Air resistance, how to calculate it?

In summary, the conversation covers topics related to air resistance and its calculation for objects such as a baseball or a car. The drag force equation is mentioned, along with the factors that affect the drag coefficient. It is also noted that the equation is simplified and in reality, the drag coefficient varies with speed and other factors. The conversation also discusses the difficulty in finding an exact solution for the velocity of an object after being hit, and how the drag coefficient changes with the Reynolds number. The conversation also briefly touches on the topic of friction and how it is affected by changing a vehicle from wheeled to maglev. Methods for measuring friction and air resistance are also mentioned.
  • #1
shizzznit
Im taking AP Physics as well as Calculas but haven't covered the topic of Air Resistance yet. I am trying to figure out what the air resistance would be on a baseball. Could someone help me out?
Much thanks
 
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  • #2
Roughly:

[itex] F_D = \frac{C_D\rho A v^2}{2} [/itex]

where FD is the drag force, CD is the drag coefficient, ρ is the density of air, A is the cross-sectional area of the ball (a regulation baseball has a circumference between 9 and 9.25 inches), and v is the velocity of the ball. The drag coefficient is a function of things like surface roughness, ball speed, and spin, varying between 0.2 and 0.5 for speeds commonly occurring during game play, but if you'd like to ballpark it (pardon the pun), 0.3 is a good number to use.
 
  • #3
What level is AP Physics?

Air resistance is usually taken into account via a differential equation; in simple situations, it grows linearly with the speed of the object.
 
  • #4
AP Physics is equivalent to first year college physics study.

Thanks very much Jamesrc
having some trouble figuring out the velocity (after the bat) but thanks for the help!
 
  • #5
There is no closed form solution for it.

You need to multiply the force times a small step size and add it to the velocity and repeat. Works best on a computer.
 
  • #6
hey I am doing an assignment for senoir school physics could you tell me how to calculate the air resistance on say the bonnet of a car?? nothing advanced just the basic formulae using the surface area of the bonnet and the force exerted by the air?? i know it has something to do with splitting it up into components??
 
  • #7
Look at

http://en.wikipedia.org/wiki/Drag_coefficient

The drag force depends on whether the air flow around the object is linear or turbulent, which then determines whether the drag force is proportional to the velocity, or the velocity squared. Also, the drag depends not only on the object size (frontal area) and shape but also on the surface roughness.

Bob S
 
  • #8
jamesrc said:
Roughly:
[itex] F_D = \frac{C_D\rho A v^2}{2} [/itex]
It turns out that in the real world it's more complicated, but instead of modifying the equation to one more realistic, Cd is redefined as a function (often implemented as an interpolated table) that varies with speed for a given medium, such as air, and a specific object (such as a bullet, http://en.wikipedia.org/wiki/External_ballistics).
 
  • #9
I was going to say this earlier, but I don't know the level of detail the OP needs.

Jamesrc suggests taking an average value of Cd, in reality you cannot do this. As Cd depends heavily on Reynolds number. Above a critical number for a ball (and it depends on the ball) the Cd will be a steady 0.2sh. Below the critical Re number the Cd will raise until it nears the critical Re number and will then drop sharply.

I did not mention it beucase if this is a generic textbook case for a baseball, his assumption of taking a middle Cd value is probably valid.
 
  • #10
hey guys i am yr 9 and doing a subject for my science fair this year: how much friction is redueced when a normal wheeled vechicle is changed into a maglev vechicle?
how can i measure friction in this case
and how can a measure air resistence
 
Sure, here's an FAQ section for the topic "Air Resistance: How to Calculate It?":

What is air resistance?

Air resistance, also known as drag, is a force that opposes the motion of an object through the air. It is caused by air particles colliding with the surface of the moving object.

How can air resistance be calculated?

Air resistance can be calculated using the formula: Drag Force (D) = 0.5 × Drag Coefficient (Cd) × Air Density (ρ) × Velocity² (v²) × Cross-sectional Area (A). This formula is derived from the principles of fluid dynamics.

What factors affect air resistance?

Several factors affect air resistance including the speed of the object, the cross-sectional area facing the airflow, the shape of the object (affecting the drag coefficient), and the air density.

How does the shape of an object affect its air resistance?

The shape of an object significantly affects its air resistance. Streamlined shapes have lower air resistance compared to blunt or irregular shapes. The Drag Coefficient (Cd) in the formula represents this effect.

Is air resistance always the same at different speeds?

No, air resistance increases with the square of the object's speed. This means that as an object moves faster, the air resistance exerted on it increases exponentially.

Can air resistance be ignored in certain calculations?

In situations where objects are moving at low speeds or where precision is not critical, air resistance can sometimes be ignored. However, for high-speed objects, like airplanes or fast cars, air resistance becomes a significant factor.

How does air density impact air resistance?

Air density, which can vary with altitude and weather conditions, affects air resistance. Higher air density increases air resistance, while lower air density reduces it.

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