Gyroscopic Precession (James Rice)

James Rice

Principle(s) Illustrated

• Forces perpendicular to the tangential motion of an object in circular motion cause a deflection of the object's path which appears to create a torque that actually shows up as if it was applied 90⁰ around the central axis from the actual point of application.

• Try saying that five times fast!

Standards

• Science and Engineering Practice

  • Construction explanations and designing solutions

• Crosscutting Concept

  • Interdependence of science, engineering, and technology

• Disciplinary Core Idea

  • If a system interacts with objects outside itself, the total momentum of the system can change; however, any such change is balanced by changes in the momentum of objects outside the system. (HS-PS2-2)

Questioning Script

Prior knowledge & experience:

Faster rotating bicycle wheels make bicycles more stable

Root question:

What keeps the wheel from falling when its hub is hung on a rope on only one side? Why does it revolve around the rope when it is spinning?

Target response:

Intuitive version:

• Forces perpendicular to the tangential motion of an object in circular motion cause a deflection of the object's path which appears to create a torque that actually shows up as if it was applied 90⁰ around the central axis from the actual point of application.

Vector version:

• I'd take up too much space writing it all out here... so just go to the HyperPhysics page on the topic.

• To boil even that one down, try this - the "right hand rule" dictates the behavior of vectors when they "multiplied" with a cross product.

• If you were to curl the fingers of your right hand around the axis of rotation for the wheel, your thumb would point along the direction of the angular momentum vector, L = Iω

• If one end of the axle is hung from a rope, then the force of gravity pulls downward on the entire wheel assembly. This force, applied through the center of mass of the wheel, creates a torque around the pivot point that joins the axle to the support at the distance, l, between the pivot point and the wheel's center of mass. This torque can be expressed as 𝜏 = mgl.

• The resulting cross product of L and mgl makes a new torque that is perpendicular to both L and mgl, which points horizontally. This is the resulting force that causes the wheel to precess.