# Homework Help: How to find velocity of air at certain point if car is moving at certain speed (Fluid Mechanics)?

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1. Nov 13, 2014

### kela582

1. The problem statement, all variables and given/known data

The shape of a car windshield is approximated in the figure below; its length is 2.0 ft and height is 1.5 ft. Obtain an equation of the windshield shape r as a function of
θ,
r(θ), in the polar coordinate system shown in the picture.

When the car moves at 55 mph, determine the velocity of the air at points A and B.

2. Relevant equations

Not really sure how to approach this...maybe Bernoulli's Equation? We've been studying the method of repeating variables, but I'm unsure how I would apply that. I really don't know where to begin with this one!

3. The attempt at a solution

(P/rho + v^2/2 + gz = constant)
But I don't know the pressure difference...I'm hopelessly stuck :( If anyone could help shed some light on this, and just give me an idea where to even begin, that would be very appreciated!

2. Nov 14, 2014

### Staff: Mentor

This seems to be a trick question. Have you heard of the "no slip" boundary condition?

Chet

3. Nov 14, 2014

### kela582

Ah, so v_A = 0.

4. Nov 14, 2014

### Staff: Mentor

Relative to the car. Relative to the ground, it's U = 55 mph.

Chet