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Physics · Glossary

What is Relative motion; frame in translation, in rotation?

Definition 18.1 University Physics — Year 1 · Chapter 18 — Non-Inertial Frames; Dynamics on Earth

Let R\mathcal R be a frame (origin OO) and R′\mathcal R' another (origin O′O') moving with respect to it. R′\mathcal R' is in translation if its axes keep fixed directions in R\mathcal R (every point of R′\mathcal R' has the velocity of O′O'); it is in rotation about a fixed axis Δ\Delta if Δ\Delta is fixed in both frames and R′\mathcal R' turns about it at the angular velocity Ω(t)\Omega(t). The velocity and acceleration of a point MM relative to R′\mathcal R', v→′\vect v' and a→′\vect a', are those a stationary observer of R′\mathcal R' measures.

Examples

Example 18.3 (Walking on a turntable)

Exercise 11.12 computed the acceleration of a person walking radially at uu on a turntable rotating at Ω\Omega: −utΩ2e→r+2uΩe→θ-ut\Omega^2\vect e_r + 2u\Omega\vect e_\theta. In the new language: relative acceleration zero, entrainment −rΩ2e→r-r\Omega^2\vect e_r, Coriolis 2Ω→∧ue→r=2uΩe→θ2\vect\Omega\wedge u\vect e_r = 2u\Omega\vect e_\theta.

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