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uniform circular motion (Definition)

When a point moves with uniform speed along the arc of a circle, its velocity is constantly changing in direction, and hence the point has an acceleration even though there is no change in the magnitude of its velocity.

Let a point move with uniform speed along the arc $mn $ of the circle whose center is $O$ in Figure 1.

Figure: 1.
Image UniformMotionFig1

When the point is at $m$, its velocity is $v$, and is perpendicular to the radius $r$. When the point has reached $n$, its velocity is $v'$, perpendicular to the radius $r'$.

${v'}$ is a vector which is obtained by adding the vector ${u}$ to the vector ${v}$ hence is the change in velocity.

If the points and are taken very near together, and are very nearly parallel and $u$ is perpendicular to , i.e., the change of velocity is toward the center of the circle. If $t$ units of time are required for the point to move from to , this change of velocity occurs in units of time, and

$\displaystyle \frac{u}{t}$

is the average rate of change of velocity. In the limiting case under consideration, i.e., when the distance between and approaches zero, this average rate of change of velocity becomes the acceleration and

$\displaystyle a = \frac{v}{t}.$

From Figure 1 it is apparent that, for small angles

$\displaystyle \frac{u}{v} = \theta$

or

$\displaystyle u = v \theta. $

Substituting this value of in the equation

$\displaystyle a = \frac{u}{t},$

we have

$\displaystyle a = \frac{v \theta}{t} = v \omega,$

$\omega$ being the angular velocity of the radius. $v/r$ may be substituted for ,

in which case

$\displaystyle a = \frac{v^2}{r}.$

Substituting $\omega r$ for in the same equation, we obtain an express for acceleration $a$ in terms of angular velocity ,

thus:

$\displaystyle a = \omega^2 r.$

This is called the centripetal acceleration. It is always directed toward the center of the circle in whose circumference the point is moving. If is measured in cm per second, is also measured in cm per second, and $u/t$ must be measured in cm per second per second. When the foot is taken as the unit of length, the acceleration is measured in feet per second per second.

If a mass particle has a uniform circular motion, it will have an acceleration toward the center, and hence a continuous force,

$\displaystyle ma = \frac{m v^2}{r},$

must act to pull it toward the center. This force is called the centripetal force; and its equal and opposite reaction, called the centrifugal force, is the force that the mass exerts away from the center. These forces vary as the mass, as the square of the speed, and, for a given speed, inversely as the radius of the curve.

This article is a derivative work of the public domain in [1].

Bibliography

[1] Randall, Harrison, Williams Neil, Colby, Walter, "General College Physics" Harper and Brothers Publishers, New York and London, 1929.



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Also defines:  centripetal force, centrifugal force

Cross-references: domain, work, square, mass, vector, magnitude, acceleration, velocity, speed
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This is version 2 of uniform circular motion, born on 2025-03-02, modified 2025-03-03.
Object id is 976, canonical name is UniformCircularMotion.
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Classification:
Physics Classification: 45.50.Dd (General motion)
 45.50.-j (Dynamics and kinematics of a particle and a system of particles)
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