Sorry for the physics questions lately but.....

[neonerd]

Can the normal force on an object ever do work?

 

[Armitage]

Only if it moves it.

 

[neonerd]

Originally posted by: Armitage
Only if it moves it.

i thought that but wanted to make sure that the normal force could move it...i got confused whether it was a force just due to gravity, or just the reaction force of say an object on the ground

 

[TheVrolok]

Originally posted by: neonerd

Originally posted by: Armitage
Only if it moves it.

i thought that but wanted to make sure that the normal force could move it...i got confused whether it was a force just due to gravity, or just the reaction force of say an object on the ground

The normal force is just the reaction force of an object due to the object it's resting (term used loosely) upon. I'm trying to think of a case where the normal force would actually be doing work. A block sliding down an incline is having most of the work done to it by gravity, but does the normal force actually contribute? I wouldn't think so. I'm not sure I can entirely answer your question because I'd hardly consider myself a physics buff, but as I sit here, for a minute and a half, I can't think of a case that would have the normal force providing work.

 

[neonerd]

oh and:

can Kinetic Enery ever be negative? I say it can't because it only goes down to zero..when an object is completely still it has PE, but no KE; and that's only when an object is elevated. I know work can be negative though, and doesn't W = KE?

that's confusing me

 

[Wheatmaster]

Originally posted by: neonerd
oh and:

can Kinetic Enery ever be negative? I say it can't because it only goes down to zero..when an object is completely still it has PE, but no KE; and that's only when an object is elevated. I know work can be negative though, and doesn't W = KE?

that's confusing me

you can't have negative PE or KE because for KE that just means no velocity and no for PE because it depends on what the reference is.

 

[Skiguy411]

Remember work is only done when the movement of the object is in the direction of that force....

 

[neonerd]

Originally posted by: Wheatmaster

Originally posted by: neonerd
oh and:

can Kinetic Enery ever be negative? I say it can't because it only goes down to zero..when an object is completely still it has PE, but no KE; and that's only when an object is elevated. I know work can be negative though, and doesn't W = KE?

that's confusing me

you can't have negative PE or KE because for KE that just means no velocity and no for PE because it depends on what the reference is.

PE you can have negative if i recall correctly....say the reference is sea level. Negative PE would be below sea level. It's how much work it actually takes to get something back to that reference point

 

[GiLtY]

Originally posted by: neonerd

Originally posted by: Wheatmaster

Originally posted by: neonerd
oh and:

can Kinetic Enery ever be negative? I say it can't because it only goes down to zero..when an object is completely still it has PE, but no KE; and that's only when an object is elevated. I know work can be negative though, and doesn't W = KE?

that's confusing me

you can't have negative PE or KE because for KE that just means no velocity and no for PE because it depends on what the reference is.

PE you can have negative if i recall correctly....say the reference is sea level. Negative PE would be below sea level. It's how much work it actually takes to get something back to that reference point

You can have negative PE depending on your reference point. The more important thing about PE is the difference in PE, because that quantity never changes (independent of reference point).

--GiLtY

 

[TheLonelyPhoenix]

Originally posted by: neonerd
can Kinetic Enery ever be negative?

Quick answer: no.

Longer answer: Depends on where you set the "zero" level... changes in energy are what's important to physics problems, not actual quantities, so its often convienent to set the zero levels at something that isn't REALLY zero... like an object sitting on the Earth's surface is said to have zero potential energy, although the potential energy is really only zero at the center of the planet. The actual quantities of energy held by something, from a conceptual standpoint, are never REALLY less than zero though.

Edit: To my knowledge, there's no real reason to dream up negative kinetic energy, so I guess you can stick to the quick answer.

 

[raptor13]

Originally posted by: Wheatmaster

Originally posted by: neonerd
oh and:

can Kinetic Enery ever be negative? I say it can't because it only goes down to zero..when an object is completely still it has PE, but no KE; and that's only when an object is elevated. I know work can be negative though, and doesn't W = KE?

that's confusing me

you can't have negative PE or KE because for KE that just means no velocity and no for PE because it depends on what the reference is.

KE can never be negetive, you're right. PE can be negative, however, depending on where you choose your reference point. At the reference point, PE is zero and if it's possible to move "below" the reference point, then PE can be negetive. In practice, PE should never be negative because you should always choose the lowest possible point of travel as your reference point.

I hope that makes sense. To summarize: KE is never negative. PE can be negative but it shouldn't be.

 

[pray4mojo]

Originally posted by: neonerd
oh and:

can Kinetic Enery ever be negative? I say it can't because it only goes down to zero..when an object is completely still it has PE, but no KE; and that's only when an object is elevated. I know work can be negative though, and doesn't W = KE?

that's confusing me

nope
just think about 1/2mv^2
mass can never be negative
v is squared so it will always be positive

 

[DrPizza]

Hmmmm..... if an object were in an elevator that was going up...

 

[neonerd]

Originally posted by: DrPizza
Hmmmm..... if an object were in an elevator that was going up...

that's in regards to Fnet, right? 🙂