ENGR 214 Chapter 15 Kinematics of Rigid Bodies

ENGR 214 Chapter 15 Kinematics of Rigid Bodies All figures taken from Vector Mechanics for Engineers: Dynamics, Beer and Johnston, 2004 1

Introduction • Kinematics of rigid bodies: relations between time and the positions, velocities, and accelerations of the particles forming a rigid body. • Classification of rigid body motions: - translation: • rectilinear translation • curvilinear translation - rotation about a fixed axis - general plane motion - motion about a fixed point - general motion 2

Motion of the plate: is it translation or rotation? Curvilinear translation Rotation 3

Translation • Consider rigid body in translation: - direction of any straight line inside the body is constant, - all particles forming the body move in parallel lines. • For any two particles in the body, • Differentiating with respect to time, All particles have the same velocity. • Differentiating with respect to time again, All particles have the same acceleration. 4

Rotation About a Fixed Axis When a body rotates about a fixed axis, any point P in the body travels along a circular path. The angular position of P is defined by . The change in angular position, d , is called the angular displacement, with units of either radians or revolutions. They are related by 1 revolution = 2 radians Angular velocity, , is obtained by taking the time derivative of angular displacement: = d /dt (rad/s) + Similarly, angular acceleration is = d 2 /dt 2 = d /dt or = (d /d ) + rad/s 2 5

Rotation About a Fixed Axis: Velocity • Consider rotation of rigid body about a fixed axis AA’ • Velocity vector of the particle P is tangent to the path with magnitude • The same result is obtained from 6

Rotation About a Fixed Axis: Acceleration • Differentiating to determine the acceleration, • • Acceleration of P is combination of two vectors, 7

Rotation About a Fixed Axis: Representative Slab • Consider the motion of a representative slab in a plane perpendicular to the axis of rotation. • Velocity of any point P of the slab, • Acceleration of any point P of the slab, • Resolving the acceleration into tangential and normal components, 8

Examples 9

Equations Defining the Rotation of a Rigid Body About a Fixed Axis • Motion of a rigid body rotating around a fixed axis is often specified by the type of angular acceleration. • Recall • Uniform Rotation, a = 0: • Uniformly Accelerated Rotation, a = constant: 10

Sample Problem 5. 1 Cable C has a constant acceleration of 9 in/s 2 and an initial velocity of 12 in/s, both directed to the right. Determine (a) the number of revolutions of the pulley in 2 s, (b) the velocity and change in position of the load B after 2 s, and (c) the acceleration of the point D on the rim of the inner pulley at t = 0. 11

Sample Problem 5. 1 • The tangential velocity and acceleration of D are equal to the velocity and acceleration of C. • Apply the relations for uniformly accelerated rotation to determine velocity and angular position of pulley after 2 s. 12

Sample Problem 5. 1 • Evaluate the initial tangential and normal acceleration components of D. Magnitude and direction of the total acceleration, 13

General Plane Motion A combination of translation & rotation 14

General Plane Motion Pure translation, followed by rotation about A 2 (to move B'1 to B' 2) Motion of B w. r. t. A is pure rotation, i. e. B draws a circle centered at A Any plane motion can be represented as a translation of an arbitrary reference point A and a rotation about A. 15

Absolute and Relative Velocity For any two points lying on the same rigid body: Note: = distance from A to B Equation can be represented graphically by a velocity diagram 16

Absolute and Relative Velocity Assuming that the velocity v. A of end A is known, determine the velocity v. B of end B and the angular velocity . Locus for v. B v. A v. B Locus for v. B/A q v. B/A The direction of v. B and v. B/A are known. Complete the velocity diagram. 17

Absolute and Relative Velocity in Plane Motion • Selecting point B as the reference point and solving for the velocity v. A of end A and the angular velocity w leads to an equivalent velocity triangle. • v. A/B has the same magnitude but opposite sense of v. B/A. The sense of the relative velocity is dependent on the choice of reference point. • Angular velocity w of the rod in its rotation about B is the same as its rotation about A. Angular velocity is not dependent on the choice of reference point. 18

Rolling Motion Consider a circular disc that rolls without slipping on a flat surface O 1 A 2 r O 2 q From geometry: A 1 s s = displacement of center 19

Sample Problem 15. 2 The double gear rolls on the stationary lower rack; the velocity of its center is 1. 2 m/s. Determine (a) the angular velocity of the gear, and (b) the velocities of the upper rack R and point D of the gear. 20

Sample Problem 15. 2 v. D/A v. B/A For any point P on the gear: For point B: For point D: 21

Sample Problem 15. 3 The crank AB has a constant clockwise angular velocity of 2000 rpm. For the crank position indicated, determine (a) the angular velocity of the connecting rod BD, and (b) the velocity of the piston P. 22

Sample Problem 15. 3 • The velocity Locus for v. D/B is obtained from the crank rotation data. • The direction of the absolute velocity The direction of the relative velocity Locus for v. D perpendicular to BD. is horizontal. is 23

Sample Problem 15. 3 • Determine the velocity magnitudes from the vector triangle drawn to scale. 24

Instantaneous Center of Rotation For any body undergoing planar motion, there always exists a point in the plane of motion at which the velocity is instantaneously zero (if it were rigidly connected to the body). This point is called the instantaneous center of rotation, or C. It may or may not lie on the body! If the location of this point can be determined, the velocity analysis can be simplified because the body appears to rotate about this point at that instant. 25

Instantaneous Center of Rotation To locate the C, we use the fact that the velocity of a point on a body is always perpendicular to the position vector from C to that point. If the velocity at two points A and B are known, C lies at the intersection of the perpendiculars to the velocity vectors through A and B. If the velocity vectors at A and B are perpendicular to the line AB, C lies at the intersection of the line AB with the line joining the extremities of the velocity vectors at A and B. If the velocity vectors are equal & parallel, C is at infinity and the angular velocity is zero (pure translation) 26

Instantaneous Center of Rotation If the velocity v. A of a point A on the body and the angular velocity w of the body are known, C is located along the line drawn perpendicular to v. A at A, at a distance r = v. A/w from A. Note that the C lies up and to the right of A since v. A must cause a clockwise angular velocity w about C. 27

Velocity Analysis using Instantaneous Center The velocity of any point on a body undergoing general plane motion can be determined easily if the instantaneous center is located. Since the body seems to rotate about the IC at any instant, the magnitude of velocity of any arbitrary point is v = w r, where r is the radial distance from the IC to that point. The velocity’s line of action is perpendicular to its associated radial line. Note the velocity has a direction which tends to move the point in a manner consistent with the angular rotation direction. 28

Velocity Analysis using Instantaneous Center 29

Instantaneous Center of Rotation C lies at the intersection of the perpendiculars to the velocity vectors through A and B. The velocity of any point on the rod can be obtained. Accelerations cannot be determined using C. 30

Sample Problem 15. 4, using instantaneous center The double gear rolls on the stationary lower rack; the velocity of its center is 1. 2 m/s. Determine (a) the angular velocity of the gear, and (b) the velocities of the upper rack R and point D of the gear. 31

Sample Problem 15. 4 Point C is in contact with the stationary lower rack and, instantaneously, has zero velocity. It must be the location of the instantaneous center of rotation. 32

Sample Problem 15. 5 using instantaneous center Crank-slider mechanism The crank AB has a constant clockwise angular velocity of 2000 rpm. For the crank position indicated, determine (a) the angular velocity of the connecting rod BD, and (b) the velocity of the piston P. 33

Sample Problem 15. 5 C is at the intersection of the perpendiculars to the velocities through B and D. 34

Absolute and Relative Acceleration Absolute acceleration of point B: Relative acceleration includes tangential and normal components: 35

Absolute and Relative Acceleration • Given determine • Vector result depends on sense of relative magnitudes of and the • Must also know angular velocity w. 36

Absolute and Relative Acceleration Draw acceleration diagram to scale: 37

Sample Problem 15. 6 The center of the double gear has a velocity and acceleration to the right of 1. 2 m/s and 3 m/s 2, respectively. The lower rack is stationary. Determine (a) the angular acceleration of the gear, and (b) the acceleration of points B, C, and D. 38

Sample Problem 15. 6 39

Sample Problem 15. 7 Crank AG of the engine system has a constant clockwise angular velocity of 2000 rpm. For the crank position shown, determine the angular acceleration of the connecting rod BD and the acceleration of point D. 40

Sample Problem 15. 7 From Sample Problem 15. 3, w. BD = 62. 0 rad/s, b = 13. 95 o. 41

Sample Problem 15. 7 Draw acceleration diagram: Drawn to scale 42

Sample Problem 15. 8 Four-bar mechanism In the position shown, crank AB has a constant angular velocity w 1 = 20 rad/s counterclockwise. Determine the angular velocities and angular accelerations of the connecting rod BD and crank DE. 43

Sample Problem 15. 8 v. B v. D Velocities Velocity diagram v. D/B v. D Shown here not to scale 44

Sample a Problem 15. 8 D/B t a. D/E t Accelerations a. D/B n a. B a. D/E n Acceleration diagram a. D/E n a. B a. D/B n a. D/E t Shown here not to scale 45