Hey Larry Mac! Cars Do Not Speed Up in the Grass!

They just slow down more slowly…

Ever had one of those things that you never noticed before, but when someone brings it to your attention, you notice it and it drives you crazy?  Frank Smith emailed me about an observation made by television commentators that was driving him nuts.  Now that he mentioned it, I keep hearing it and it’s driving me nuts, too.

Not to denigrate Larry Mac and the other television commentators.  I’ve learned a lot from Mr. McReynolds.   There’s a perfectly good physics explanation for why he (and others) keep telling us that cars speed up when they get into the grass on a racetrack.

Assuming that the driver has the presence of mind to take his foot off the gas, this is impossible.

Why Cars Slow Down

Let’s say you’ve got a car doing 100 mph coming onto the frontstretch.  The force of the engine pushes the car forward and the forces of friction push in the opposite direction as the car is moving.  Frictional forces include friction between the various parts in the motor and drivetrain, air resistance (which is  friction between the car surfaces and the air molecules) and friction between the tires and the track.  The force of the engine must be greater than or equal to the force of friction in order for the car to move to the right (as shown in my drawing).  If the force of the engine is just equal to the force of friction, the car will move at constant speed (no acceleration).  If the force of the engine is greater than the force of friction, the car will accelerate to the right.

Now let’s say the driver takes his foot off the brake and lets the car coast.  We’ve removed the force of the engine, so all we’re left with is the frictional force.  The frictional force causes the car to slow down (decelerate) until it comes to a stop.  This is just an example of Newton’s Second Law:  F=ma, or (in words)

The larger the net force, the larger the acceleration.  It’s a vector equation, so the direction of the acceleration is in the direction of the net force.

Bring on the Grass

Now let’s look at a case in which the driver lets up on the gas, the car travels on the frontstretch for a little while, then goes into the grass.  The frictional forces in the engine/drivetrain and the air resistance remain the same, regardless of what surface the car is traveling on.

It is impossible for the car to speed up unless there is a force pushing it to the right.

Our NASCAR  television commentators are not stupid.  They’re reporting what they see and it does look like the car starts to speed up when it transitions from asphalt to grass.

This is an example of relative motion.  Have you ever sat at a train track and focused only on the train going past in front of your car?  If you only look at the moving train, you can make it feel as though the train were standing still and you were moving in the opposite direction as the train.   That’s relative motion.  To you, the train is moving to the left at 50 mph.  To someone on the train, you’re moving to the right at 50 mph.

We’ve had this issue arise before during Carl Edwards’ restart penalty at the first Richmond race:  If the car that is supposed to start the race spins his tires, his lack of acceleration can make the car beside him look like it’s accelerating, even though it’s actually moving at constant speed.  I’ve got some animations at the link above to show you how it’s very easy for your eyes to be fooled.

We judge speed relative to what’s around it.  If you’re going 65 mph on the expressway and another car is going 60 mph, you’re going 5 mph faster than the other car.  If we just took your cars, without anything else surrounding you, it would look exactly the same to you as if you were going 200 mph and the other car was going 195 mph.  What gives you the ability to distinguish 200 mph from 65 mph are the stationary objects you pass.  In the Richmond case, you can’t just look at the 99 and the 2 cars and make a valid observation about their speeds – you have to look at their cars relative to something standing still, like the lines on the wall that indicate the restart lane.

The same thing is happening in our car-in-the-grass scenario.  The big difference between the two surfaces is the coefficient of friction between asphalt and grass.  The coefficient of friction between tires and asphalt is about 0.7-0.8.  The coefficient of friction between tires and grass is 0.35 – roughly half what it is when the car is on the asphalt.

The frictional force due to the tires is proportional to the coefficient of friction.  The grass has a lower coefficient of friction, so the frictional force decreases when the car travels into the grass.  The car doesn’t speed up – it slows down at a slower rate.  It looks like it speeds up because your brain is watching the car on the asphalt and expecting the car to keep decelerating the same way.  When the car moves onto the grass and the frictional force changes, the deceleration changes and it looks to us like the car is speeding up, even though it isn’t.

This all assumes, again, that the driver isn’t on the gas.  But if  you’re spinning out into the grass and your foot is pushing down on the accelerator, you’ve got much bigger problems than not understanding basic physics.