Basic Physics

# Uniform circular motion – problems and solutions

1. An object moves in a circle with the constant angular speed of 10 rad/s. Determine (a) Angular speed after 10 seconds (b) Angular displacement after 10 seconds.

Known :

Wanted :

(a) Angular speed (ω) after 10 seconds.

(b) Angle (θ) after 10 seconds

Solution :

(a) Angular speed (ω) after 10 seconds

Object in uniform circular motion so that angular speed is constant, 10 rad/s.

(b) Angular displacement (θ)

Constant angular speed 10 radians/second means the object around 10 radians each second. After 10 seconds, the object around 10 x 10 radians = 100 radians.

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2. A particle moves in a circle with the constant speed of 10 m/s. Radius of circle = 1 meter. Determine (a) Particle’s speed after 5 seconds (b) Particle’s displacement after 5 seconds (c) Centripetal acceleration.

Known :

Radius of circle (r) = 1 meter

Particle’s speed (v) = 10 m/s

Solution :

(a) Particle’s speed after 5 seconds

The motion of object is in the uniform circular motion so that speed is constant, 10 m/s.

(b) Particle’s displacement after 5 seconds

10 meters/second means each 1 second, particle’s displacement = 10 meters. After 5 seconds, particle’s displacement = 5 x 10 meters = 50 meters.

(c) Centripetal acceleration (ar)

ar = v2 / r = 102 / 1 = 100 / 1 = 100 m/s2

3. A ball attached to one end of a cord, is revolved in a circle with radius of 2 meters at the constant speed of 60 rpm. Determine (a) the magnitude of the angular speed after 2 seconds (b) the angular displacement after 1 minute.

Known :

Radius of circle (r) = 2 meters

Angular speed (ω) = 60 rpm = 60 revolutions / 1 minute

= 60 revolutions / 60 seconds = 1 revolution / second = 2π radians / second

Solution :

(a) Angular speed (ω) after 2 seconds

The angular speed is constant so after 2 seconds, the angular speed (ω) = 6.28 radians / second

(b) Angular displacement (θ)

The angular speed = 1 revolution/second means each 1 second, ball experience 1 revolution. After 60 seconds, ball moves 60 revolutions.

The angular speed = 6.28 radians/second means each 1 second, the ball moves with the angle of 6.28 radians. After 60 seconds, the ball moves 376.8 radians.

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4. A bike wheel rotates 120 revolutions in 60 seconds. What is the angular speed?

Solution :

(a) revolutions per minute (rpm)

120 revolutions / 60 seconds = 120 revolutions / 1 minute = 120 revolutions / minute = 120 rpm

(b) degrees per second (o/s)

1 revolution = 360o, 120 revolutions = 43200o

120 revolutions / 60 seconds = (120)(360o) / 60 seconds = 43200o / 60 seconds = 720o/second