csx

May 8, 2013
1,075
211
Status
Pre-Health (Field Undecided)
the obj weighs 100 N and it is traveling upwards at constant velocity of 5m/s. whats the power required to keep it in motion. Ignore air resistance.

I said power is 0 watts because if velocity is constant, a=0, meaning F=0. And if F=0 then P=Fv=0.

Answer is P=Fv=100nx5m/s=500W

i dont get it...I mean...100N is the objects weight not the thing thats applying force

conversely, the followup question says the same object same speed moves horizontally this time...now the answer is 0.

i dont get it
 
Last edited:

justadream

7+ Year Member
Apr 29, 2011
2,171
815
Status
@csx

Isn't this an AAMC question?

Just because something has constant velocity doesn't mean you don't need to apply work to keep it constant.

Think about a rocket. Let's say the rocket goes up at 10m/s. If you don't apply ANY force, will the rocket continue going at 10m/s? NO!

Power = Fv. Force is mg.

You can draw a FBD. Some force going up needs to balance gravity (mg) going down.
 
OP
csx

csx

May 8, 2013
1,075
211
Status
Pre-Health (Field Undecided)
no its not.
i mean...intuitively my logic didnt make sense...but i went with the #'s
 

texan2414

5+ Year Member
Jul 24, 2013
1,200
733
Status
Medical Student (Accepted)
I think you confused the NET force with the force required to keep the object moving in the fwd direction.
 
About the Ads