Why is the work negative in this KA practice problem?

[ucevla]

See screenshot for the problem.

If the plaque were to slide a distance D to the right, what could be said about the work done on the clot by the lower pressure blood on the right side of the clot?

The answer says the work is negative. Why is that?

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[freak7]

Work done by the right side

 

[ucevla]

How? Isn't blood flowing from high pressure to low pressure? Ie left to right? There is force moving right to left?

 

[freak7]

How? Isn't blood flowing from high pressure to low pressure? Ie left to right? There is force moving right to left?

Imagine you're the blood on the right side and your big strong buddy is the blood on the left side. You're both pushing on a large object, let's say a car (the clot). Now you're buddy is going to win every time because he's stronger than you and push the car a distance D. However you're still exerting force on the car, but in the opposite direction of the car's movement. The work done on the car by you in that case would be negative because you're pushing in the opposite direction of the movement of the car.

 

[ucevla]

Great! Thanks!