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Failed first test on forces&kinematicsneed some help with corrections

  1. Oct 5, 2012 #1
    unfortunatly I failed my first physics exam. I have to go back through and correct the ones I missed. I need some help on the ones im stuck on. any help and explanations is greatly appreciated.

    1.) three blocks are being accelerated upward by a force applied to the bottom block. the mass of the bottom block is 3kg, the middle block is 6kg, the top block is 9kg. The blocks are accelerating at 3.2 m/s. what is the net force on the middle block? would it just be mass x acceleration? (6x3.2)?

    2.) A person can swim with a speed of 3 m/s in still water, and wants to get to the other side of a 400m wide river whos water flows 2 m/s downstream. if she swims acress in such a way that the time taken is the smallest possible, how far downstream is she swept on the opposite bank? ( i calculated 133.3 seconds and a distance of 267m) I need to find out to how to get her speed relative to the earth?? my choices are 1.6,2.2,3.6,4.3 or 5.0. thought it was distance/time but none of those answers worked.

    3.) Two blocks are being pulled along by a constant force of 25N(pulling on B). Block A has a mass of 2kg and black B has a mass of 5kg. (I calculated the tension which is 7.14N) Now it says the blocks encounter a region of friction and the same 25n force is applied but now they are moving with constant velocity. what is the net force on block A? what is the coefficient of friction?

    4.)A bucket of water is swung around in a vertical circle with enough speed so the water stays in the bucket. the total mass of water + bucket= 5kg. what is the minimum speed so that water does not leave? If the bucket handle is only capable of applying a force of 172N what is the maximum speed you can swing the bucket?
  2. jcsd
  3. Oct 5, 2012 #2


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    Pick one of these and elaborate on what you have tried and where you are having difficulty.
  4. Oct 6, 2012 #3
    OK I'm stuck on 3. So to find the net force on the block A I used Fnet=ma. Since it states its moving at constant velocity, the acceleration =0. So the net force would be 0? Is this correct? What about finding the coefficient of friction if acceleration is 0? I know the force of friction is the normal force X mu. But if acceleration is 0, wouldnt that set Fnet=o ?
  5. Oct 6, 2012 #4


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    If the blocks aren't accelerating the net force on them is indeed Fnet=0. However you still have the 25N force acting on the blocks so what other force is missing?
  6. Oct 6, 2012 #5
    I think its the friction. so the friction should be 25N to keep the net force 0 right? By that, we can calculate the coefficient easily. Block A can be deduced to have no net force by common sense, since velocity is constant.
  7. Oct 7, 2012 #6
    ok so the other force acting on it would be friction. I used Fnet=ma which m(a) is 0. so it would be T-Friction=0. friction is mu x normal. so i could solve for mu which is the coefficient of friction would is equal to 0.36.

    I am now stuck on the bucket swug in a vertical cirlce. I used the equation Fnet=ma, in which a is (v^2/r) So the minimum speed turned out to be 2.7 m/s. I cant seem to determine the maximum speed if the handle is only capable of applying 172N. I have tried Fnet=ma ----> Tension+fg=m(v^2/r) but my answers does not match ones provided. can somone tell me where im going wrong?

    As for the swimmer problem, to finding her speed relative to the earth, I broke it up into x and y compenets and got the distance downstream =267m. and the time it takes her to get across is 133.3 secs. To find her speed I added to the distance vectors (the 400m^2 wide river + distance downstream 267^2) and got the hypotnuse distance. I used Speed=distance/time. speed=479.82/133.3sec = 3.6 m/s. Does this sound correct?
    Last edited: Oct 7, 2012
  8. Oct 7, 2012 #7
    How did you solve the bucket problem without a radius? That wasn't included in your original post.
  9. Oct 8, 2012 #8


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