Angular Velocity and Radian Measure

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Angular Velocity and Radian Measure

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    A. Knowledge Map Concept connections.

    13. If a circle with radius r is rotating

    about its center at a constant angular

    velocity w, measured in radians per

    time unit, what’s the tangential

    linear velocity v of any point on the


    12. What’s 3. What’s linear 11. What

    angular velocity? velocity? are radians?

    10. What’s

    arc length?

    9. What are 6. What’s an arc? angle degrees?

    5. What’s a circle? 8. What’s an angle?

    4a. What’s rotation?

    4. What’s a 2. What’s a line? 7. What’s a ray? line segment?

    1. What’s a point?

    Mass. Math. Curriculum Framework 12.M.1: Describe the relationship between degree and radian measures, and use radian measure in the solution of problems, in

    particular problems involving angular velocity [and acceleration].

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B. Knowledge Map Notes.

1. What's a point?

    A point is a location. In the Cartesian Plane every point can be represented by an ordered pair, (x,y).

    2. What's a line?

    A line is a set of points extending in opposite directions without bounds. On the Cartesian Plane each of its (x,y) points satisfy the equation Ax + By = C, for some A, B, and C.

    3. What's linear velocity?

    Linear velocity is the rate of change of distance along a line. So velocity v = d / t, where d is distance

    traveled, and t is the time it took. Velocity has both magnitude, called speed, and direction, plus or minus along the line.

    4. What's a line segment?

    A line segment is a subset of the points on a line between and including two distinct endpoints. 4a. What’s rotation?

    If one point on a rigid object is held motionless and motion is imparted to another point on the object, then the object will rotate about the motionless point, which is then called the center of rotation. 5. What's a circle?

    A circle is the set of all points the same distance from one point, called the center. The center is not part of the circle. A line segment between any point on the circle to the center is called a radius. A line segment between any two points on the circle is called a chord. A chord that includes the center is called a diameter, and has a length of two radii. The distance around a circle, its perimeter, is called its circumference. All circles are similar (have the same shape), so the circumference of a circle divided by the length of one of its diameters is a constant, called ;. ; = C / D is an irrational fraction that can be approximated to a

    pretty good engineering precision by the rational fraction 355/113. It’s easy to remember this approximate

    rational fraction from the numeral sequence 113355.

    6. What's an arc?

    An arc is a subset of the points on a circle between and including two distinct endpoints. An arc that’s a half circle is called a semicircle. A minor arc is smaller than a semicircle, and a major arc is larger than a semicircle.

    7. What's a ray?

    A ray is a set of points that includes a point on a line, called the ray's endpoint, and all that line's points on

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only one side of the endpoint.

    8. What's an angle?

    Start with two rays with the same endpoint, called the vertex. An angle is space formed by rotating the terminal ray about the vertex from an initial position collinear with the initial ray, to a final or terminal position. The angle is considered positive if the rotation was counter-clockwise, negative if clockwise. Angles with the same initial ray that differ by integer n rotations have the same terminal ray.

    9. What are angle degrees?

    Divide the circumference of a circle into 360 arcs of equal length, then the measure of each arc, and its central angle, is one degree. The planets Jupiter and Saturn were gods to the ancient Sumerians who first divided a circle into 360? more than 5000 years ago. In one year Jupiter moves 30? in the sky, and 30 years is the time Saturn takes to move once around the sky, its period. In one year Saturn moves 12? in the sky, and 12 years is Jupiter’s period. The Sumerians also gave us sexagesimal, base-60, numbers, still used in

    minutes and seconds of hours and degrees. The least common multiple of 12 and 30 is 60. 10. What's arc length?

    Arc length (s) is a fraction of the circumference (C) of a circle: s = C ? (T? / 360?), where T? is the degree

    measure of the arc and its central angle.

    11. What are radians?

    Arc length, s = C ? (T? / 360?) = 2 ? ; ? r ? (T? / 360?) => s / r = T? ? (; / 180?). Since ( s / r ) is directly

    proportional to T?, we use it as another measure of an arc and its central angle. While s / r is dimensionless

    ( length / length ), it is based on the radius so the unit "rad-ian" is assigned to it. So T radians := s / r =

    T? ? (; /180?) => T / T? = 1 = ; / 180? => ; radians = 180?.

    12. What's angular velocity?

    Angular velocity is the rate of change of an angle. So angular velocity w = T / t, where T is the changing

    angle measured in rotations, degrees, or radians, and t is the time. Angular velocity has both magnitude

    and direction, plus or minus around the vertex. Counter-clock-wise is positive.

    13. If a circle with radius r is rotating about its center at a constant angular velocity w, measured in

    radians per time unit, what's the tangential linear velocity v of any point on the circle?

    w = T / t , but T = s / r radians = the measure of the angle that’s changing, so

    w = s / r / t or w = s / t / r, but s / t = v, so

    w = v / r, implying that v = r ? w.

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C. Toolkit Student stumbling blocks and their remedies.

    Note: Class is assumed to be Honors Trigonometry.

    N5. Confusion over why ; = C / D is an irrational fraction. Remedy: review definitions of ratios, fractions, and rational numbers. A rational number can be expressed as a ratio of integers. Integers are the natural numbers, their opposites, and zero. A ratio is a fraction whose numerator and denominator are any numbers, not just integers. So if C / D is irrational, then either C or D or both is/are irrational.

    N8. Confusion over extended angle definition, esp. multiple angles that use same initial and terminal rays, and positive and negative angles. Remedy: demonstrate multiple examples on blackboard. Have pairs of students create and graph angles with same initial ray and with and without same terminal ray; one student poses the problem, the other solves, then they switch for the next.

N11-1. Confusion over the statement that s / r is dimensionless and directly proportional to T? in the

    equation s / r = T? ? (; / 180?). Remedy: Review direct proportionality as y = kx, and point out that the

    constant of proportionality in this case, π / 180?, has dimensions 1/degrees. So the degrees in T? are

    cancelled by the 1/degrees in the constant of proportionality leaving s / r dimensionless.

N11-2. Confusion over what exactly is one radian. Remedy: T radians = s / r, so if T = 1 radian then s / r

    = 1 and s = r. So one radian is a central angle whose arc length equals one radius. Draw one. Then draw a circle and annotate with special angle measures in degrees and radians: 0, 180? = ;, 90? = ; / 2, 45? = ; /

    4, 15? = ; /12, 30? = ; / 6, 60? = ; / 3, and 75? = 5; /12. Fill in quadrants II IV by adding n 90? = n ; / 2,

    n = 0, 1, 2, to the quadrant I angles.

    N12. Confusion over what angular velocity is. Remedy: Think of a Merry-Go-Round (Carousel) or Ferris wheel. Both rotate about their centers at a constant change of angle: rotations/minute, degrees/sec, radians/sec. Tachometers measure the angular velocity of an engine in rotations per minute, rpm.

    N13. Confusion over what linear velocity means for a point on a rotating circle. Remedy: Think of the “linear” part as along the curved line of the circle, the curved path of the point. Then linear velocity is how

    fast the point’s path length is changing with time.

D. Applications Problems and solutions.

    1. From MA Math Curriculum Frameworks, November 2000, p.65, referring to Standard 12.M.1:

    In one hour, the minute hand on a clock moves through a complete circle, and the hour hand moves through 1/12 of a circle. Through how many radians do the minute and the hour hand move between 1:00 p.m. and 6:45 p.m. on the same day? Express you answers to the nearest hundredth of a radian.

Solution: 6:45 1:00 = 5 hours + 45/60 hour = 5.75 hours

    angular displacements:

    minute hand: 5.75 hours ? 2; radians / hour ? – 36.13 radians

    hour hand: 5.75 hours ? 2; radians / circle ? 1/12 circle / hour ? – 3.01 radians

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2. Retrieved and modified 8/26/2007 from

    Kara and Jami took a ride on the Bear Mountain Carousel in Bear Mountain State Park, NY. Jami sat on a horse in the outside row, and Kara sat in the row closest to the center of the carousel. The rows are 17.6 feet and 12.4 feet from the center respectively. The angular speed of the carousel is 4.3 revolutions per minute.

    Calculate the linear speed of each rider in feet per second to the nearest tenth. Who is traveling at a faster rate? Why?

Solution: Kara's linear speed: 5.6 ft/sec = 12.4 ft ? 4.3 rpm ? 2; radians/rotation ? 1 min/60 sec

    Jami's linear speed: 7.9 ft/sec = 17.6 ft ? 4.3 rpm ? 2; radians/rotation ? 1 min/60 sec

    Jami has a faster linear speed because she is farther from the center of the carousel.

3. In London, England, the London Eye Ferris wheel takes 30 minutes to complete one revolution. It

    rotates at a constant angular velocity and never stops, so it passes by the entrance and exit ramps at a slow walk speed of 26 cm/sec so passengers can walk on and off the capsules as it rotates.

What is the London Eye’s diameter in feet and inches to the nearest inch?

Solution: v = r w => r = v / w = (26 cm/sec) / (1 rev / 30 min)

    D = 2?(26 cm/sec)?(30 min / 1 rev)?(1 in / 2.54 cm)?(1 ft / 12 in)?(60 sec/min)?(1 rev/ 2; rad)

    D = 488.74352603023 ft = 488 ft + 0.74352603023 ft ? (12 in / ft) ? 488 ft 9 in.

    4. Challenge Problem. Requires use of cosine function (Why?), so you need to review, or look ahead.

    Search the Internet for the latitude and longitude of the center of Lowell, MA, the average radius of the Earth, and how long it takes the Earth to rotate once with respect to the stars (sidereal period). Then find Lowell’s angular and linear velocities, in radians / minute and miles / hour to the nearest hundredths.

Solution: From,

    Lowell’s latitude and longitude are 42.633N and 71.316W.


    Earth’s: average radius ? (6378.1 km + 6356.8 km) / 2

     ? (100,000 cm / km) ? (in / 2.54 cm) ? (ft / 12 in) ? (mi / 5280 ft)

     ? 3956.55 miles.

     sidereal rotation period ? 23.9345 hours

    Lowell’s angular velocity ? (1 rot / 23.9345 hrs) ? (2; rad / 1 rot) ? (60 min / hr)

     ? 15.75 radians / minute

    Equatorial linear velocity ? 3956.55 miles ? 15.75 radians / min ? (hr / 60 min)

     ? 1,038.59 mph

    Lowell’s linear velocity ? 1,038.59 mph ? cos(42.633?) ? 764.10 mph

     (because latitude circle radius is smaller than equator’s)

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