ME 2320 Lecture Notes - Lecture 10: Virtual Displacement, Virtual Work, Rigid Body
23 May 2018
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10.1A
Forces that do no work:
●reaction at a frictionless pin when the body supported rotates about the pin
●reaction at a frictionless surface when the body in contact moves along the surface;
●the reaction at a roller moving along its track;
●the weight of a body when its center of gravity moves horizontally;
●the friction force acting on a wheel rolling without slipping (since at any instant the point of
contact does not move)
Forces that do work
●weight of a body
●the friction force acting on a body sliding on a rough surface
●most forces applied on a moving body
p. 585
CHAPTER 10: Solving Problems on Your Own
In this section: method of virtual work
U dscosαd=F
U dθd=M
Principle of virtual work: If a system of connected rigid bodies is in equilibirum, the total virtual wokr of
the external forces applied to the system is zero for any virtual displacement of the system.
●Virtual displacement: A machine of mechanism in equilibirum has no tendency to move.
However, we can cause--or imagine-- a small displacement. Since it does not actually occur, such
a displacement is called a virtual displacement.
●Virtual work: The work done by a force or couple durin ga virtual displacement is called virtual
work.
●You need to consider only the forces that do work during the virtual displacement.
●Forces that do no work during a virtual displacement that are consistent with the constraints
imposed on the system are
a. Reactions at supports
b. Internal forces at onnections
c. Forces exreted by inextensible cords and cables
None of these forces need be consifered when you use the method of virtual work.
Document Summary
Reaction at a frictionless pin when the body supported rotates about the pin. Reaction at a frictionless surface when the body in contact moves along the surface; The reaction at a roller moving along its track; The weight of a body when its center of gravity moves horizontally; The friction force acting on a wheel rolling without slipping (since at any instant the point of contact does not move) The friction force acting on a body sliding on a rough surface. Most forces applied on a moving body: 585. In this section: method of virtual work dscos . Principle of virtual work: if a system of connected rigid bodies is in equilibirum, the total virtual wokr of the external forces applied to the system is zero for any virtual displacement of the system. Virtual displacement : a machine of mechanism in equilibirum has no tendency to move. However, we can cause--or imagine-- a small displacement.
