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Homework Help: Basic physics, I have no idea

  1. Jun 24, 2011 #1
    1. The problem statement, all variables and given/known data

    1. A frictionless slope has a hypotenuse of 2.0m and a height of 1.0m. A box of mass 40kg is pushed up along the hypotenuse with a force of 250N. Since work done is defined as the product of the force and the distance moved by the object in the direction of the force, we can say that work done is 2.0 x 250, which gives us 500J. However, all of the work done should be converted to G.P.E. and G.P.E. is only 1.0 x g (10N/kg) x 40 = 400J. Where is the 100J? There is no friction of any kind.

    2. If you rub a comb and place it near a trickling stream of water, you will notice that the water bends towards the comb. This is mainly due to polarity. But water has 2 polarities, a negative one and a positive one, so it could bend towards the comb and at the same time, move away from the comb. But why does it bend towards?

    3. If you rub a balloon and stick it to the curtain, after some time, will it drop? Why will it drop?

    2. The attempt at a solution

    1. Where is the 100J? Work done - G.P.E.?

    2. 2 polarities, so why does it bend?

    3. My guess is it will not drop since there is electrostatic attraction between the balloon and the curtain and since the balloon is an insulator, electric charges are not able to move from balloon to curtain.
  2. jcsd
  3. Jun 24, 2011 #2

    Andrew Mason

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    What is the force that would be required just to balance gravity? If the applied force is greater than this force, what happens to the box? What kind of energy does the box have as a result of this additional force (ie additional to the force required to balance gravity)?

    This is an example of induced charge. I think explanations that say it has to do with the fact that water is a polar molecule are wrong. As you correctly point out, a polar molecule is still neutral.

    Water is not a good insulator. It conducts electricity, largely due to the impurities (eg. dissolved salts) in the water. This means that some electrons and ions are free to move in the presence of an electric field. What happens to these charges when passing near the charged comb?

  4. Jun 24, 2011 #3
    1. Work can go into many things, a change in GPE being only one. In your problem there would also be a change in kinetic energy which is where the extra 100J went.

    2. The comb will gain and excess of one type of charge. The molecules in the water will rotate to align themselves with the charge on the comb, with the like charged side of the molecule away and the unlike charged side of the molecule close to the comb. Then the water moves towards the comb and the stream bends.

    3. I dont have enough knowledge to answer this for you, sorry.
  5. Jun 24, 2011 #4

    Andrew Mason

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    I don't think this is correct. It think that the difference between the attractive force on the + side of the water molecule and repulsive force on the - side across the distance of a molecule would be insignificant. There would also be a lot of work required to flip the molecules to align with the field.

  6. Jun 25, 2011 #5
    1. I understand qns 1 now. Since the box is moving, it must have kinetic energy, which is 100J.

    2. Water has impurities, I see, thanks.

    3. I guess qns 3 is kinda difficult. From what I know, crop sprayers make use of this kind of principle to ensure that pesticide particles are evenly distributed over the crops. The particles are charged and when a droplet approaches a plant leaf, it will induce an opposite charge on the leaf and is attracted to it. This is what it said on my textbook. But, why will the plant leaf be induced? It is an insulator with no free moving electrons.
  7. Jun 27, 2011 #6
    Le Bump.
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