In this position, the truck is stable—there is no rotational motion. As before, the gravitational force acts at the center of mass. The force from the ground is not applied at the center of mass, but notice that if you extend a line from that arrow, it would pass right through the center of mass. That means this force also has a zero torque arm, so it creates no torque.
But wait. Why does the ground force point that way? It’s actually a combination of two forces: First, there is the upward-pushing force from the ground. This is an interaction between the tire and the ground that prevents the truck from falling through the dirt. (It’s the same force that pushes up on your coffee cup to keep it from passing through the table.) This is called the “normal” force, because it’s perpendicular to the surface.
Second, there is the frictional force from the back tires. This force pushes forward, parallel to the ground. Combine those two and the net force points diagonally up to the right. This is the whole key to pulling a wheelie: You need to hit the perfect frictional force to aim that arrow right at the center of gravity. That’s all driver skill.
You can see that the net force causes the truck to accelerate forward. Oh, there is a way to do a wheelie without accelerating. If you had the center of mass directly over the back wheel, then the force from the ground would push straight up and again produce zero torque. However, in this case I think the monster truck is accelerating.
The Secret to the Front Flip
Now for the finale! So for a truck moving to the right, you need to create a clockwise torque to make it somersault forward. That’s pretty tough. The key is to have only the back wheels hit the ramp. And that’s exactly what the Mad Scientist driver does, as you can see in the video (at 2:00)—as he approaches the ramp, he guns it into a wheelie. Here is a force diagram:
Since the front wheels are off the ground, they miss the small ramp—there is no force exerted on the front wheels. Then, when the back wheels hit, the ramp pushes both back and up. The direction of this ramp force produces the necessary clockwise torque. This increases the rotational motion of the truck in the clockwise direction. Once again, after it leaves the ramp, it continues to rotate in the same direction as long as it stays aloft. In this case, Mad Scientist completes a full rotation before landing. Honestly, it’s quite impressive.
