Assume that a car is travelling at 70mph, the accelerator is jammmed at 70mph. Now say it was to go up a hill with agradient of 1/10 and down the other side, would it still be doing 70mph on the way up and down the hill?

#### DoctorBeaver

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##### speed of a car
« Reply #1 on: 15/05/2007 10:04:31 »
With or without cruise control?

#### daveshorts

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##### speed of a car
« Reply #2 on: 15/05/2007 11:21:45 »
If the accelerator is set to the power required to make you go at 70mph on the flat. This means that the power produced is enough to overcome air resistance at this speed. If you go up hill you will need to increase your potential energy as well as overcome air resistance so the car will slow down until the air resistance plus the rate at which you are gaining potential energy is the same as the power the engine is producing.

When you start going downhill you are loosing potential energy, so you will accelerate again, to above 70mph

#### DoctorBeaver

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##### speed of a car
« Reply #3 on: 15/05/2007 11:33:57 »
If I accelerate my car from 70mph I can watch my petrol gauge nosedive

#### another_someone

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##### speed of a car
« Reply #4 on: 15/05/2007 13:03:20 »
At constant power, speed would reduce as you climb a hill, and increase as you descend - you should be more than aware of this as a cyclist.

#### another_someone

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##### speed of a car
« Reply #5 on: 15/05/2007 13:06:00 »
If I accelerate my car from 70mph I can watch my petrol gauge nosedive

If by that you mean your car is very inefficient on fuel above 70mph, then that implies that it has poor aerodynamics (that is not necessarily a criticism, since at low speeds, aerodynamics are less important than other factors).

If you simply mean that you consume fuel under acceleration, but that is misleading, since the real wastage of fuel tends to happen under braking (unless you have a vehicle capable of regenerative braking).

#### paul.fr

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##### speed of a car
« Reply #6 on: 15/05/2007 16:49:38 »
At constant power, speed would reduce as you climb a hill, and increase as you descend - you should be more than aware of this as a cyclist.

Not really, where i live is flat. When i go for long bike rides, all of the area is flat and full of canal paths and farmers roads.

The only time i encounter anything resembling a hill, is when i go to work on my bike. Even then, it's only a humback bridge. Which i don't notice as i always ride flat out in top gear when going to work.

#### DoctorBeaver

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##### speed of a car
« Reply #7 on: 16/05/2007 10:39:49 »
If I accelerate my car from 70mph I can watch my petrol gauge nosedive

If by that you mean your car is very inefficient on fuel above 70mph, then that implies that it has poor aerodynamics (that is not necessarily a criticism, since at low speeds, aerodynamics are less important than other factors).

If you simply mean that you consume fuel under acceleration, but that is misleading, since the real wastage of fuel tends to happen under braking (unless you have a vehicle capable of regenerative braking).

When cruising on the motorway at around 70mph I get about 30-35mpg. While I'm accelerating the gauge shows 10-15mpg, or less if I really give it some welly! Under braking or going downhill with my foot off the throttle the gauge goes past 50mpg.

#### another_someone

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##### speed of a car
« Reply #8 on: 16/05/2007 14:45:03 »
When cruising on the motorway at around 70mph I get about 30-35mpg. While I'm accelerating the gauge shows 10-15mpg, or less if I really give it some welly! Under braking or going downhill with my foot off the throttle the gauge goes past 50mpg.

The numbers are deceptive.

The cruising mpg is as you see it.

The acceleration mpg is meaningless on its own, because while you are accelerating you are converting fuel into kinetic energy.  The real issue is, after having converted that fuel into kinetic energy, how much of the kinetic energy is actually used to get you from A to B (the normally desired purpose of driving a car), and how much is thrown away as heat.

If you brake, although you may not actually be burning fuel at that instant (or very little of it), you are throwing away the energy value of the fuel you burnt earlier.  Thus it is actually the braking that is most wasteful on fuel, even though the fuel you are wasting is not burnt at that instant, but was burnt earlier in developing the kinetic energy that you are now throwing away as heat.

#### DoctorBeaver

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##### speed of a car
« Reply #9 on: 17/05/2007 10:09:29 »
So are you telling me I shouldn't brake? Could I use that as my defence if I get caught speeding?

Anyway, if QM is right, they can't prove both that I was in that place and doing that speed.

#### another_someone

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##### speed of a car
« Reply #10 on: 17/05/2007 10:36:34 »
So are you telling me I shouldn't brake? Could I use that as my defence if I get caught speeding?

The speed limits are not optimised for fuel economy.

As has been mentioned above, optimum fuel economy can be obtained by gaining speed on the downhill stretches and allowing your inertia to help take you over the following hill.  You will note that a substantial proportion of speed cameras are located near the bottom of hills, so forcing drivers to dump their kinetic energy as heat just before they could best make use of it to climb the next gradient, hence requiring them to apply more power than they might otherwise do to climb the next gradient.

Anyway, if QM is right, they can't prove both that I was in that place and doing that speed.

The uncertainty is actually between position and momentum.  Since momentum is a combination of velocity and mass, can one not increase the accuracy of knowledge of the velocity if one is willing to sacrifice precision of knowledge about the mass of the object, and thus still satisfy the uncertainty principle?

#### DoctorBeaver

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##### speed of a car
« Reply #11 on: 17/05/2007 13:35:08 »
Spoilsport!

#### The Naked Scientists Forum

##### speed of a car
« Reply #11 on: 17/05/2007 13:35:08 »