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Homework Help: Forces acting on two balls

  1. Dec 5, 2004 #1
    There are two balls which are apart from each other but there is a line going from the balls diagonally up and these two join. The topic is electricity so I have said the two balls have like charges as they are repelling.

    See www.kgmm.co.uk/balls.GIF[/URL] for diagram.

    The question is what forces r acting on the balls, I have labelled gravity on both but aht others r there? Presumably a repulsion force from ball to ball?


    Last edited by a moderator: Apr 21, 2017
  2. jcsd
  3. Dec 5, 2004 #2
    You have pretty much answered your own question :smile:. The lines joining the balls - are they strings?
  4. Dec 5, 2004 #3
    Doesn't say:

    "The diagram below shows 2 small plastic balls. The balls are charged. Draw the foces acting on each ball"

  5. Dec 5, 2004 #4
    "In the text you read that electrons are removed from atoms by rubbing. An electron needs energy to remove it from an atom. Explain, using scientific terms how rubbing provides this energy"

    Do I just need to say that by rubbing you are changing the charge because of the movement of protons to the atom whcich means that it is positiive instead of negative due to the electrons.?

    Or is that wrong?


  6. Dec 5, 2004 #5
    The forces acting on the balls are gravity, trying to pull them down, the electric charge, trying to push them apart, and the tension in the string.

    If the balls aren't moving, then you know that the forces must be in balance.

    What can you say about the force provided by a string? How might you vary the strength of the force and the direction it acts in?
  7. Dec 5, 2004 #6
    The point is we don't know if it is string, presumably they would have mentioned it was string if it was important. But anyway.

    The force applied to the two balls must be the same to both of them as the balls are not moving and is the same string. Do you mean by reducing the the length of the string the forces would be greater as they would be closer to each other.

  8. Dec 5, 2004 #7
    Partly that, but what I was trying to get at is that a string can only PULL something. It can't push it, or edge it sideways.

    The force on an object exerted by a string can only act directly along the line of the string. If you represent the force by an arrow, then you have to draw the arrow pointing along the string.

    So in this case, you know that gravity acts straight down, and the electric repulsion between the balls acts horizontally (assuming the balls are both at the same height). The force along the string has to balance both of these forces out to keep the ball in equilibrium.

    Can you figure out how a force acting at the angle of the string, might balance both the vertical and the horizontal force at the same time?
  9. Dec 5, 2004 #8
    I don't, I understand that the strong has to balance the vertical and horizontal force, but can't work out how the horizontal AND vertical forces are balanced at the point of string.

    Thanks again

  10. Dec 5, 2004 #9
    This is normally called a 'parallelogram of forces'. In this case it is a rectangle of forces - a rectangle is a special simple case of a parallelogram, where the angles happen to be right angles.

    If the balls were 4 cm lower than the hanging point and 3 cm to one side of it, then you could draw a rectangle 4 cm high, and 3 cm wide to represent the forces acting. The force acting (tension) in the string would be the length of the diagonal (5 cm in this case as I chose 3 and 4 specially to make the numbers come out nicely) and you could measure or calculate the angle of the string to the vertical (just under 37 degrees in my example).

    This might be going into this deeper than you are expected to go. Maybe all they expected you to work out was that as the balls don't hang straight down, something must be pushing them apart. And you'd already worked that out when you made the opening post in this thread. :smile:
    Last edited: Dec 5, 2004
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