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a ( t ) = d v ( t ) d t = A ω 2 cos ω t i ^ A ω 2 sin ω t j ^ .

From this equation we see that the acceleration vector has magnitude A ω 2 and is directed opposite the position vector, toward the origin, because a ( t ) = ω 2 r ( t ) .

Circular motion of a proton

A proton has speed 5 × 10 6 m/s 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 t = 2.0 × 10 −7 s = 200 ns? At t = 0, the position of the proton is 0.175 m i ^ and it circles counterclockwise. Sketch the trajectory.


From the given data, the proton has period and angular frequency:

T = 2 π r v = 2 π ( 0.175 m ) 5.0 × 10 6 m / s = 2.20 × 10 −7 s
ω = 2 π T = 2 π 2.20 × 10 −7 s = 2.856 × 10 7 rad / s .

The position of the particle at t = 2.0 × 10 −7 s with A = 0.175 m is

r ( 2.0 × 10 −7 s ) = A cos ω ( 2.0 × 10 −7 s ) i ^ + A sin ω ( 2.0 × 10 −7 s ) j ^ m = 0.175 cos [ ( 2.856 × 10 7 rad / s ) ( 2.0 × 10 −7 s ) ] i ^ + 0.175 sin [ ( 2.856 × 10 7 rad / s ) ( 2.0 × 10 −7 s ) ] j ^ m = 0.175 cos ( 5.712 rad ) i ^ + 0.175 sin ( 5.712 rad ) j ^ = 0.147 i ^ 0.095 j ^ m .

From this result we see that the proton is located slightly below the x -axis. This is shown in [link] .

A graph of y position as a function of x position is shown. Both x and y are measured in meters and run from -0.2 to 0.2. A proton is moving in a counterclockwise circle centered on the origin is shown at 11 different times. At t = 0 s the particle is at x = 0.175 m and y = 0. At t = 200 nanoseconds, the particle is at a position given by vector 0.147 I hat minus 0.95 j hat meters.
Position vector of the proton at t = 2.0 × 10 −7 s = 200 ns . 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.


We picked the initial position of the particle to be on the x- axis. This was completely arbitrary. If a different starting position were given, we would have a different final position at t = 200 ns.

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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:

a T = d | v | d t .

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:

a = a C + a T .

The acceleration vectors are shown in [link] . Note that the two acceleration vectors a C and a T are perpendicular to each other, with a C in the radial direction and a T in the tangential direction. The total acceleration a points at an angle between a C and a T .

The acceleration of a particle on a circle is shown along with its radial and tangential components. The centripetal acceleration a sub c points radially toward the center of the circle. The tangential acceleration a sub T 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.
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.

Questions & Answers

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Practice Key Terms 4

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