Thursday, June 1Welcome

A NASCAR Viewer’s Guide to Phoenix Raceway

Ross Chastain gave a great physics class at Martinsville. Here’s how he passed his five cars in the final half-lap of the race.

Requires rotational force to rotate

Imagine swinging a tennis ball tied to a string overhead. Since there is a string, the ball moves in a circle.

That string provides the force that spins the ball. This rotational force is always towards the center of rotation. Physicists call it “centripetal force”, but I think “rotational force” is more easily understood.

A turn on a racetrack utilizes exactly the same physics, but without strings.

And those 3,675-pound cars require far more turning power than a tennis ball.

The amount of force required to rotate is proportional to the spinning mass multiplied by the velocity times the velocity divided by the turning radius.

Physics tells us:

  • Faster rotation requires more rotational force.
  • To make tight turns (like Martinsville) you need more turning power.
  • A heavier car needs more turning power.

Let’s calculate the force normally required in Martinsville. The speed of the pole was 96.078 mph, but the speed of each car varied throughout the lap.

  • During practice, the car reached 114-119 mph on the straight.
  • Drivers entered the turns at 75 to 85 mph, depending on the age of the driver and tires.
  • Most drivers slowed to about 60 mph before accelerating again to exit the turn.

Suppose a driver drives a 202-foot radius turn in Martinsville at an average speed of 80 mph. This requires 7,775 pounds of rotational force.

Four tires have to produce all of the nearly four tons of rotational force.

Sir Isaac Newton discovered that force equals mass times acceleration. Chastain is 5 feet 9 inches. I estimate his weight to be about 160 pounds, which brings the total car and driver weight to 3,675 pounds for him.

Dividing the force by the mass, the acceleration of a car spinning at 80 miles per hour in Martinsville is typically about 2.1 G’s. where G is the earth’s gravitational acceleration.

A head that weighs about 10 pounds feels like it weighs 20 pounds when subjected to 2G of acceleration.

Compare that to a space shuttle astronaut who experienced about 3G during takeoff.

Ross Chastain’s turning model

On the final lap, Chastain had to overtake two cars. However, none of the cars he needed to pass were close enough for him to catch up.

Chastain floored it heading into Turn 3. Instead of relying solely on the tires for turning power, Chastain used the walls to help turn. This gave him enough rotational power to spin faster.

On lap 499 Chastain’s lap time was 20.758 seconds. The 500th lap time was 18.845 seconds. As my colleague Dustin Long pointed out, this is the fastest lap for a stock car at Martinsville Speedway.

Chastain ran the first half normally. About halfway through the 499th lap he was 10.379 seconds.

This meant he completed the second half of the lap in 8.466 seconds.he had to run average 112 mph from the midpoint of the backstretch to the start/finish line.

He didn’t run 112 mph the whole time. Let’s assume he entered the turn at his 122 mph. This puts him 37-47 mph faster than anyone else. Rotational force he is 18,079 pounds and acceleration is almost 5G.

Isn’t 5G dangerous?

Humans can tolerate 5G for a short time. A 10 lb head feels like he is 50 lbs at 5 G acceleration. But that’s not the main issue.

The human body is optimized for the 1 G acceleration provided by the mass of the Earth. When your body accelerates faster, it has to work harder to keep the blood circulating.Without adequate blood flow, your organs can’t get enough oxygen.

A warning sign of too much G is loss of peripheral vision and the ability to see color. When the brain senses lack of oxygen, it shuts down the least important functions first.

However, if high acceleration continues, the person accelerating will eventually lose consciousness. Fighter pilots wear pressure suits to keep circulation normal.

Much of what we know about how the human body can withstand high acceleration is thanks to Air Force Colonel John Stapp. He conducted an experiment on himself in his 1950s and endured 25G for just over a second of his, exerting a maximum force of 46.2G.

Unfortunately, his experiments permanently damaged his eyesight. But he lived another 45 years, and in 1999 he died at the age of 89.

Ross Chastain not only made it to Championship Four, but he also provided great answers to every student who asked their math and science teacher.

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