I am the blue car in this scenario. If I am stopped on the highway and am passed by a car going fast (about 65 mph), I feel my car rock in the left/right direction. I think I understand why this happens: the wake of the red car in the air pushes my car back and forth.

![stopped car being passed by fast car in opposite direction][1]

But if I'm moving slowly forward (say 5 mph or less), I don't feel anything when the car passes. I would think that, if anything, the force would be *stronger*, because the velocity difference is now 70 mph. I'm moving slowly enough that the rocking would still be quite noticeable. Why don't I feel the rocking any more in this scenario?

![slowly-moving car being passed by fast car in opposite direction][2]

**Update:** As suggested by RedGrittyBrick, I recorded some accelerometer data from my phone to try to better understand what's happening. My phone was mounted to the dash with a mount that holds it in place; it was approximately positioned so that the x (left/right), y (up/down), and z (forward/back) coordinates match that of the car. From watching the x-axis data as it was recorded, I think his explanation is the most likely one: it is so slight that I don't notice it being so significant when moving slowly.

If you want to view the data yourself, you can see [the raw data][3] yourself. There's 17 minutes of raw data there, not all of it useful; IIRC it was towards the end, look for where the data is least noisy to find where I'm stopped; if you see a slight left-then-right on the x axis, that's me being passed. If anyone knows of a good way to view and analyze this data (or point me in the direction of what tool(s) to use to do that myself), maybe you can get a more accurate interpretation of it (and I can assist in telling you what was going on in the real-world in association with the data).

What I'd like to know specifically is: in objective terms, how strong is the rocking from when I'm moving slowly compared to when I'm stopped? There was too much vibration for me to see it real-time, but maybe it can still be extracted from the data?


  [1]: https://i.sstatic.net/wgdvT.png
  [2]: https://i.sstatic.net/niSpW.png
  [3]: https://drive.google.com/file/d/0B72y7-pdB73bQ3J2RGtraDc0Y0k/view?usp=sharing