# Fluid mechanics question

1. Aug 5, 2009

### AxiomOfChoice

Suppose you're driving along a straight, smooth, level stretch of highway with a constant velocity $v$. There is a cup of some fluid in your cup holder. How would you go about finding the angle $\theta$ that the surface of the fluid makes with the bottom of the cup as a function of $v$? And what if you started out with a velocity $v_0$ at time $t=0$ and then accelerated at a constant rate $a$? Can we find $\theta(t)$ without much sweat?

2. Aug 5, 2009

### jpreed

The angle of the liquid in the cup is determined by the acceleration of the car.

If you are traveling at constant velocity the angle of the liquid inside the cup will be zero. Think about when you fly on a plane at 350 mph, the liquid is still flat (unless the plane is slightly tilted).

If you start at zero velocity and begin to accelerate constantly, then the angle of the liquid in the cup will be

$$\theta = \tan^{-1} \left (\frac{a}{g}\right)$$

where $$a$$ is your acceleration and $$g$$ is gravity.

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