Final answer:
An acrobat can't stop spinning in midair due to the conservation of angular momentum, which ensures their total angular momentum remains constant in the absence of external torques. They can modify their spin rate by changing body shape but cannot stop spinning until an external force acts on them.
Step-by-step explanation:
The reason why an acrobat can't stop himself from spinning while he is in midair is due to the conservation of angular momentum. Angular momentum is a physical quantity that represents the rotational inertia and angular velocity of an object; it is conserved in a closed system where no external torques are acting. This means that in the absence of external forces, an acrobat or a figure skater, as shown in the reference video, must maintain the same total angular momentum throughout their aerial maneuver.
When an acrobat is spinning in midair, they do have some control over their spin rate. By changing their body configuration, such as extending or tucking in their arms and legs, they can alter their moment of inertia, which consequently changes their angular velocity. This is because the product of angular velocity and moment of inertia must stay constant if no external torque is present. However, the total spin cannot be stopped without an external force (such as grabbing onto something or by air resistance, which is generally too weak to have a noticeable effect during a short duration maneuver).
Therefore, although an acrobat can change the rate at which they spin, they cannot halt their overall rotational motion until they make contact with the ground or another surface that can exert an external force on them.