You are watching: A particle traveling around a circle at constant speed will experience an acceleration.
Uniform circular motion is a specific type of motion in which an object travels in a circle with a constant speed. For example, any point on a propeller spinning at a constant rate is executing uniform circular motion. Other examples are the second, minute, and hour hands of a watch. It is remarkable that points on these rotating objects are actually accelerating, although the rotation rate is a constant. To see this, we must analyze the motion in terms of vectors.
Centripetal Acceleration
In onedimensional kinematics, objects with a constant speed have zero acceleration. However, in two and threedimensional kinematics, even if the speed is a constant, a particle can have acceleration if it moves along a curved trajectory such as a circle. In this case the velocity vector is changing, or
This is shown in (Figure). As the particle moves counterclockwise in time
on the circular path, its position vector moves from
to
The velocity vector has constant magnitude and is tangent to the path as it changes from
to
changing its direction only. Since the velocity vector
is perpendicular to the position vector
the triangles formed by the position vectors and
and the velocity vectors and
are similar. Furthermore, since
and
the two triangles are isosceles. From these facts we can make the assertion
or
Figure 4.18 (a) A particle is moving in a circle at a constant speed, with position and velocity vectors at times
and
(b) Velocity vectors forming a triangle. The two triangles in the figure are similar. The vector
points toward the center of the circle in the limit
We can find the magnitude of the acceleration from
The direction of the acceleration can also be found by noting that as
and therefore
approach zero, the vector
approaches a direction perpendicular to
In the limit
is perpendicular to
Since
is tangent to the circle, the acceleration
points toward the center of the circle. Summarizing, a particle moving in a circle at a constant speed has an acceleration with magnitude
The direction of the acceleration vector is toward the center of the circle ((Figure)). This is a radial acceleration and is called the centripetal acceleration, which is why we give it the subscript c. The word centripetal comes from the Latin words centrum (meaning “center”) and petere (meaning to seek”), and thus takes the meaning “center seeking.”
Figure 4.19 The centripetal acceleration vector points toward the center of the circular path of motion and is an acceleration in the radial direction. The velocity vector is also shown and is tangent to the circle.
Let’s investigate some examples that illustrate the relative magnitudes of the velocity, radius, and centripetal acceleration.
Example
Creating an Acceleration of 1 gA jet is flying at 134.1 m/s along a straight line and makes a turn along a circular path level with the ground. What does the radius of the circle have to be to produce a centripetal acceleration of 1 g on the pilot and jet toward the center of the circular trajectory?
StrategyGiven the speed of the jet, we can solve for the radius of the circle in the expression for the centripetal acceleration.
SolutionSet the centripetal acceleration equal to the acceleration of gravity:
Solving for the radius, we find
Significance
To create a greater acceleration than g on the pilot, the jet would either have to decrease the radius of its circular trajectory or increase its speed on its existing trajectory or both.
Check Your Understanding
A flywheel has a radius of 20.0 cm. What is the speed of a point on the edge of the flywheel if it experiences a centripetal acceleration of
134.0 cm/s
Centripetal acceleration can have a wide range of values, depending on the speed and radius of curvature of the circular path. Typical centripetal accelerations are given in the following table.
Typical Centripetal AccelerationsObjectCentripetal Acceleration (m/s2 or factors of g)Earth around the Sun 

Moon around the Earth 

Satellite in geosynchronous orbit  0.233 
Outer edge of a CD when playing 

Jet in a barrel roll  (2–3 g) 
Roller coaster  (5 g) 
Electron orbiting a proton in a simple Bohr model of the atom 

Equations of Motion for Uniform Circular Motion
A particle executing circular motion can be described by its position vector
(Figure) shows a particle executing circular motion in a counterclockwise direction. As the particle moves on the circle, its position vector sweeps out the angle
with the xaxis. Vector
making an angle
with the xaxis is shown with its components along the x– and yaxes. The magnitude of the position vector is
and is also the radius of the circle, so that in terms of its components,
Here,
is a constant called the angular frequency of the particle. The angular frequency has units of radians (rad) per second and is simply the number of radians of angular measure through which the particle passes per second. The angle
that the position vector has at any particular time is
.
If T is the period of motion, or the time to complete one revolution (
rad), then
Figure 4.20 The position vector for a particle in circular motion with its components along the x and yaxes. The particle moves counterclockwise. Angle
is the angular frequency
in radians per second multiplied by t.
Velocity and acceleration can be obtained from the position function by differentiation:
It can be shown from (Figure) that the velocity vector is tangential to the circle at the location of the particle, with magnitude
Similarly, the acceleration vector is found by differentiating the velocity:
From this equation we see that the acceleration vector has magnitude
and is directed opposite the position vector, toward the origin, because
Example
Circular Motion of a ProtonA proton has speed
and is moving in a circle in the xy plane of radius r = 0.175 m. What is its position in the xy plane at time
At t = 0, the position of the proton is
and it circles counterclockwise. Sketch the trajectory.
SolutionFrom the given data, the proton has period and angular frequency:
The position of the particle at
with A = 0.175 m is
From this result we see that the proton is located slightly below the xaxis. This is shown in (Figure).
Figure 4.21 Position vector of the proton at
The trajectory of the proton is shown. The angle through which the proton travels along the circle is 5.712 rad, which a little less than one complete revolution.
SignificanceWe picked the initial position of the particle to be on the xaxis. This was completely arbitrary. If a different starting position were given, we would have a different final position at t = 200 ns.
Nonuniform Circular Motion
Circular motion does not have to be at a constant speed. A particle can travel in a circle and speed up or slow down, showing an acceleration in the direction of the motion.
In uniform circular motion, the particle executing circular motion has a constant speed and the circle is at a fixed radius. If the speed of the particle is changing as well, then we introduce an additional acceleration in the direction tangential to the circle. Such accelerations occur at a point on a top that is changing its spin rate, or any accelerating rotor. In Displacement and Velocity Vectors we showed that centripetal acceleration is the time rate of change of the direction of the velocity vector. If the speed of the particle is changing, then it has a tangential acceleration that is the time rate of change of the magnitude of the velocity:
The direction of tangential acceleration is tangent to the circle whereas the direction of centripetal acceleration is radially inward toward the center of the circle. Thus, a particle in circular motion with a tangential acceleration has a total acceleration that is the vector sum of the centripetal and tangential accelerations:
The acceleration vectors are shown in (Figure). Note that the two acceleration vectors
and
are perpendicular to each other, with
in the radial direction and
in the tangential direction. The total acceleration
points at an angle between
and
Figure 4.22 The centripetal acceleration points toward the center of the circle. The tangential acceleration is tangential to the circle at the particle’s position. The total acceleration is the vector sum of the tangential and centripetal accelerations, which are perpendicular.
Example
Total Acceleration during Circular MotionA particle moves in a circle of radius r = 2.0 m. During the time interval from t = 1.5 s to t = 4.0 s its speed varies with time according to
What is the total acceleration of the particle at t = 2.0 s?
StrategyWe are given the speed of the particle and the radius of the circle, so we can calculate centripetal acceleration easily. The direction of the centripetal acceleration is toward the center of the circle. We find the magnitude of the tangential acceleration by taking the derivative with respect to time of
using (Figure) and evaluating it at t = 2.0 s. We use this and the magnitude of the centripetal acceleration to find the total acceleration.
SolutionCentripetal acceleration is
directed toward the center of the circle. Tangential acceleration is
Total acceleration is
and
from the tangent to the circle. See (Figure).
Figure 4.23 The tangential and centripetal acceleration vectors. The net acceleration
is the vector sum of the two accelerations.
SignificanceThe directions of centripetal and tangential accelerations can be described more conveniently in terms of a polar coordinate system, with unit vectors in the radial and tangential directions. This coordinate system, which is used for motion along curved paths, is discussed in detail later in the book.
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Summary
Uniform circular motion is motion in a circle at constant speed.Centripetal acceleration
is the acceleration a particle must have to follow a circular path. Centripetal acceleration always points toward the center of rotation and has magnitude
Nonuniform circular motion occurs when there is tangential acceleration of an object executing circular motion such that the speed of the object is changing. This acceleration is called tangential acceleration
The magnitude of tangential acceleration is the time rate of change of the magnitude of the velocity. The tangential acceleration vector is tangential to the circle, whereas the centripetal acceleration vector points radially inward toward the center of the circle. The total acceleration is the vector sum of tangential and centripetal accelerations.An object executing uniform circular motion can be described with equations of motion. The position vector of the object is
where A is the magnitude
which is also the radius of the circle, and
is the angular frequency.
Conceptual Questions
Can centripetal acceleration change the speed of a particle undergoing circular motion?