Power problem Need help

[alisa7217]

This problem is simple and i cant figure out what I am doing wrong. I am supposed to find the speed of a bucket that is being lifted out of a well with a power output of 118W. The mass of the bucket is 4.5 kg and the weight of the rope is ignore. Dividing the power by the force should give me the velocity but it doesnt. I used 118 over 4.5*9.81 and the answer is wrong. Can anyone tell me what i am doing wrong?

[christinono]

This problem is simple and i cant figure out what I am doing wrong. I am supposed to find the speed of a bucket that is being lifted out of a well with a power output of 118W. The mass of the bucket is 4.5 kg and the weight of the rope is ignore. Dividing the power by the force should give me the velocity but it doesnt. I used 118 over 4.5*9.81 and the answer is wrong. Can anyone tell me what i am doing wrong?
Power = work/time
work = fdcos(angle)

[alisa7217]

I dont have the time so i am using P=fd/t=F(d/t)=Fv which is p/f=V. The force should be m*a and a should = gravity right?

[christinono]

Yes, that looks right. :smile:

[alisa7217]

but the answer is wrong when i submit it...

[christinono]

I dont have the time so i am using P=fd/t=F(d/t)=Fv which is p/f=V. The force should be m*a and a should = gravity right?
Wait a minute...
how did U get from P=W/t to P=fd/t??? :confused:

[christinono]

You can also use
Work = \frac{1}{2}mv_f^2 - \frac{1}{2}mv_i^2

[OlderDan]

Part of the work being done by the power source is increasing the potential energy of the bucket, and part is increasing the kinetic energy. You need to consider both.

[christinono]

It's asking for the FINAL velocity, right? The velocity can't be constant.
Use W=Fd and the change in kinetic energy formula.

[OlderDan]

Alisa,

Your original statement is correct, and so is your calculation if the system has achieved a state of constant velocity and constant force. What is the form of the answer you are supposed to get? Perhaps the problem is not so easy and you are supposed to be looking at something other than the constant velocity condition.

[alisa7217]

I still havent figure it out and its due it 10 minutes

[OlderDan]

Then believe in what you did and try to defend it until you understand what (if anything) you did wrong.