How many walls can this battery-powered crane lift before recharging?

[MichaelTam]

Homework Statement

Exercise

Relevant Equations

PE=mgh,

A new, innovative type of crane is battery powered. Its battery has a total energy capacity U , expressed in Joule. Determine how many walls, N of mass m , the crane can lift before the battery has 20% left and needs to be recharged. The walls need to be lifted from /h_1/ to /h_2/ a new location at height . You may assume that energy is only used when the walls are lifted and you may disregard any energy losses.

 

[MichaelTam]

 

[MichaelTam]

My solution is incorrect, but I still can’t find out the error….

 

[PeroK]

My solution is incorrect, but I still can’t find out the error….

What about the mass of each wall?

 

[MichaelTam]

N=0.8*U/(g*(h_2-h_1)), you may see it at the second post

 

[PeroK]

N=0.8*U/(g*(h_2-h_1)), you may see it at the second post

I see it now.

What about the mass of each wall?

 

[MichaelTam]

each wall has a mass of m but N is the sum of those mass.

 

[PeroK]

each wall has a mass of m but N is the sum of those mass.

N is the number of walls.

 

[MichaelTam]

But I cannot express N as variable m

 

[MichaelTam]

the last exercise tells me m=1000kg, but I don’t know if I can substitute that into this situation or not.

 

[PeroK]

But I cannot express N as variable m

What does that mean? The walls have mass $m$ which determines the energy needed to lift them.

 

[MichaelTam]

‘That’ means the last exercise it mention m is equal to 1000kg, I think I doesn’t work in this exercise.

 

[MichaelTam]

I use the equation of
1.Potential energy = N*m*g*(h_2-h_1)
2.Energy of the battery can be used = (1-20%)*U
(1) = (2)
Then I find N=N=0.8*U/(m*g*(h_2-h_1)) but how can I express m in terms of other variable?

 

[PeroK]

I use the equation of
1.Potential energy = N*m*g*(h_2-h_1)
2.Energy of the battery can be used = (1-20%)*U
(1) = (2)
Then I find N=N=0.8*U/(m*g*(h_2-h_1))

That's the answer.

but how can I express m in terms of other variable?

I don't understand this question.

 

[MichaelTam]

I found the system is coming up with a error, the solution can be include m now, I’m sorrry….