Circular motion
Uniform circular motion
Circular motion kinematics correlates directly to linear kinematics. Equations for position, velocity and acceleration are similar in form to equations for angle, angular velocity and angular acceleration.
The left column gives the equations for linear kinematics in one dimension. The equations on the right are the corresponding equations for circular motion.
Theta is defined as an angle in a circle of radius r. We have defined the counterclockwise direction to be positive.
The acceleration vector for circular motion, in polar coordinates, is mathematically equivalent to the acceleration vector in Cartesian coordinates. In both cases, the components are independent, in that they are perpendicular to each other. Acceleration in polar coordinates is used for rotating systems as a convenience.
It is important to note that these linear accelerations are two representations of acceleration, differing in coordinate system only. The acceleration of an object can be defined by either form. We choose the form that is most convenient for the motion of the object in question.
Angular acceleration, on the other hand, is a different kind of quantity. It defines the motion of rotation, not of translation.
Let's review vectors a bit.
Practice problem
1. Which way does the vector v2 - v1 point?
A. NE
B. SE
C. SW
D. NW
E. None of the above
Consider the diagram below. Which direction does v2 - v1 point? How about v3 - v2? How about v4 - v3?
Derive radial acceleration
For uniform circular motion, the magnitudes of the velocity vectors are equal: v1 = v2. The velocity vectors are at right angles to the radial vectors, so the triangle made by the velocity vectors is similar (in the geometric sense) to the triangle made by the radial vectors.
These similar triangles provide a method for deriving a convenient form of the radial acceleration. For circular motion, the direction of the radial acceleration always points toward the center of the circle.
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