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The rotating-frame acceleration identity for a uniform rotation about an axis

Example

Let ν>0 and define R:R→SO(3) by the rotation about the third standard axis through angle νt:

R(t)=(cos⁡νt−sin⁡νt0sin⁡νtcos⁡νt0001)(t∈R).

Then R(t)∈SO(3) for every t, and Ω=RTR′ is

Ω(t)=ν(0−10100000).

The angular-velocity vector of Angular velocity of a moving frame and the rotating-frame acceleration identity is ω(t)=(0,0,ν), independent of t. For any twice continuously differentiable y:R→R3, put x(t):=R(t)y(t) and let c:R→R3 be identically zero. That lemma gives

R(t)Tx′′(t)=y′′(t)+2 ω×y′(t)+ω×(ω×y(t))(x=Ry).

Facts & Assumptions

Given: A number ν>0 and the rotation matrix R(t) displayed above, defined for t∈R; write ct:=cos⁡νt and st:=sin⁡νt.

[L1]

The identity ct2+st2=1 holds for every real t; sine and cosine have derivatives (sin⁡)′=cos⁡ and (cos⁡)′=−sin⁡, so repeated differentiation gives continuous derivatives of all orders. Vector-valued formulas are differentiated componentwise (Sine and cosine defined by their real power series, Parity and the Pythagorean identity for sine and cosine, The derivatives of sine and cosine are cosine and minus sine, The derivative and the Riemann integral of a vector-valued function: an intrinsic derivative and a componentwise integral).

[L2]

For twice continuously differentiable R:I→SO(3) and y, with Ω=RTR′ and ω(t) the unique vector with Ωz=ω×z for all z, Angular velocity of a moving frame and the rotating-frame acceleration identity gives RTx′′=y′′+2 ω×y′+ω′×y+ω×(ω×y)+RTc′′ when x=Ry+c.

[L3]

The cross product in R3 of the standard basis vectors is e1×e2=e3, e2×e3=e1, e3×e1=e2, and it is bilinear and alternating (The cross product in R3).

Verification

technique · direct; compute $\Omega$ and check the identity on two explicit curves defined on $\mathbb R$
1.1L1givenalgebra

By [L1], R′=ν(−st−ct0ct−st0000), and the product RTR′ equals ν(ctst0−stct0001)(−st−ct0ct−st0000)=ν(−ctst+stct−ct2−st20st2+ct2stct−ctst0000)=ν(0−10100000), using ct2+st2=1; hence reading off the entries of Ω as in clause 2 of [L2] gives ω(t)=(0,0,ν) and Ωz=ω×z for every z. Also RTR=I3 and det⁡R=ct2+st2=1, so R(t)∈SO(3); [L1] gives its required C2 regularity.

2.1step 1.1L1L2L3algebra

Take y(t)=e1 for t∈R, so y′=y′′=0, x(t)=R(t)e1=(ct,st,0) and x′′=−ν2(ct,st,0) by [L1]. Then RTx′′=−ν2RT(ct,st,0)=−ν2e1; the right-hand side is ω×(ω×e1)=(0,0,ν)×(0,ν,0)=(−ν2,0,0) by [L3] and step 1.1. The two sides agree.

3.1step 1.1step 2.1L1L2L3algebra∎

Take y(t)=te1 for t∈R, so y′=e1, y′′=0, x(t)=t(ct,st,0) and x′′=(−2νst−tν2ct, 2νct−tν2st, 0) by [L1]. Multiplying by RT gives RTx′′=(−tν2, 2ν, 0); the right-hand side is 2(0,ν,0)+t(−ν2,0,0)=(−tν2,2ν,0) by [L3] and step 1.1, using 2 ω×e1=2(0,ν,0) and ω×(ω×te1)=t(−ν2,0,0). The two sides agree, exhibiting the two rotational terms separately.

Remarks

  • Both checks are finite computations with the convention Ω=RTR′ and angular velocity ω=(0,0,ν). The other convention R′RT gives the same vector in this fixed-axis example, so the example does not test the distinction between conventions.

  • The identity is kinematic. The terms are named "Coriolis" and "centrifugal" only in a mechanical interpretation, which requires the force and mass hypotheses of the consuming physical items.

Depends on

Dependency tree · 2 levels

Sources