Motorcycle Jumps
How do motorcyclists change their angle of inclination to land a jump?
We'll start off by visualizing the scenario. Take a look at Figure 1. At the start of the jump, the motorcycle is angled up, with its front wheel higher than its back wheel. If the motorcyclist continues like this, it will become very difficult to land smoothly. That's why, as the video continues, the motorcycle rotates as the front wheel becomes lower in height than the back wheel. Once the motorcyclist is in this position, he will be able to land. Today's article focuses on how the motorcyclist was able to change his motorcycle's angle of inclination while in the air. It may seem like the motorcycle will do this on its own as it falls, but this is a common misconception. Before we move on, it may be helpful to review the previous article, Spinny Skaters.
Figure 1
Figure 2
Before I can explain how the angle of inclination changes, we need to talk about angular momentum. A quick summary is that angular momentum can be seen as the rotational counterpart for linear momentum. Linear momentum is easier to explain. Velocity is the rate of change of position with respect to time. Simply put, it's how fast something moves in a direction. If we multiply the velocity by the mass of an object, we get its linear momentum. Similarly, we have angular momentum. This is the product of angular velocity and the moment of inertia. Angular velocity is just how fast an object spins in some direction. The moment of inertia is the resistance to the change in rotation, and it can be seen as a version of mass for rotational motion. All in all, angular momentum is a quantity that describes the rotational motion of an object.
An interesting quality of angular momentum is that it is conserved in the absence of a non-zero external torque. This means that as long as there is no outside force pushing to change the rotational speed of an object, its angular momentum will always be the same. Let's think about the motorcycle again. The answer to our question is that the motorcyclist can hold the brakes. When the bike launches off the ramp, the wheels have an angular momentum going "forward". If the motorcyclist holds the brakes while in the air, the frame of the bike will try to clamp down and slow down the wheels. In doing this, since the angular momentum is conserved, some of the forward angular momentum will be transferred to the bike frame. Because the bike frame has angular momentum, and therefore angular velocity, it can be rotated forward to be oriented for landing.