Physics Asked on January 6, 2021
To explain what I mean let’s say I have an object that is moving and I know its speed and direction (angle). I can get the X and Y components of speed using speed*cos(angle)
and speed*sin(angle)
.
Is this possible with rotation?
I have a sphere that is rolling. Each 2*PI*R
that it moves forwards, it rotates 2*PI
, obviously. The general formula is angle = distance/R
.
If it’s rolling along the X axis (for example, from point (0,0) to (x,0)) I know it rotates angle
along the Y axis.
But what if it’s rolling along the (1,1) vector? That’s a 45º degree angle but doing rot_x = cos(45) * angle
and rot_y = sin(45) * angle
doesn’t work, does it? Is it possible to get each rotations component to know how the sphere should be rotating?
The transformation of the ${x,y,z}$ components of the vector is
$${hat x, hat y,hat z} = {x,y cos (alpha)-z sin (alpha),y sin (alpha)+z cos (alpha)}$$
for the rotation around the $x$-axis by an angle $alpha$,
$${bar x, bar y,bar z} = {x cos (alpha )+z sin (beta),y,z cos (beta)-x sin (beta)}$$
for the rotation around the $y$-axis by an angle $beta$ and
$${ acute{x}, acute{y},acute{z}} = {x cos (delta)-y sin (delta),x sin (delta)+y cos (delta),z}$$
for the rotation around the z-axis by an angle $delta$.
Answered by Gendergaga on January 6, 2021
Rotational quantities, θ, ω, and α, are usually represented by vectors along the axis of rotation. For a rolling object, they are perpendicular to the translational velocity and parallel to the surface on which rolling occurs. The arc length, s = r θ, does not change if the surface is tilted.
Answered by R.W. Bird on January 6, 2021
If the location of where you want to measure speed is $pmatrix{x & y}$ from the instant center of rotation, then
$$ pmatrix{v_x v_y} = pmatrix{ -y,omega x , omega} $$ where $omega$ is the angular speed.
In the case of a rolling ball, the center of rotation is at the contact point if there is no slipping, and thus the center of the ball is at $pmatrix{0 & R}$ relative to the contact point. So the velocity vector of the ball (center) is
$$ pmatrix{v_x v_y} = pmatrix{-R, omega 0} $$
The above are just the 2D projection of the transformation laws for velocity
$$ vec{v} = vec{omega} times vec{r} $$ where $times$ is the vector cross product.
Answered by John Alexiou on January 6, 2021
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