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Work and net workby mandy9008
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#1
Jun2110, 05:53 PM

P: 127

1. The problem statement, all variables and given/known data
A shopper in a supermarket pushes a cart with a force of 39 N directed at an angle of 25° below the horizontal. The force is just sufficient to overcome various frictional forces, so the cart moves at constant speed. (a) Find the work done by the shopper as she moves down a 48 m length aisle. (b) What is the net work done on the cart? Why? (c) The shopper goes down the next aisle, pushing horizontally and maintaining the same speed as before. If the work done by frictional forces doesn't change, would the shopper's applied force be larger, smaller, or the same? What about the work done on the cart by the shopper? 2. Relevant equations W=Fxcosө 3. The attempt at a solution a. W=39N (48m) cos25 W=1696.6J b. I thought that this would also be 1696.6J because work done by the shopper equals the net work done on the cart 


#2
Jun2110, 06:06 PM

Mentor
P: 41,326




#3
Jun2110, 06:09 PM

P: 127

So there are gravitational forces on the cart as well?



#4
Jun2110, 06:27 PM

Mentor
P: 41,326

Work and net work
Hint: "the cart moves at constant speed" What does that tell you about the net force? 


#5
Jun2110, 08:54 PM

P: 127

if it is moving at a constant speed then the net work will be zero, right? I remember my professor saying that in class, but he never explained why. I am guessing that there is some force that is equal and opposite of the work done by the shopper, which could be explained by Newton's third law



#6
Jun2110, 10:48 PM

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P: 1,925

Hello mandy9008,
When contemplating the issue, ask yourself these questions: (a) If an object is moving at a constant velocity, what does that tell you about its acceleration a? (b) Knowing that F_{net} = ma, and if you also happen to know that a = 0, what does that tell you about F_{net} (where F_{net} can be the sum of multiple forces)? Thus what can you conclude about all the individual forces that make up F_{net}? (Hint: if you were thinking that it means each of the individual forces making up F_{net} is zero, that's not generally true. But you can say something about how they combine together.) (c) If an object does not change velocity, what does that tell you about the object's change in kinetic energy? (d) If an object's kinetic energy and potential energy do not change, what does that say about the total mechanical work done on that object by the all forces acting on that object? But you are on the right track. There must be some sort of force (or component of which that is doing work) that is equal in magnitude and opposite in direction to the component of the shopper's force that happens to be doing work. To find out which forces do work and how much, find the component of each force that is parallel to the displacement vector. (For example, if a force is completely perpendicular to the displacement, you can ignore it because it's not doing any work. If a force is at some sort of angle to the displacement vector, you need to find the component of that force which is parallel to the displacement vector to calculate how much work the force does.) 


#7
Jun2210, 06:08 AM

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P: 41,326

It's Newton's 2nd lawnot the thirdthat tells you that the net force must be zero. ΣF = ma. Since the velocity is constant, you know that a = 0, thus ΣF = 0. 


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