Sign of Work (Negative or Positive)

[Ineedhelpwithphysics]

Homework Statement

A pump is required to lift 850 kg
of water per minute from a well 14.7 m
deep and eject it with a speed of 17.7 m/s

Relevant Equations

W = F*d
W = delta KE

Im confused I did

850*9.8*14.7 = 1.2x10^5

Why is it not negative since the force is opposite to the motion?

 

[Chestermiller]

The force that the pump exerts on the water is upward, and the motion is upward.

 

[kuruman]

What exactly is the question as was given to you? Which work are you trying to calculate? When calculating work you have to specify the force that does the work and the system on which this work is done.

Assuming that the system is the water, the pump does positive work on the water as @Chestermiller explained but gravity does negative work on the water. In any case, in the work-energy theorem equation that you wrote down, W is the net work done on the water, that is the positive work done by the pump and the negative work done by gravity. You just calculated the work done by gravity with the wrong sign.

 

[PeroK]

Homework Statement: A pump is required to lift 850 kg
of water per minute from a well 14.7 m
deep and eject it with a speed of 17.7 m/s
Relevant Equations: W = F*d
W = delta KE

Im confused I did

850*9.8*14.7 = 1.2x10^5

Why is it not negative since the force is opposite to the motion?

You can't get a negative number by multiplying positive numbers. Your equation represents force and displacement in the same direction. The force is positive and the displacement is positive.

 

[Orodruin]

Sign of what work? Always state the full question exactly as given. In this case it could be either the work done by the pump or the work done by gravity. If you don’t specify, we cannot know.