Static equilibrium (conceptual)

[kermitthefrog]

In order for an object to be in static equilibrium

a)the net force in all directions must be zero
b)the net torque must be zero
c) the object's velocity must be zero
d) a and b are correct
e) all of the above


2. I know that the net force in all directions must be equal to zero, also the sum of the net torque must be equal to zero. I am confusted about the object's velocity being zero.

3. An object can have non-zero velocity and not be accelerating(ie it has a constant velocity), which would fulfill the idea that no net force is acting upon it. However in my class text, it keeps stating that the object is at rest. I'm not sure if this is just a specific example of equilibrium, or if all objects in equilibrium must be at rest (ie velocity= 0). From the searches that I have done I can't tell if the object's velocity must be zero...any explanations?



3.attempt at solution, because I think you can have constant velocity in which you are not accelerating, I believe the answer is D, both choice A and B are correct.

 

[Redbelly98]

It's a good question, and boils down to whether the object must be at rest or not.

A quick google search gives conflicting answers. Wolfram claims that zero net force and torque are sufficient for static equilibrium, whereas wikipedia and physicsclassroom.com claim the object must be at rest as well. You can look at these three links, and hopefully decide which is most consistent with your class's use of static equilibrium:

http://en.wikipedia.org/wiki/Static_equilibrium
http://scienceworld.wolfram.com/physics/StaticEquilibrium.html
http://www.physicsclassroom.com/class/vectors/U3l3c.cfm