# I'm stumped

A(n) 560 kg elevator starts from rest. It moves
upward for 4.61 s with a constant acceleration
until it reaches its cruising speed of 2.13 m/s.
The acceleration of gravity is 9.8 m/s2.
Find the average power delivered by the
elevator motor during this period. Answer in
units of kW.

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Tide
Homework Helper
What exactly have you tried to do with this problem on your own?

Well, I found that the acceleration is equal to .462039 until the speed is constant. I'm not sure what steps to take from here or if I'm even on the right track.

Gokul43201
Staff Emeritus
Gold Member
Correct so far...

Now, how do you calculate power ? What formula is there for that ?

the equation is P=(F*d)/t

Gokul43201
Staff Emeritus
Gold Member
That will work. Now what is F = ? and what is d = ? You know what t is.

If I knew that I would not have asked the question.

Gokul43201
Staff Emeritus
Gold Member

Okay, why don't you draw the free-body diagram for the elevator ? What are the different forces acting on it ? What is the net force equal to ? And you know the mass and acceleration. Use Newton's Third Law to find the relation between the net force and the acceleration. From this, you can find the value of F.

You know the final velocity, total time taken calculate the acceleration. Multiply it withh mass it will be the work done by it divide it by total time taken.
Another way,
calculate the height at which it reached multiply it with mass and acc. due to gravity it will be the work done divide it wih time.

Gokul43201
Staff Emeritus
Gold Member
aekanshchumber said:
You know the final velocity, total time taken calculate the acceleration. Multiply it withh mass it will be the work done by it divide it by total time taken.
No, mass*acceleration is not the work done. And one more thing you're forgetting here is the force needeg to bear the weight of the elevator. You must include that too.

Another way,
calculate the height at which it reached multiply it with mass and acc. due to gravity it will be the work done divide it wih time.
What about the work done to increase the kinetic energy ? If you include that, this method will work.

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