Fly in a Car

[Gamingphreek]

This is kind of puzzeling. DOes this have anything to do with a cars "gravity".

If you are driving down the road at like 70mph and you have a fly flying around in the car. Why is that fly keeping up with the car, why isn't it splat against the back windshield?

I know if you have a ball and are throwing that in the car or something, inertia keeps it moving, but the fly doesn't have that inertia or momentum.

-Kevin

 

[imported_Hamzter]

If the car's travelling at constant speed, the fly will remain stationary with respect to the car, since it's in equilibrium (no wind resistance, since the car is pretty much sealed). However, if the car is accelarating, the fly should travel towards the windows, although the fly may well be able to keep up with the cars accelaration and stay near the centre of the car. It's fairly simple mechanics, remember an object in equilibrium will remain at constant velocity, since there's no wind resistance acting on the fly should be able to stay in the centre of the car no problem.

When you throw a ball in a car, it has momentum, not inertia (inertia is the change in momentum). Flys also have momentum, just alot less of it.

 

[anthrax]

Originally posted by: Gamingphreek
This is kind of puzzeling. DOes this have anything to do with a cars "gravity".

If you are driving down the road at like 70mph and you have a fly flying around in the car. Why is that fly keeping up with the car, why isn't it splat against the back windshield?

I know if you have a ball and are throwing that in the car or something, inertia keeps it moving, but the fly doesn't have that inertia or momentum.

-Kevin

You have to remember that the fly is actually flying in the air in the car... When a car, acclerates....the contents of the car is accerates along with it.... the car applies a force on its passenger via it seats. the car applies a force on the fly via the air in the car... The air is inturn acclerated by the sides of the car. (the car acts as a container)

since u are inside the car and the air and the fly is being acclerated at the same rate. The Fly should not travel towards the rear window even when the car is acclerating.

 

[iwantanewcomputer]

Originally posted by: anthrax

Originally posted by: Gamingphreek
This is kind of puzzeling. DOes this have anything to do with a cars "gravity".

If you are driving down the road at like 70mph and you have a fly flying around in the car. Why is that fly keeping up with the car, why isn't it splat against the back windshield?

I know if you have a ball and are throwing that in the car or something, inertia keeps it moving, but the fly doesn't have that inertia or momentum.

-Kevin

You have to remember that the fly is actually flying in the air in the car... When a car, acclerates....the contents of the car is accerates along with it.... the car applies a force on its passenger via it seats. the car applies a force on the fly via the air in the car... The air is inturn acclerated by the sides of the car. (the car acts as a container)

since u are inside the car and the air and the fly is being acclerated at the same rate. The Fly should not travel towards the rear window even when the car is acclerating.

yes it should, you are right that the air is accelerating the fly foreward, but not at the same rate as the car or air inside because the fly is more dense.

 

[DrPizza]

Let's see if I can clear this up.


Let's take a look at something a little easier than air:
On a flat surface, dashboard for example, place an object. At low acceleration, the object doesn't slide. Why? Because the force of friction is greater than the force required to accelerate the object at the car's acceleration.

Let's say the car is accelerating at 1 meter per second each second. (increasing the velocity by 1 m/s every s, or abbreviated by the physics folks as 1 m/s^2

Now, we use the coefficient of static friction for the object against the dashboard (which would probably have to be determined experimentally)
Why do we want the static friction? Because we don't want the object sliding on the dashboard (this is the kinetic coefficient of friction which will generally be lower)

Force of friction = coef * Normal force
F(f) = mu*F(n)
(on a level surface, normal force is equal in magnitude to the weight.)

So, the car is accelerating at 1 m/s^2
the acceleration of the car is caused by a force in the direction of the motion of the car. This happens where the tire meets the road. The tire pushes backward against the road as it tries to spin, and <equal and opposite forces> the road pushes back against the tire. Anyway, the car is accelerating at 1 m/s^2.

For the object to stay on the same spot on the dashboard, it needs to have the same acceleration as the car.
since, F=ma, and a = 1.0 m/s^2, then it needs a force equal to it's mass times 1.0 m/s^2
This force is provided by the friction between the object and dashboard.

Let's say it has a mass of 10 kg, and a coefficient of static friction of .30 (so I don't have to use so many symbols)
Then, it needs a force of m*a = 10kg *1m/s^2 = 10 kgm/s^2 or 10 newtons.

The force of static friction that can be provided is F(f) = .30 * weight.
the weight, from the acceleration of gravity, is 98.1 Newtons (F=mg)
.30 * 98.1 = 29.4 Newtons. So, up to 29.4 newtons of force can be provided by the static friction. Only 10 is necessary for the acceleration, therefore, the object doesn't slide on the dashboard.

**Note: if you double the mass of the object, you double the amount of force necessary for acceleration. However, you double the normal force as well, therefore you double the frictional force. It all balances out.

Now, if you attempt to accelerate the car at 3m/s^2 (roughly increase your speed by 6 mph every second, or 0 to 60 in 10 seconds (actually, more like 9.1 seconds))

Available force of friction is the same. Using F=ma, with the 10kg object sitting on the dashboard, it needs a force of 30 newtons now to accelerate. Unfortunately, static friction provides only 29.4 Newtons. Therefore, the object starts to slide.

Now, once it starts sliding, this is kinetic friction. The coefficient of kinetic friction will be lower, let's say around .20
So, friction is providing a force of .20*98.1Newtons = 19.6 Newtons.
F=ma
F/m = a
19.6N/10kg = 1.96 m/s^2
The object will accelerate at 1.96 m/s^2.
However, the car is accelerating at 3.00 m/s^2. Relative to the car, the object is moving backwards, at an acceleration just over 1 m/s^2.
So, if you're leaning backward in your seat, and accelerate the car at 3m/s^2. Expect a whole lot of coffee to be landing in your lap.

Now, back to the fly...

The force for acceleration of the fly will be provided by air resistance. Simply because without motion, there is no air resistance, the fly is going to end up moving toward the back of the car. Air resistance is proportional to (often used) velocity squared. If v=0, air resistance = 0.

However, the air itself is also going to be accelerating toward the back of the car (relative to the car) Because of inertia, the air doesn't want to accelerate. This results in higher density of air at the rear of the car and lower density of air at the front of the car. An interesting experiment to verify this: Hang a balloon on a string from the ceiling of the car, and have a helium balloon floating at about the same height. As the car accelerates forward, the regular air balloon will go backwards (as will a fly clinging to its surface, and a fly that's hovering in the air. However, as the car accelerates forward, the helium balloon will actually move forward in the car. I've got a video of this on my computer that I downloaded somewhere on the net, but don't have the time to search for it now (where it's located on the net.) It's a pretty common demonstration in college physics.

 

[DrPizza]

holy shit, that was a long post.

 

[DrPizza]

Oh, and another look at your original question, suppose you're in a convertible and the fly is flying around down by your feet. No problem, as that air is moving at the same speed as the car. But, in that convertible, the fly decides to fly up above your head. Good-bye fly.

 

[pm]

An interesting thing to do in a car is to have a helium balloon floating freely in the car. IE. take a small helium balloon and tie it to a nut and put the center of the backseat. Accelerating moves the balloon forward, braking makes it go backward... as the air in the car moves around.

Pretty neat. Even when you know what will happen, seeing it happen is weird.

 

[DrPizza]

Originally posted by: pm
An interesting thing to do in a car is to have a helium balloon floating freely in the car. IE. take a small helium balloon and tie it to a nut and put the center of the backseat. Accelerating moves the balloon forward, braking makes it go backward... as the air in the car moves around.

Pretty neat. Even when you know what will happen, seeing it happen is weird.

yeah, as I posted above

 

[pm]

Originally posted by: DrPizza

Originally posted by: pm
An interesting thing to do in a car is to have a helium balloon floating freely in the car. IE. take a small helium balloon and tie it to a nut and put the center of the backseat. Accelerating moves the balloon forward, braking makes it go backward... as the air in the car moves around.

Pretty neat. Even when you know what will happen, seeing it happen is weird.

yeah, as I posted above

True. I missed it in your post.

 

[DrPizza]

Originally posted by: pm

Originally posted by: DrPizza

Originally posted by: pm
An interesting thing to do in a car is to have a helium balloon floating freely in the car. IE. take a small helium balloon and tie it to a nut and put the center of the backseat. Accelerating moves the balloon forward, braking makes it go backward... as the air in the car moves around.

Pretty neat. Even when you know what will happen, seeing it happen is weird.

yeah, as I posted above

True. I missed it in your post.

probably because my post was rambling... I'd type for a minute, work with students for 3 or 4 minutes... type, work, type..

 

[Valkerie]

Flying cars do exist. The only reason they're not being made public is because civil engineers refuse to take the neccessary time to figure out a way in which flying cars will replace cars with wheels. It's a budget problem, and more of an adaptive problem as well. If you don't believe me, do the research.

 

[Gamingphreek]

Originally posted by: Valkerie
Flying cars do exist. The only reason they're not being made public is because civil engineers refuse to take the neccessary time to figure out a way in which flying cars will replace cars with wheels. It's a budget problem, and more of an adaptive problem as well. If you don't believe me, do the research.

Ummmm.... reading the post might help

Thanks for the respnses everyone. I learned a lot

 

[Mrpilot007]

Let me put it simply. Think of the air in your car with a fly in it as to a bowl of milk with a cherio in it. As you walk to the sink the cherio does not fall out onto the floor nor does the milk go flying out just because you got up from the couch. It slowly floats to the edge of the bowl. As does the fly float along in your car and does not splat against the window the moment you drive away.

 

[piasabird]

The Air in the car is also traveling at 70 miles an hour.

 

[piasabird]

The earth is traveling at an incredible speed but you are not flying off the face of the earth.