Respuesta :

When forces are in equilibrium. 

A rock lying on the ground exerts a force on the eartth equal to its mass times acceleration due to gravity. 
The earth exerts an identical reactionary force stopping the rock from moving. 

A force is being applied but there is no work being done.
There is a gravitational force that is holding the rock in place, which is larger than the force that you are applying. Your muscular exertions will only produce waste heat. So, even though force is defined as that which can cause an acceleration of a mass, in some circumstances an insufficient force will cause no acceleration.

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