For every action, so states Isaac Newton's third law of motion, there is an equal and opposite reaction. And in NASCAR, when that action has to do with Dale Earnhardt Jr., the equal and opposite reaction usually comes from Kyle Busch.

... the bottom line is that Earnhardt's free agency and subsequent move to the Hendrick stable is effectively what led to Busch's ouster, and Busch has thoroughly outperformed his successor in every way since the move was made.

That's certainly the way it seemed last week at Dover International Speedway, when the current and former Hendrick Motorsports drivers traded barbs through the media, then briefly exchanged words in the drivers' meeting prior to the Autism Speaks 400, and added an entire new level of intrigue to the saga involving Earnhardt's change of crew chief. Who knew that when Rick Hendrick hired NASCAR's most popular driver, he was also laying the groundwork for NASCAR's most intense feud.

This latest chapter began Friday, when Busch was asked what he knew about Lance McGrew, whom he's worked with briefly in the past, and replaced Tony Eury Jr. atop the No. 88 pit box last weekend. "He's got his hands full having to deal with what's going on," Busch said, "and if Junior doesn't run well, then he's going to be the problem again. It's never Junior. It's always the crew chief."

Within minutes, that comment was splashed across the Web, routing Junior Nation into another state of anti-Busch fury. And yet, Busch taking potshots at Earnhardt's somewhat disproportionate popularity-to-performance ratio is nothing new. He's done it before. He'll surely do it again. And in all honesty, it's hard to blame him when he does it.

Now, you may not like Kyle Busch. You may think he's childish and whiny and immature. Fine. But if there's anyone in NASCAR who has the right to feel wronged, who should and does take every opportunity to point to his sport's equivalent of a scoreboard so everyone can see what the tally shows, it's the driver of the No. 18 car. No, he didn't fit at Hendrick. In retrospect, that much seems obvious, and you have to wonder how much longer he might have remained there even if Earnhardt had not become available. But the bottom line is that Earnhardt's free agency and subsequent move to the Hendrick stable is effectively what led to Busch's ouster, and Busch has thoroughly outperformed his successor in every way since the move was made.

That's not a knock against Earnhardt or Hendrick, whose closer-than-we-realized personal history -- going back to Robert Gee, Earnhardt's maternal grandfather and a longtime Hendrick employee -- made their association a natural. But since the switch, Busch has 11 race wins on NASCAR's top series, and has firmly established himself as a championship contender. Earnhardt has one victory and is trying to "rebuild Rome," as he aptly puts it. If you worked in a public arena like NASCAR, and you were outperforming the guy who had effectively caused you to lose your last job, wouldn't you take every opportunity to remind people of it, too?

Kyle on Junior

Kyle Busch's reaction to the crew chief change on the No. 88: "You've got to make the most popular drive in the sport competitive ..."

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That's what Busch is doing, and it's completely understandable. But who, really, is the target here? Clearly, Busch and Earnhardt are not friends; in those rare times when they've both been in the media center together after a race, there's a stony silence between them. But is Busch really taking digs at Earnhardt, who in all fairness has rarely had a bad thing to say about him publicly? Is he taking shots at Hendrick? Or is he needling all those fans who see him as public enemy No. 1, who gave Busch holy hell after he and Earnhardt spun out while racing for the win at Richmond early last year, who never give the kid enough credit for his talent and seemed to ignore the lopsided performance results of the last year and a third? Busch told me last year that he has much bigger issues with Junior Nation than Junior himself. And we all know the guy loves to poke the beast from time to time, whether that beast is NASCAR or folks with No. 88 tattoos.

And yet, exchanges like this probably don't help Busch in the long run -- especially when he backs them up by running 23rd as he did at Dover, fighting tire and splitter problems and placing 11 spots lower than you-know-who. For all his issues on-track, Earnhardt remains a smart, savvy guy capable of seeing the big picture. His public responses to Busch's barbs have always been measured and mature. Now, that may be because he hasn't had the performance results to back up any mud-slinging he might really want to do. But in the public eye, he gets a lot of credit for not taking the bait, which he refused to do at Dover.

"He's always had a chip on his shoulder for me," Earnhardt said when informed of Busch's remarks. "I expect anytime he gets an opportunity to throw a jab in there, he's going to do it. That's just his personality. We're working toward trying to figure out how we can make our deal work and that has nothing to do with Kyle. We'll see how this weekend goes and then move forward next week."

That ability to take the high road is one of Earnhardt's more impressive traits. Of course, we still don't know what he said to Busch in the drivers' meeting Sunday. Questions on that subject were immediately shut down by Earnhardt's public relations man after the race, so the extent of the conversation remains unknown. It seems very doubtful that the two were making dinner plans for Pocono.

And so, the mystery remains. And if Earnhardt keeps struggling and Busch keeps winning, it seems only a matter of time before the driver of a certain yellow Toyota takes advantage of an opportunity to remind everyone of that fact. Maybe that's not exactly the classy, mature thing to do. Maybe it's not helping Kyle Busch make any new fans. But it's understandable, coming from an uber-confident 24-year-old who's been through the experience of getting canned. And until Earnhardt steps his performance up to Busch's level -- definitely possible, but far from guaranteed -- all those Busch bashers are really left with nothing to say in response.

The opinions expressed are solely those of the writer.