Dear Car Talk: I have a delicate question.


In a late 1960s/early '70s round-headlight Ford van, how possible is it for two people in the back of this van to actually get the van visually moving up and down?


I have a sister who says she saw a van that I was in reacting this very way in an apartment parking lot close to 5-5:30 p.m. I know this did not happen because, while I was in this older van, I was sitting in the front passenger seat. If you need this for your calculation, I weigh about 120 pounds, and the man in the driver's seat weighs at most 160-170.


Ok, if this scenario did take place ... as far as two people in back of the van ... my thoughts are that no way are they going to be able to get that older van to visually move up and down so that someone on the outside could see it.


My sister said it was moving. She said, "Very much visually up and down." This was a generic work van for upholstery type work. Again, this did not take place. I would just like to put to rest the inconceivable thought that it's even possible that my sister saw what she says she saw. — Calli


Dear Calli: Upholstery, huh? So I bet it was nice and comfortable in back.


I'm afraid it's more than possible. It's so possible that a "rockin' van" is a cliche. When we test shocks on a customer's car, the way we do it is by simply pushing down on each corner of the car. And if the shocks are worn out, with each push, the movement of the car increases, until it's bouncing up and down. That's called simple harmonic motion.


Think about a swing set at the park. You push it once, and it goes out and comes back. And if you catch it at its furthest point and push it again, you add to that momentum, and it goes further out and comes further back the next time. Shocks can behave the same way. Even more so when they're worn out.


I know I can create that "up and down" motion on the corner of a car all by myself. So certainly, two people can do it. In fact, if you really want to entertain your sister, Calli, you can also get the van moving side to side.


If you want a plausible defense, if you were laughing and moving around in your seats in an older van with worn-out shocks, you could probably make the van move that way, too. So maybe tell your sister that Upton the Upholsterer has a particularly keen sense of humor.


And then suggest he may want to get his shocks checked and replaced if necessary. Newer shocks might provide more cover next time.


Ray Magliozzi was the co-host of NPR's Peabody award-winning "Car Talk." Questions can be sent to Ray Magliossi c/o King Features, 628 Virginia Drive, Orlando, FL 32803, or at CarTalk.com.