Finding Force and work on a moving object

[Ltpenguin]

Homework Statement

A cable lifts a 5000kg elevator at a constant velocity of 2.0m/s [up]

I) Find the force acting on the elevator.
II) Find displacement of the elevator in 15s.
III) find the work done by the cable on the elevator over a time period of 15s.

Homework Equations

W=FΔd
Δd=ta

The Attempt at a Solution

I)
F=mg+ma
F=(5000*9.8)+(5000*2)
F=59'000 N
II)
30 m up
III)
W=FΔd
W=(59,000)(30)
W=1.77 million joules

*updated is this correct now?*

 

[paisiello2]

No.

-

 

[haruspex]

Unfortunately there is no simple yes or no answer because the question is ambiguous. Does "the force" mean the force from the cable or the net of all forces?

 

[Ltpenguin]

Unfortunately there is no simple yes or no answer because the question is ambiguous. Does "the force" mean the force from the cable or the net of all forces?

Alright so i will guess that "the force" means the force from the cable so...
F=mg
F=5000 x 9.8m/s
F=49'000 N

Is that correct? or am i missing something with the 2m/s the elevator is moving. Thanks for the help.

 

[TrueColor]

Hello!
The elevator is moving with constant velocity, so the net force acting on it is 0. (Ans 1.)
Also, work done by the cable equals the work against gravity (forces are equal!). (Ans.3)

i)The force is $$|mg|=|F_{upward}|$$

ii) 15s * 2m/s = 30m

iii) $$mgh = mg * 30m = 5000kg * 9.8m/s^2 * 30m = 1.47*10^6 J$$

 

[Ltpenguin]

Alright so i will guess that "the force" means the force from the cable so...
F=mg
F=5000 x 9.8m/s
F=49'000 N

Is that correct? or am i missing something with the 2m/s the elevator is moving. Thanks for the help.

I figured this out i believe:
F=mg+ma
F=(5000*9.8)+(5000*2)
F=59'000 N
i think that is correct. The Force placed by the tension cable to keep the elevator at rest is 49'000 Newtons to move the elevator at 2m/s it takes 59'000 N

*updated first post check it out*

 

[paisiello2]

Still no.

 

[Ltpenguin]

Still no.

What do you think it could be? i point in the right direction well help a ton thank you :)

 

[paisiello2]

If you bother to stick in the units, you'll see that they are inconsistent the way that you formulated the problem.

I think you had it on your 2nd try.

 

[Ltpenguin]

If you bother to stick in the units, you'll see that they are inconsistent the way that you formulated the problem.

I think you had it on your 2nd try.

do you mean by 2m/s vs 9.8m/s²

 

[Ltpenguin]

I think you had it on your 2nd try.

wait so it is right?

 

[paisiello2]

Is what right?

You gave 3 different answers. One of them is right, though.

 

[haruspex]

F=mg+ma
F=(5000*9.8)+(5000*2)

What kind of physical entity is represented by a in the first equation?
What kind of physical entity is represented by the 2 in the second equation?

 

[Ltpenguin]

What kind of physical entity is represented by a in the first equation?
What kind of physical entity is represented by the 2 in the second equation?

a is the acceleration of the elevator, 2m/s

 

[haruspex]

a is the acceleration of the elevator, 2m/s

2m/s is a speed, not an acceleration.

 

[Ltpenguin]

Alright so this is it
F=mg
F=5000 x 9.8m/s
F=49'000 N
The cable exerts 49k N force on the elevator, making the net force zero so it can move at a constant velocity.

 

[paisiello2]

Yes.

 

[slland]

w = 1 470 000 J ?
Is this correct?

 

[haruspex]

w = 1 470 000 J ?
Is this correct?

Yes.