- #1
Quesadilla
- 95
- 13
Homework Statement
This is an exercise on classical mechanics, filed under the section on oscillatory motion (according to the lecture notes).
A car is driven with constant speed [itex]30 km/h[/itex] along a bumpy road. The height of the road may be described as [itex]y = y(x) = H_0 \sin(kx), x>0[/itex]. Now set [itex]H_0 = 0.15 m[/itex] and [itex]k = 2 m^{-1}[/itex]. Describe the car's vertical movement.
Homework Equations
Equation of motion for SHM :
[tex]\ddot{x} + {\omega}_0^2 x = 0 [/tex] which has solution
[tex] x(t) = A \cos(\omega_0 t + \phi) [/tex]
could be of relevance, I suppose.
The Attempt at a Solution
My interpretation of the question is that I should find the height [itex]y[/itex] as a function of time [itex]t[/itex]. I attempted to find [itex]x(t)[/itex] after which [itex]y(t)[/itex] would follow from the given relationship between [itex]y[/itex] and [itex]x[/itex].
Using the chain rule, I got
[tex]\dot{y} = \frac{dy}{dt} = \frac{dy}{dx} \frac{dx}{dt} = kH_0 \cos(kx) \dot{x}.[/tex]
Given constant speed 30 km/h, which we could convert to [itex] 30 / 3.6 = 25/3 [/itex] m/s, we can use the Pythagorean identity to get
[tex] \left(\frac{25}{3}\right)^2 = \dot{y}^2 + \dot{x}^2 [/tex] which implies
[tex] \dot{x} = \frac{\frac{25}{3}}{\sqrt{1 + k^2H_0^2 \cos(kx)}}. [/tex]
Separation of variables now yields
[tex] \int_0^t dt = \frac{25}{3} \int_0^x \frac{dx}{\sqrt{1 + k^2H_0^2 \cos(kx)}}. [/tex]
where the latter integral is supposedly an elliptic integral of the first order, which are mentioned in passing in the lecture notes, but with which I do not have any real familiarity.
This would give [itex] t(x) [/itex], so I would have to take some sort of inverse of the elliptic integral to get [itex] x(t) [/itex].
I think I am taking the wrong approach to the problem, or maybe I am making some logical error somewhere in my thought process. Any comments or hints are most welcome. Many thanks in advance.
Remark: This is my first post here and therefore I am not quite sure how to write LaTeX in the posts. I tried looking in other treads and follow their example, but in the preview I only see the "code" written as plain text.
Last edited: