Power particle-like object problem

1. Mar 10, 2008

critch

Hi, Ive been trying to complete this question for almost an hour, i was hoping you could provide some advice

At a certain instant, a particle-like object is acted on by a force F = (5.0 N) i hat - (1.0 N) j hat + (10.0 N) k hat while the object's velocity is v = - (2.0 m/s) i hat + (4.0 m/s) k hat. 1)What is the instantaneous rate at which the force does work on the object?

2)At some other time, the velocity consists of only a j component. If the force is unchanged, and the instantaneous power is -12 W, what is the velocity of the object just then?

I tried taking the dot product of the force and velocity, making sure i only multiplied the like terms of the unit vectors. then i took the hypotenuse using pythagoras, but the answer i get is 41.2 Watts and its not right? please could someone help?

2. Mar 11, 2008

Shooting Star

1) Taking the dot product is enough. I didn't quite understand what you have done after that.

2) Suppose the velo is v = vj. Find F.v and equate it to the instantaneous power given.