Undergrad Question Regarding Force Being Equal, Even Moving Upwards (if V=0)?

[Ascendant0]

I understand based on the equation F = ma that if there is no acceleration, the forces on the object all balance out to 0 in all directions.

What I don't get is for example, slowly lowering a heavy stone slab at a constant velocity v, and raising it way above my head as high as I can at a constant velocity v, would be considered to be the same force (since my force on it would be equal to the normal force since there's no acceleration). Obviously, it's going to be a LOT harder raising that stone slab overhead as it would be slowly lowering it. It would take a lot more effort, yet according to the force equation, I'm still applying the same force. That to me is a bit confusing.

I'm thinking I'm just viewing "force" conceptually wrong here, but if someone could help me make more sense of why this is the case in the above example, I'd greatly appreciate it.

 

[Dale]

It would take a lot more effort

Your perceived effort is a very poor measurement of force. It is exceptionally difficult for humans to do constant velocity motions or to accurately gauge the force applied during different movements. The perceived effort will also involve the motion of your own body, not just the external object.

You should always avoid human-perception based explanations. Replace a human with a spring or something similarly easy to analyze. A spring will elongate by the same amount if you are raising or lowering a mass at constant velocity.

In short, the physics definition is correct, but humans are complicated.

 

[A.T.]

I'm thinking I'm just viewing "force" conceptually wrong here,

Correct. At the very least, you have to consider the work done, which is positive when raising something, and negative when lowering something at constant speed.

However, for physiological reasons, doing negative work with your muscles doesn't recharge your body (like regenerative braking would). You still consume energy, and it might even be more exhausting to your muscles to operate like this.