Work, distance and time problem

[kieslingrc]

1. A 133 watt motor is used to lift a 148 kg container to a height of 6 m. How much time does it take to lift the container with the motor?




2. work= Fd; P= work/t; F=ma



3. I am thinking way too much into this. I converted the watts into J/s,[1W=1J/s] = 133 J/s. I converted the weight into mass [mass = weight / gravity] = 15.1 kg. Now I am stuck because I need to know the force, [F = ma] but don't have the information for acceleration. I totally confused myself and don't know what to do. I believe I need to divide the work by the power to solve for time, but I am stuck on figuring out the work

 

[LawrenceC]

Work is force times distance. You know the force, you know the distance.

 

[kieslingrc]

Work is force times distance. You know the force, you know the distance.

Just figured that out. I told you I was reading too far into this, lol.

 

[kieslingrc]

So what I have now is: time = work / power; t = 888J / 133J/s leaving 6.6766...s (the joules cancel out) which we round to one decimal place for this question, and my answer would be 6.7 seconds. Is this correct?

 

[LawrenceC]

You are off by a factor of g. You already have the mass of 148 kg. Why are you dividing by g?

 

[LawrenceC]

The work is not 888J.

 

[kieslingrc]

You are off by a factor of g. You already have the mass of 148 kg. Why are you dividing by g?

148kg is not the mass, it is the weight. I forgot that this weight (148) is also the force in Newtons. I did not need to convert it to mass, it was already in the form I needed it. [F = W = mg]. If all I had was the mass, I would have multiplied the mass times gravity to obtain the Force, or weight.

 

[LawrenceC]

A kilogram is a unit of mass, not weight.

 

[kieslingrc]

Good point, force is either pounds or Newtons. Thanks for pointing that out.

 

[kieslingrc]

Reworking the problem:
F= 1450.4N (148*9.8)
W= 8702.4J (1450.4N*6m)
T= 65.4s (8702.4J/133J/s)

I think I am still missing something.

 

[LawrenceC]

Like what? Power is the rate at which work is done. You calculated how much work there is to do. What you did in your last post is correct.

 

[kieslingrc]

Woohoo, thank you for your guidance! I always want to understand the work I am doing and it kills me when I can't figure something out. I have spent 2 hours on this one problem. I really appreciate you pointing out my errors and explaining why. Thats something you can't get when taking an online class.

 

[LawrenceC]

Good fortune to you in your endeavor. Bye.