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**CURVILINEAR MOTION: CYLINDRICAL COMPONENTS (Section 12.8)**

Today’s Objectives: Students will be able to determine velocity and acceleration components using cylindrical coordinates. In-Class Activities: Check homework, if any Reading quiz Applications Velocity Components Acceleration Components Concept quiz Group problem solving Attention quiz

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**A) transverse velocity. B) radial velocity. **

READING QUIZ 1. In a polar coordinate system, the velocity vector can be written as v = vrer + vθeθ = rer + rqeq. The term q is called A) transverse velocity. B) radial velocity. C) angular velocity. D) angular acceleration. . Answers: 1. C 2. C 2. The speed of a particle in a cylindrical coordinate system is A) r B) rq C) (rq)2 + (r)2 D) (rq)2 + (r)2 + (z)2 .

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APPLICATIONS The cylindrical coordinate system is used in cases where the particle moves along a 3-D curve. In the figure shown, the boy slides down the slide at a constant speed of 2 m/s. How fast is his elevation from the ground changing (i.e., what is z )? .

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**APPLICATIONS (continued)**

A polar coordinate system is a 2-D representation of the cylindrical coordinate system. When the particle moves in a plane (2-D), and the radial distance, r, is not constant, the polar coordinate system can be used to express the path of motion of the particle.

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**POSITION (POLAR COORDINATES)**

We can express the location of P in polar coordinates as r = rer. Note that the radial direction, r, extends outward from the fixed origin, O, and the transverse coordinate, q, is measured counter-clockwise (CCW) from the horizontal.

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**VELOCITY (POLAR COORDINATES)**

The instantaneous velocity is defined as: v = dr/dt = d(rer)/dt v = rer + r der dt . Using the chain rule: der/dt = (der/dq)(dq/dt) We can prove that der/dq = eθ so der/dt = qeθ Therefore: v = rer + rqeθ . . Thus, the velocity vector has two components: r, called the radial component, and rq, called the transverse component. The speed of the particle at any given instant is the sum of the squares of both components or v = (r q )2 + ( r )2

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**ACCELERATION (POLAR COORDINATES)**

The instantaneous acceleration is defined as: a = dv/dt = (d/dt)(rer + rqeθ) . After manipulation, the acceleration can be expressed as a = (r – rq2)er + (rq + 2rq)eθ .. . . The magnitude of acceleration is a = (r – rq2)2 + (rq + 2rq)2 The term (r – rq2) is the radial acceleration or ar. The term (rq + 2rq) is the transverse acceleration or aq ..

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**CYLINDRICAL COORDINATES**

If the particle P moves along a space curve, its position can be written as rP = rer + zez Taking time derivatives and using the chain rule: Velocity: vP = rer + rqeθ + zez Acceleration: aP = (r – rq2)er + (rq + 2rq)eθ + zez .. .

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**Find: Velocity and acceleration at q = 30°. **

EXAMPLE Given: r = 5 cos(2q) (m) q = 3t2 (rad/s) qo = 0 Find: Velocity and acceleration at q = 30°. Plan: Apply chain rule to determine r and r and evaluate at q = 30°. . . .. t ò o = Solution: q = q dt = 3t2 dt = t3 At q = 30°, q = = t3. Therefore: t = s. q = 3t2 = 3(0.806)2 = 1.95 rad/s 6 p .

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EXAMPLE (continued) q = 6t = 6(0.806) = rad/s2 r = 5 cos(2q) = 5 cos(60) = 2.5m r = -10 sin(2q)q = -10 sin(60)(1.95) = m/s r = -20 cos(2q)q2 – 10 sin(2q)q = -20 cos(60)(1.95)2 – 10 sin(60)(4.836) = -80 m/s2 .. . Substitute in the equation for velocity v = rer + rqeθ v = er + 2.5(1.95)eθ v = (16.88)2 + (4.87)2 = m/s .

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EXAMPLE (continued) Substitute in the equation for acceleration: a = (r – rq2)er + (rq + 2rq)eθ a = [-80 – 2.5(1.95)2]er + [2.5(4.836) + 2(-16.88)(1.95)]eθ a = er – 53.7eθ m/s2 a = (89.5)2 + (53.7)2 = m/s2 .. .

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**CONCEPT QUIZ .. . 1. If r is zero for a particle, the particle is**

A) not moving. B) moving in a circular path. C) moving on a straight line. D) moving with constant velocity. . Answers: 1. D 2. C 2. If a particle moves in a circular path with constant velocity, its radial acceleration is A) zero B) r. C) -rq D) 2rq. .. .

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GROUP PROBLEM SOLVING Given: The car’s speed is constant at 1.5 m/s. Find: The car’s acceleration (as a vector). Hint: The tangent to the ramp at any point is at an angle f = tan-1( ) = ° Also, what is the relationship between f and q? 12 2p(10) Plan: Use cylindrical coordinates. Since r is constant, all derivatives of r will be zero. Solution: Since r is constant the velocity only has 2 components: vq = rq = v cosf and vz = z = v sinf .

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**GROUP PROBLEM SOLVING (continued)**

Therefore: q = ( ) = rad/s q = 0 v cosf r . .. vz = z = v sinf = m/s z = 0 r = r = 0 . .. .. . a = (r – rq2)er + (rq + 2rq)eθ + zez a = (-rq2)er = -10(0.147)2er = er m/s2

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**Example SOLUTION: Evaluate time t for q = 30o.**

Evaluate radial and angular positions, and first and second derivatives at time t. Rotation of the arm about O is defined by q = 0.15t2 where q is in radians and t in seconds. Collar B slides along the arm such that r = t2 where r is in meters. After the arm has rotated through 30o, determine (a) the total velocity of the collar, (b) the total acceleration of the collar, and (c) the relative acceleration of the collar with respect to the arm. Calculate velocity and acceleration in cylindrical coordinates. Evaluate acceleration with respect to arm.

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SOLUTION: Evaluate time t for q = 30o. Evaluate radial and angular positions, and first and second derivatives at time t.

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**Calculate velocity and acceleration.**

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**Evaluate acceleration with respect to arm.**

Motion of collar with respect to arm is rectilinear and defined by coordinate r.

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**C) greater than its transverse component. **

ATTENTION QUIZ 1. The radial component of velocity of a particle moving in a circular path is always A) zero. B) constant. C) greater than its transverse component. D) less than its transverse component. Answers: 1. A 2. B 2. The radial component of acceleration of a particle moving in a circular path is always A) negative. B) directed toward the center of the path. C) perpendicular to the transverse component of acceleration. D) All of the above.

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End of the Lecture Let Learning Continue

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