Name: finding forces with virtual work in linked bodies
Uploaded: 2023-09-22T11:49:36.000Z
Duration: 1 min 6 s
Channel: JoVE

Принцип виртуальной работы может быть применен к машинам с несколькими соединенными жесткими телами под нагрузкой для решения связанных с этим задач.
Рассмотрим коленно-рычажные тиски, в середине которых прикладывается сила, в результате чего сила реакции оказывается деревянным бруском на тисках.
На диаграмме свободного тела трением пренебрегают, и учитывается положительное виртуальное смещение.
Выберите систему осей координат, а также определите положительные и отрицательные приращения.
Силы реакции в левой точке и нормальная сила не работают во время этого смещения.
В результате необходимо вычислить только виртуальную работу, выполненную прикладными и противодействующими силами.
Выражая координаты в терминах тета угла и дифференцируя, можно определить общую виртуальную работу обеих сил.
Если свести общую виртуальную работу к нулю, то получится уравнение для силы реакции, оказываемой деревянным бруском на тиски.

<ol>
	<li>Хиббелер, Р.К. (2016). Инженерная механика: Статика. Четырнадцатое издание, Нью-Джерси: Пирсон. Раздел 11.1, стр. 586-587.</li>
	<li>Бир, Ф.., Джонстон, Э.Р., Мазурек, Д.Ф., Корнуэлл,.Д. и Селф, Б.. (2016). Векторная механика для инженеров. Одиннадцатое издание, Нью-Йорк: McGraw-Hill Education. С. 578-579.</li>
</ol>

Виртуальная работа для системы связанных жестких тел

Виртуальная работа - это мощный метод, используемый для решения задач, связанных с несколькими связанными жесткими телами. Когда система находится в равновесии, виртуальная работа равна нулю. Это позволяет рассчитать результирующие силы, когда система испытывает виртуальное перемещение. При анализе такой системы в первую очередь используйте схему свободного тела, где независимая координата представляет конфигурацию связей, и отметьте ее прогиба после положительного виртуального перемещения.

Затем измерьте координаты положения от фиксированной точки на схеме свободного тела и направьте их в направлении сил, совершающих работу. Это позволяет выразить каждое виртуальное перемещение в терминах независимой координаты. Следующий шаг - записать уравнение виртуальной работы, которое помогает определить результирующие силы и моменты. Важно отметить, что если сила или момент направлены в направлении положительного виртуального перемещения, то их работа определяется как положительная виртуальная работа; в противном случае это отрицательная виртуальная работа.

Основным преимуществом использования этого метода является его способность решать задачи, связанные с системой нескольких связанных жестких тел. Каждая из этих систем имеет только одну степень свободы, поскольку расположение связей может быть полностью определено с использованием только одной координаты. Это значительно упрощает сложные вычисления и помогает лучше понять, как различные переменные связаны друг с другом в сложных системах.

В целом, аналитическое решение задач равновесия, связанных с несколькими связанными жесткими телами, может быть эффективно выполнено с помощью виртуальной работы, что позволяет не только получить точные ответы, но и ясное представление о их внутреннем устройстве.

Виртуальная работа — это мощный метод, используемый для решения задач, связанных с несколькими соединенными твердыми телами. Когда система находится в равновесии, виртуальная работа ...

virtual-work-for-a-system-of-connected-rigid-bodies

Education

JoVE Core

Mechanical Engineering

Машиностроение

Unit 1

Раздел 1

Virtual Work

Виртуальная работа

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

Virtual Work for a System of Connected Rigid Bodies

finding forces with virtual work in linked bodies

Virtual work helps find forces in a system of connected rigid bodies. It is a useful method when several links move together and the system is in equilibrium. In that case, the total virtual work is zero.

To analyze the system, start with a free-body diagram. Use one independent coordinate to describe the link configuration. Then mark the deflected position that comes from a positive virtual displacement. This makes the small change in position clear before any calculations begin.

Next, measure position coordinates from a fixed point on the diagram. Direct those coordinates toward the forces that do work. This lets each virtual displacement be written in terms of the independent coordinate. After that, write the virtual work equation to determine the resulting forces and moments.

The sign of the work depends on the direction of motion. If a force or couple moment acts along the positive virtual displacement, the work is positive. If it acts opposite that direction, the work is negative.

This method is especially helpful for systems with several connected rigid bodies because they often have only one degree of freedom. That means the full link arrangement can be described by one coordinate. Virtual work simplifies the calculations and also shows how the variables in the system are related.

Finding Forces with Virtual Work in Linked Bodies

finding-forces-with-virtual-work-in-linked-bodies

work

Work in Force and Motion

Work connects force, motion, and energy in mechanics. It happens when a force acts on an object and the object moves along the line of action of that force. Work can also be described as energy transfer caused by applying a force, which leads to a change in the object’s motion or position. Work is calculated as the force multiplied by the part of the displacement in the same direction as the force. It can also be written as the dot product of the applied force and the object’s displacement. In...

work of a couple moment

Couple Moment Work in Rotation

Mechanical engineering studies motion, energy, and force, and it uses the idea of a couple moment to describe how two equal and opposite forces act on a rigid body. A couple moment is created when those forces act at two points on the body that are separated by a distance. When a rigid body moves because of a couple, its displacement can be split into two parts. First, the two points move by the same amount to their final positions. Then the body rotates about one of those points. During the...

virtual work

Virtual Work in Force Equilibrium

Virtual work is a mechanics principle used to check equilibrium with imaginary motion. If a body is in static or dynamic equilibrium, the total virtual work from external forces and couple moments must be zero for any virtual displacement. A virtual displacement is an imagined small movement. It can be a shift or a rotation. The virtual work done by a force equals the force dotted with the virtual displacement in the force’s direction. For a couple moment, virtual rotational work is found by...

virtual work for a system of connected rigid bodies

Virtual work helps find forces in a system of connected rigid bodies. It is a useful method when several links move together and the system is in equilibrium. In that case, the total virtual work is zero. To analyze the system, start with a free-body diagram. Use one independent coordinate to describe the link configuration. Then mark the deflected position that comes from a positive virtual displacement. This makes the small change in position clear before any calculations begin. Next,...

principle of virtual work: problem solving

principle-of-virtual-work-problem-solving

Virtual Work for Finding Unknown Forces

The principle of virtual work helps find unknown forces and moments in equilibrium problems. It is an important idea in mechanics and engineering. The method uses a small, hypothetical motion called a virtual displacement. If the system is in equilibrium, the work done by the forces during that displacement is zero. To apply this principle, the positions of the internal and external forces must be written in terms of one common coordinate parameter. That parameter can be an angle, a distance,...

mechanical efficiency of real machines

Friction’s Role in Machine Efficiency

Mechanical efficiency shows how well a machine turns input work into output work. It is calculated as the ratio of output work to input work. An ideal machine has an efficiency of one, which means the input work equals the output work. Real machines are never perfectly ideal because some energy is lost. Friction is one of the main causes of that loss. In a toggle vise used to compress wooden blocks, the sliding block rubs against the horizontal plane and creates friction. To find the...

conservative forces

Conservative Forces and Path-Independent Work

Conservative forces are forces whose work depends only on an object’s starting and ending positions. The path between those positions does not change the amount of work done. This idea is important in mechanical engineering because it helps describe how energy moves through a system. A conservative force also conserves energy in the sense that its work is path independent. Gravity is one example. Electrostatic forces and springs modeled by Hooke’s law are also examples. A ball rolling down a...

potential energy

Gravitational and Elastic Energy Stored

Gravitational and elastic potential energy are forms of stored energy from conservative forces. A conservative force, such as gravity or an elastic force, gives a body the capacity to do work. This capacity is measured as potential energy and depends on the body’s position relative to a fixed reference point, or datum. For gravitational potential energy, the datum is the level where the energy is taken as zero. If a body is above this reference plane, its weight has positive gravitational...

potential-energy criterion for equilibrium

potential-energy-criterion-for-equilibrium

Stable Equilibrium Through Potential Energy

Potential energy helps determine when a mechanical system is stable at equilibrium. When a system is acted on by gravitational and elastic forces, its total potential function is the algebraic sum of gravitational potential energy and elastic potential energy. If the system is already in equilibrium and is given a small displacement, the work done on the system equals the negative of the change in potential energy between the starting and ending positions. For a virtual displacement, which is...

stability of equilibrium configuration

Types of Equilibrium in Mechanical Systems

Types of equilibrium in mechanical systems are stable, neutral, and unstable. These three cases help explain how a structure or object responds when it is pushed slightly away from balance. They are important for predicting whether a system will return to its original position, stay where it is, or move farther away. Stable equilibrium occurs when a system returns toward its original position after a small displacement. In this case, the potential energy is at a minimum. A cantilever beam...

one-degree-of-freedom system

Spring-Mass Motion and Energy Stability

A one-degree-of-freedom system uses a single independent variable to describe its state and behavior. In mechanical engineering, this kind of model helps explain how parts in a larger system respond to force. It also gives engineers a way to predict motion before a design is built in real life. A simple harmonic oscillator is one example of a one-degree-of-freedom system. A mass attached to a spring moves along one axis, and that position determines the system’s state. The spring force makes...

stability of equilibrium configuration: problem solving

stability-of-equilibrium-configuration-problem-solving

Equilibrium Stability Through Potential Energy

Stability of equilibrium in physics and engineering can be studied by using a system’s potential energy. An equilibrium configuration is a state where the system can remain at rest. Stability describes whether the system tends to return to that state after a small disturbance. Problem solving begins by finding the equilibrium points of the system. This is done by setting the derivative of the potential energy function equal to zero and solving for the independent variable values that satisfy...

key terms and concepts

Learn how virtual work is used with connected rigid bodies to find forces and moments using a free-body diagram, one coordinate, and equilibrium.

Finding Forces with Virtual Work in Linked Bodies | JoVE K12

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

mechanical engineering

Finding Forces with Virtual Work in Linked Bodies

Description

Transcript