Negatif External force work

Mathieu_Courchesne

How can we explain a negatif external force work?

 

I got a model where I impose a velocity to a node of a spring pull to simulate load on a rope attached to a structure.

 

See the image attached.

 

 

Thanks

Polyvios Romanidis

Hello,

Work is the energy transferred to or from an object via the application of force along a displacement.  That means that generally work can be either positive or negative.

The work, generally, is given by the following equation:

If we simplify it for one direction this can be written as:

In this equation F must be replaced with its sign whether is positive or negative compared with the direction the body is moving.  Using this equation work can be negative, for example, when the displacement is in a specific direction and the force pulls in the exact opposite.

Some other examples of negative work can be produced within models that constrain prestrain elements or non-balanced boundary conditions, even with the use of gravity.  The following example examines a single element with a high initial velocity in z-axis that decelerated by gravity.

In the above model the initial velocity, and hence the displacement, are positive, while the gravity force is negative.  That leads to negative work from the gravity (external force).  In the following diagrams external force work and displacement of the body are given.