How can the window washer move at constant velocity with unbalanced forces?

[ThomasMagnus]

A window washer pulls himself upward using the bucket pulley apparatus shown. How hard must he pull downward to raise himself at a constant speed? The mass of the person and the bucket is 65kg.

My attempt:

When doing pulley problems, I like to set 'moving to the right' as positive. In this case, there are two forces acting toward the right side and one to the left.

The force that the washer pulls with will be equal to the force that the rope applies on the bucket

The forces acting on the bucket: F_T-mg=0 (constant acceleration therefore a=0)
Forces acting on the right side: F_T=0

Add the two equation together:
2F_T=mg
mg=637N
F_T=319N


My question is: How can he be moving at constant velocity if there is an unbalanced force acting on the right side? Is it because there is no net force acting on him? (i.e F_T-mg=0) If he is moving at constant velocity, can there be a net force on one side of the pulley, but not on another. Or if he is moving at constant velocity, does there have to be a net force of zero on the entire system?


If the sum of the forces acting on the washer and bucket is zero, then is he at rest or is he moving at constant velocity? My guess is if the forces are balance on the bucket, then he will move at constant velocity because of the force he is applying on the other side of the rope (other wise he would be at rest)


Thanks!

 

[srmeier]

In order for the man to move at constant velocity the net force on him must be zero. He can't move at a constant velocity if there is an unbalanced force on the right (think about it: the right side of the rope would accelerate but the left side wouldn't? would the rope not rip in this case?). If the man doesn't apply a force to the right side would he accelerate? If so, in which direction?

 

[tiny-tim]

Hi ThomasMagnus!

2F_T=mg
mg=637N
F_T=319N

This is correct, but I have no idea what you mean by forces acting towards the right.

The free body diagram for washer-and-bucket has only three forces, all vertical: two equal tension forces upward, and the weight downward.

Since the acceleration is zero, 2T = mg.

Or if he is moving at constant velocity, does there have to be a net force of zero on the entire system?

Yes.

If the sum of the forces acting on the washer and bucket is zero, then is he at rest or is he moving at constant velocity?

Either.

 

[ThomasMagnus]

Where I was getting mixed up is with the coordinate axis. Should I choose any forces that will act in the direction of the acceleration (or in this case motion) to be positive?

Thanks

 

[tiny-tim]

Hi ThomasMagnus!

(just got up :zzz: …)

You can do either … since the acceleration in this case is zero, it doesn't matter.

(Usually, it's easier to use the direction in which the acceleration is positive, so that your F = ma equation has both sides positive instead of negative! )

 

[ashishsinghal]

The forces acting on the bucket: F_T-mg=0 (constant acceleration therefore a=0)
Forces acting on the right side: F_T=0

Add the two equation together:
2F_T=mg
mg=637N
F_T=319N

You first say that F_T=0 and then say F_T=319N.
This does not make any sense.
Also do you mean upward direction as right?