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Energy conservation problem 
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#1
Mar2510, 03:27 PM

P: 120

1. The problem statement, all variables and given/known data
A string exerts a tension force of 55N on a 10kg block as it moves a distance of 5m up a frictionless incline. The string pulls parallel to the incline. A) Find the total external work done on the blockearth system as the block moves 5m up the incline. B)Find the change in gravitational potential energy for the blockearth system as the block moves 5m up the incline. C)Find the change in kinetic energy for the blockearth system as the block moves 5m up the incline. 2. Relevant equations [tex]KE=\frac{1}{2}mv^{2}[/tex] [tex]PE_{g}=mgh[/tex] Our teacher tells us to round gravity up to 10m/s/s. 3. The attempt at a solution A)The external force should be the tension, so: [tex](55N)(5m) = 275J[/tex] B)I used trig (345 triangle) to find the height to go up 3m. [tex]mgh_{f}mgh_{i}=\Delta KE[/tex] [tex](10)(10)(3)(10)(10)(0)=300J[/tex] C)[tex]KE_{f}KE_{i}=\DeltaKE[/tex] I am not sure what to do here because I don't know how to solve for velocity. 


#2
Mar2510, 03:30 PM

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PF Gold
P: 3,440

What does the workenergy theorem say about change in kinetic energy?



#3
Mar2510, 04:50 PM

P: 120

If this is right, then I can set 275J equal to [tex]KE_{f}KE_{i}[/tex] [tex]\frac{1}{2}mv_{f}^{2}\frac{1}{2}mv_{i}^{2}=275J[/tex] [tex]5v_{f}^{2}5v_{i}^{2}=275J[/tex] But then I will have two variables [tex]v_{f}[/tex] and [tex]v_{i}[/tex] I think I am on the wrong track. 


#4
Mar2510, 05:06 PM

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Energy conservation problem



#5
Mar2510, 05:59 PM

P: 120

I have the Normal force (80N) by the ramp on the block, the tension force (55N) , and a weight force (80N in ydir, 60N in xdir). (no friction) Is the other force doing work the Normal force? If so, then I am not sure how to calculate the work because the distance is perpendicular. 


#6
Mar2510, 07:31 PM

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P: 3,440

Yes there are three forces. As you say, the normal force does zero work. So what is the net work, i.e. the total work done by all three forces?



#7
Mar2510, 08:09 PM

P: 120

The problem is defining the system as blockearth, so if I am not mistaken, the weight force is the only force not doing work on the system. That would leave Normal and Tension forces. Normal is perpendicular, so equal to zero, Tension is 275J, so total work is [275+0] 275 Joules. Do I need to add in the weight force? I think it should be included in the blockearth system, not as external work. 


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