ليه أحسن سرعة 90 كلم/س؟

[Ibtal]

لاحظ اليديو اللى حطة الزميل و اتفرج على الحتة اللى خرطوم الدخان محطوط ورا العربية... حتلاقى الدخان بيروح فى اتجاة الشنطة (عكس اتجاه الهوا) و بعدها يطلع لفوق و يرجع الى الوراء

اتجاة الدخان العكسى ناتج عن الفراغ اللى ورا العربية و دة بيبداء يتكون بعد 90 و دة السبب ان المقاومة بعد 90 اكبر من قبلة بشكل كبير نتيجة تاثير الفراغ على اندفاع السيارة للامام

MMamdouh

Very good observation Mr. Mamdouh. I do not know why you did not go to engineering study to enhance your engineering sense.

So the forces on the car are the friction with the ground and the aerodynamic Drag.
These two forces are counteracted by the engine torque on the wheel axes.

Of course there is the lift force upward effective at high speed which can be minimized by reducing the gap between the car bottom and the ground and by using the spoilers.

Now we talk only about the aerodynamic Drag (which is a function of the square of the car speed) and this we can divide into three components:

1) The frontal air pressure.
The more the front of the car is blunt the higher this force is.
2) The friction of the air on the surface of the car
This depends on the aerodynamic shape of the car and the car surface roughness and obstacles
3) The negative pressure created just behind the car and this is what you have observed in the video. This component of Drag is very effective at high speed.
You could notice something in car racing that cars are always trying to benefit from this vacuum by trailing each other which help a lot the trailing car also the front car but by lesser amount. Also you see that in highways (in US and Europe of course) the trucks are trailing each other and they always take turn to be in the front.

[MMamdouh]

Very good observation Mr. Mamdouh. I do not know why you did not go to engineering study to enhance your engineering sense.

my score at high school was not good enough... i scored 82.5% and the faculty of engineering minimum requirment was 88.5%

So the forces on the car are the friction with the ground and the aerodynamic Drag.
These two forces are counteracted by the engine torque on the wheel axes.

Of course there is the lift force upward effective at high speed which can be minimized by reducing the gap between the car bottom and the ground and by using the spoilers.

Now we talk only about the aerodynamic Drag (which is a function of the square of the car speed) and this we can divide into three components:

1) The frontal air pressure.
The more the front of the car is blunt the higher this force is.
2) The friction of the air on the surface of the car
This depends on the aerodynamic shape of the car and the car surface roughness and obstacles
3) The negative pressure created just behind the car and this is what you have observed in the video. This component of Drag is very effective at high speed.
You could notice something in car racing that cars are always trying to benefit from this vacuum by trailing each other which help a lot the trailing car also the front car but by lesser amount. Also you see that in highways (in US and Europe of course) the trucks are trailing each other and they always take turn to be in the front.

that's an impeccable illustration

MMamdouh

[Ibtal]

my score at high school was not good enough... i scored 82.5% and the faculty of engineering minimum requirment was 88.5%


that's an impeccable illustration

MMamdouh

Thank you and I am sorry for writing in english but it was faster in such subject.
Another question which is somewhat related is the Golf Ball. Why does it have lot of dents and not smooth?
May be we should open another title so we do not spoil the car subject.

[MMamdouh]

i write much faster in english myself so it is not a problem for me at all

i have no idea about the dents on golf balls... i only know that air pressure rifle pellets are slimmer from the middle for directional stability



and the same thing applies to the fact that the pellet is revolving around its axle while it is shot

seems like aerodynamics is far more deep than we thought

MMamdouh

[Ignition-Kid]

Thank you and I am sorry for writing in english but it was faster in such subject.
Another question which is somewhat related is the Golf Ball. Why does it have lot of dents and not smooth?
May be we should open another title so we do not spoil the car subject.

You Mean dimples right ??? If so then you should check this out

Aerodynamics in Sports Equipment, Recreation and Machines - Golf - Instructor

[Ibtal]

i write much faster in english myself so it is not a problem for me at all

i have no idea about the dents on golf balls... i only know that air pressure rifle pellets are slimmer from the middle for directional stability



and the same thing applies to the fact that the pellet is revolving around its axle while it is shot

seems like aerodynamics is far more deep than we thought

MMamdouh

Let me just put those two theories:

First, any thing that rotates around itself creates negative pressure on one side and positive pressure on the opposite side and this generates a force sometimes we call it Lift.
Example:
In football, when you shoot the ball with a spin the ball makes a curve due to this force.

Second, when you move a body in the air or any other fluid and this body has different length on one side than the other side you get also this lift force. This is actually the theory of flying.
Example:
We talked about the lift under the car because the top of the car is longer than the bottom.

For the golf ball, the ball is required to go far and in order to do that you have to give it an initial high velocity (strong hit). Due to this high speed a negative pressure is generated behind the ball because of the vacuum and this means a high drag force.
So although you hit very strongly you create an opposite force.
The reason of the negative pressure is that the air flow separates from the surface of the ball due to high friction. So the trick is to prevent the air flow from separating and this can be done by creating turbulence in the air flow by introducing the dents on the surface of the ball. This is to reduce the difference between the negative and the positive pressure thus reducing the Drag force

[Ibtal]

You Mean dimples right ??? If so then you should check this out

Aerodynamics in Sports Equipment, Recreation and Machines - Golf - Instructor

Thank you I will read it because it is an interesting subject

[eng.octooo]

معلومات جديدة وحلوة بالنسبة ليا

[ahmed_h_g]

بصراحه أنا كنت عايز نفس السؤال والحمد لله لقيته باجاباته كمان
الف شكر ...