[VIDEO] Jay Leno Can’t Do Flips in a Car and Not Expect Us to Do the Physics

Rhett Allain reports: Bob Riggle is 80 years old and he has a car. This car has a 2,500 horsepower engine mounted in the rear. But what happens when you have this much power? Yes, you can see in the video that there are two events. First, the car does a “wheelie” and second the car rolls over.

Fortunately no one was injured, but at least this is a great opportunity for a physics lesson.

Center of Mass and Wheelies

There are some forces acting on this car so let’s start with a diagram.

There are essentially three forces on the car in this case.

• The gravitational force pulls down. We can model this force as though it was only pulling down at one point. We call this point the center of mass (technically, it would be the center of gravity—but on the surface of the Earth these two points are at the same place).
• There is the force that the ground pushes up on the car. Since the car is not accelerating in the vertical direction, this ground force must be equal to the gravitational force.
• The friction force pushes on the tire at the point of contact with the ground. This force pushes the car in the direction that it is accelerating.

But how does this car stay tilted up like that? Shouldn’t the gravitational force make it fall back down? Clearly, it doesn’t. Perhaps the best way to understand this wheelie is to consider fake forces. We normally consider forces as interactions between objects (between the ground and the car or between the Earth and the car). However, it’s sometimes useful to create other forces that are due to accelerations. Now, these are fake forces in that they are not a real interaction. But as viewed in an accelerating reference frame (like inside the car), it is as though there is this real acceleration force.

Since the car accelerates to the left (in the above diagram), the fake force is to the right and keeps the car in wheelie up position.

But what about torque? If you want to rotate an object, you need torque. One expression for torque would be (this is just the scalar form—for simplicity):

In this expression, F is the force, r is the distance from the point of rotation to the point where the force is applied and θ is the angle between these two things. For the total torque about the wheel, it’s really just the torque due to the gravitational force and the torque due to the fake force.

If you put the engine in the front of the car (where it usually is) then the center of mass moves closer to the front. This means the gravitational torque will be much larger (since r is larger). If you get the center of mass closer to the back wheel, the torque from the fake force doesn’t need to be as high to get a wheelie. Read the rest of this entry »