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Virtual Work MEAM 535 Principle of Virtual Work Aristotle Galileo (1594) Bernoulli (1717) Lagrange (1788) 1. Start with static equilibrium of holonomic system of N particles 2. Extend to rigid bodies 3. Incorporate inertial forces for dynamic analysis 4. Apply to nonholonomic systems University of Pennsylvania 1 MEAM 535 Virtual Work Key Ideas (a) Fi Virtual displacement e2 Small Consistent with constraints Occurring without passage of time rPi Applied forces (and moments) Ignore constraint forces Static equilibrium e Zero acceleration, or O 1 Zero mass Every point, Pi, is subject to The virtual work is the work done by a virtual displacement: . e3 the applied forces. N n generalized coordinates, qj (a) Pi δW = ∑[Fi ⋅δr ] i=1 University of Pennsylvania 2 € MEAM 535 Example: Particle in a slot cut into a rotating disk Angular velocity Ω constant Particle P constrained to be in a radial slot on the rotating disk P F r How do describe virtual b2 Ω displacements of the particle P? b1 O No. of degrees of freedom in A? Generalized coordinates? B Velocity of P in A? a2 What is the virtual work done by the force a1 F=F1b1+F2b2 ? University of Pennsylvania 3 MEAM 535 Example l Applied forces G=τ/2r B F acting at P Q r φ θ m F G acting at Q P (assume no gravity) Constraint forces x All joint reaction forces Single degree of freedom Generalized coordinate, θ Motion of particles P and Q can be described by the generalized coordinate θ University of Pennsylvania 4 MEAM 535 Static Equilibrium Implies Zero Virtual Work is Done Forces Forces that do work Applied Forces or External Forces Forces that do no work Constraint forces (a) Fi Static Equilibrium Ri Implies sum of all forces on each particle equals zero N (a) ∑[Fi + Ri ].δri = 0 i=1 € University of Pennsylvania 5 MEAM 535 The Key Idea Constraint forces do zero virtual work! 0 N (a) ∑[Fi + Ri ].δri = 0 i=1 € N (a ) δW = ∑Fi .δri = 0 i=1 € University of Pennsylvania 6 MEAM 535 Principle of Virtual Work If a system of N particles (P1, P2,…, PN) is in static equilibrium, the virtual work done by all the applied (active) forces though any (arbitrary) virtual displacement is zero. Converse: If the virtual work done by all the applied (active) forces on a system of N particles though any (arbitrary) virtual displacement is zero, the system is in static equilibrium. Proof: Assume the system is not in static equilibrium. (a) for some Pi [Fi + Ri ] ≠ 0 Can always find virtual displacement so that University of Pennsylvania 7 € MEAM 535 Constraints: Two Particles Connected by Rigid Massless Rod (x2 , y2) e F2 F2 R2 F 1 F1 R1 (x1 , y1) 2 2 2 (x1 – x2) +(y1 – y2) = r R1 = -R2 = αe University of Pennsylvania 8 MEAM 535 Rigid Body: A System of Particles A rigid body is a system of infinite particles. The distance between any pair of particles stays constant through its motion. Each pair of particles can be considered as connected by a massless, rigid rod. The internal forces associated with this distance constraint are constraint forces. The internal forces do no virtual work! University of Pennsylvania 9 MEAM 535 Contact Constraints and Normal Contact Forces Rigid body A rolls and/or slides on rigid body B n Contact Forces P1 T1 P2 r N2 B 1 N1 T r2 2 contact O C N1 = -N2 = αn normal T1 = -T2 = βt A P B P Contact Kinematics v 2 .n = v 1 .n C P C P P1 and P2 are in v 2 .n = v 1 .n contact implies: δr2 .n = δr1 .n University of Pennsylvania 10 MEAM 535 Normal and Tangential Contact Forces A 1. Normal contact forces t P Normal contact forces are constraint forces 1 T1 Equivalently, normal forces do no virtual work N2 N1 T2 2. Tangential contact forces P2 If A rolls on B (equivalently B rolls on A) A P2 B P1 C P2 C P1 v = v , v = v B then, tangential contact forces are constraint forces In general (sliding with friction), tangential forces will contribute to virtual work A P B P v 2 = v 1 University of Pennsylvania 11 MEAM 535 Principle of Virtual Work for Holonomic Systems A system of N particles (P1, P2,…, PN) is in static equilibrium if and only if the virtual work done by all the applied (active) forces though any (arbitrary) virtual displacement is zero. A holonomic system of N particles is in static equilibrium if and only if all the generalized (active) forces are zero. Only “applied” or “active” forces contribute to the generalized force The jth generalized force is given by Why? University of Pennsylvania 12 MEAM 535 Principle of Virtual Work vs. Traditional Approach Traditional Principle of Virtual Work P2 1. Generalized Coordinates F 1 2. Identify forces that do work F2 3. Analyze motion of points at P1 which forces act T Free Body Diagram 4. Calculate generalized forces F2 F1 T 2 equations for each particle University of Pennsylvania 13 MEAM 535 Partial Velocities In any frame A n speeds Pi rPi a2 a1 Define the jth partial velocity of Pi O A a3 N (a) Pi The jth generalized force is Q j = ∑[Fi ⋅ v j ] i=1 University of Pennsylvania 14 € MEAM 535 Example Illustrating Partial Velocities Three Degree-of-Freedom Robot Arm differentiating University of Pennsylvania 15 MEAM 535 Example (continued) Equations relating the joint velocities and the end effector velocities ˙ ˙ ˙ ˙ ˙ ˙ ˙ x = −l1θ1 s1 − l2 (θ1 + θ 2)s12 − l3(θ1 + θ 2 + θ 3)s123 P ˙ ˙ ˙ ˙ ˙ ˙ ˙ y = l1θ1 c1 + l2(θ1 + θ 2 )c12 + l3 (θ1 + θ 2 + θ 3 )c123 in matrix form € u 1 x˙ −(l1s1 + l2s12 + l3s123 ) −(l2s12 + l3s123 ) −l3s123 u = 2 y˙ (l1c1 + l2c12 + l3c123) (l2c12 + l3c123) l3c123 u 3 The three partial velocities€ of the point P (omitting leading superscript A) are columns of the “Jacobian” matrix University of Pennsylvania 16 MEAM 535 Example 1 Generalized speed: l G=2τ/r B u=dθ/dt Q r φ θ m F Generalized Active Forces P F = -Fa1 x No friction, gravity Kinematic Analysis Need Partial Velocities University of Pennsylvania 17 MEAM 535 Example 1 (continued) Generalized speed: l G=2τ/r B u=dθ/dt Q r Velocities φ F θ m P x Generalized Active Forces F = -Fa1 The system is in static equilibrium No friction, gravity if and only if Q1=0 University of Pennsylvania 18 MEAM 535 Example 2 Assumptions No friction at the wall homogeneous rod, Gravity (center of mass is at length 3l Q midpoint, C) Massless string, OP h C O θ Q l φ 2l P C P University of Pennsylvania 19 MEAM 535 Example 3 A solid circular cylinder B of mass M rolls down a fixed plane wedge A without slipping. The mass m is connected to a massless string which passes over a pulley C and wraps around a massless spool attached to the cylinder at the other end. (a) Determine the relationship between m and M for static equilibrium. (b) Find the tensions in the string on either side of the pulley C. University of Pennsylvania 20 MEAM 535 Conservative Holonomic Systems All applied forces are conservative Or There exists a scalar potential function such that all applied forces are given by: The virtual work done by the applied forces is: University of Pennsylvania 21 MEAM 535 Statement A conservative, holonomic system of N particles (P1, P2,…, PN) is in static equilibrium if and only if the change in potential energy though any (arbitrary) virtual displacement is zero. University of Pennsylvania 22 MEAM 535 HW Problem What are the generalized forces? z q2 a3 y a2 b2 a1 e2 C b3 C* q4 q q3 x 1 b1 P q5 e3 e1 University of Pennsylvania 23 .
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