EQUATIONS OF MOTION: ROTATION ABOUT A FIXED AXIS • Today’s Objectives: • Students will be able to: • Analyze the planar kinetics of a rigid body undergoing rotational motion. In-Class Activities: • Check Homework • Reading Quiz • Applications • Rotation About an Axis • Equations of Motion • Concept Quiz • Group Problem Solving • Attention Quiz
READING QUIZ 1. In rotational motion, the normal component of acceleration at the body’s center of gravity (G) is always __________. A) zero B) tangent to the path of motion of G C) directed from G toward the center of rotation D) directed from the center of rotation toward G 2. If a rigid body rotates about point O, the sum of the moments of the external forces acting on the body about point O equals? A) IGa B) IOa C) m aG D) m aO
Pin at the center of rotation. APPLICATIONS The crank on the oil-pump rig undergoes rotation about a fixed axis, caused by the driving torque, M, from a motor. As the crank turns, a dynamic reaction is produced at the pin. This reaction is a function of angular velocity, angular acceleration, and the orientation of the crank. If the motor exerts a constant torque M on the crank, does the crank turn at a constant angular velocity? Is this desirable for such a machine?
APPLICATIONS (continued) The pendulum of the Charpy impact machine is released from rest when = 0°. Its angular velocity () begins to increase. Can we determine the angular velocity when it is in vertical position ? On which property (P) of the pendulum does the angular acceleration () depend ? What is the relationship between P and ?
EQUATIONS OF MOTION: ROTATION ABOUT A FIXED AXIS (Section 17.4) When a rigid body rotates about a fixed axis perpendicular to the plane of the body at point O, the body’s center of gravity G moves in a circular path of radius rG. Thus, the acceleration of point G can be represented by a tangential component (aG)t = rG aand a normal component (aG)n = rG w2. Since the body experiences an angular acceleration, its inertia creates a moment of magnitude, Iga, equal to the moment of the external forces about point G. Thus, the scalar equations of motion can be stated as: Fn = m (aG)n = m rG w2 Ft = m (aG)t = m rG a MG = IG a
EQUATIONS OF MOTION (continued) Note that the MGmoment equation may be replaced by a moment summation about any arbitrary point. Summing the moment about the center of rotation O yields MO = IGa + rG m (aG) t = [IG + m(rG)2] a From the parallel axis theorem, IO = IG + m(rG)2, therefore the term in parentheses represents IO. Consequently, we can write the three equations of motion for the body as: Fn = m (aG)n = m rG w2 Ft = m (aG)t = m rG a MO = IO a
PROCEDURE FOR ANALYSIS Problems involving the kinetics of a rigid body rotating about a fixed axis can be solved using the following process. 1. Establish an inertial coordinate system and specify the sign and direction of (aG)n and (aG)t. 2. Draw a free body diagram accounting for all external forces and couples. Show the resulting inertia forces and couple (typicallyon a separate kinetic diagram). 3. Compute the mass moment of inertia IG or IO. 4. Write the three equations of motion and identify the unknowns. Solve for the unknowns. 5. Use kinematics if there are more than three unknowns (since the equations of motion allow for only three unknowns).
EXAMPLE Given: The uniform slender rod has a mass of 15 kg. Find:The reactions at the pin O and the angular acceleration of the rod just after the cord is cut. G Plan: Since the mass center, G, moves in a circle of radius 0.15 m, it’s acceleration has a normal component toward O and a tangential component acting downward and perpendicular to rG. Apply the problem solving procedure.
EXAMPLE (continued) Solution: FBD & Kinetic Diagram rG = Equations of motion: + Fn = man = mrGw2 Ox = 0 N + Ft = mat = mrGa -Oy + 15(9.81) = 15(0.15)a (1) + MO = IG a + m rG a (rG) (0.15) 15(9.81)= IG a + m(rG)2a Using IG = (ml2)/12 and rG = (0.15), we can write: IG a + m(rG)2a = [(15×0.92)/12 + 15(0.15)2] a = 1.35 a
EXAMPLE (continued) rG = After substituting: 22.07 = 1.35 a a = 16.4 rad/s2 From Eq (1) : -Oy + 15(9.81) = 15(0.15)a Oy = 15(9.81) − 15(0.15)16.4 = 110 N
CONCEPT QUIZ 1. If a rigid bar of length l (above) is released from rest in the horizontal position (q = 0), the magnitude of its angular acceleration is at maximum when A) q = 0 B) q = 90 C) q = 180 D) q = 0 and 180 2. In the above problem, when q = 90°, the horizontal component of the reaction at pin O is __________. A) zero B) m g C) m (l/2) w2 D) None of the above
GROUP PROBLEM SOLVING Given: Wsphere = 30 lb, Wrod = 10 lb Find: The reaction at the pin O just after the cord AB is cut. Plan: Draw the free body diagram and kinetic diagram of the rod and sphere as one unit. Then apply the equations of motion.
msphere(3)(0)2 30 lb 10 lb (IG)roda (IG)spherea mrod(1.0)(0)2 Ox = Oy mrod(1.0a) msphere(3a) GROUP PROBLEM SOLVING (continued) Solution: FBD and kinetic diagram; Equations of motion: Fn = m(aG)n: Ox = (30/32.2)(3)(0)2 + (10/32.2)(1.0)(0)2 Ox = 0 lb
msphere(3)(0)2 30 lb 10 lb (IG)roda (IG)spherea mrod(1.0)(0)2 Ox = Oy mrod(1.0a) msphere(3a) GROUP PROBLEM SOLVING (continued) Ft = m(aG)t: - Oy + 30 + 10 = (30/32.2) (3a) + (10/32.2) (1.0a) Oy = 40 – 3.106 a MO = Ioa: 30(3.0) + 10(1.0) = [ 0.4 (30/32.2) (1)2 + (30/32.2) (3)2 ]sphere a + [ (1/12) (10/32.2) (2)2 + (10/32.2) (1)2 ]rod a 100 = 9.172 a Therefore, a = 10.9 rad/s2, Oy = 6.14 lb
m m (a) (b) T 10 lb 10 lb ATTENTION QUIZ 1. A drum of mass m is set into motion in two ways: (a) by a constant 40 N force, and, (b) by a block of weight 40 N. If aa and ab represent the angular acceleration of the drum in each case, select the true statement. A) aa > ab B) aa < ab C) aa = ab D) None of the above 2. In case (b) above, what is the tension T in the cable? A) T = 40 N B) T < 40 N C) T > 40 N D) None of the above
End of the Lecture Let Learning Continue