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Homework Help: Focre of Brick on Window

  1. Nov 30, 2004 #1
    Force of Brick on Window

    Ok, I figured i'd had this question sussed but it doesn't seem so now that I know the answer. So, I know the answer, but I want to know why it's that way.

    A thrown brick hits a window, breaking the glass, and ends up on the floor inside the room. Even though the brick broke the glass, we know that

    a.) the force of the brick on the glass was bigger than the force of the glass on the brick. * What I thought was the answer

    b.) The force of the brick on the gass was the same size as the force of the glass on the brick * The alleged answer

    c.) the force of the brick on the glass was less than the force of the glass on the brick

    d.) the brick didn't slow down as it broke the glass.

    Ok, so here's my logic. (I guess since i'm new, it's important that I mention that i'm also in first year chemistry, a 1/2 year course that I have had immesurable trouble with, and as such, physics, a full year course, has the short end of the stick, really short. Now the exam's on Dec. 10th and i'm scrambling, of course. Typical teenager...I know)

    I picked a.) because, in my eyes, the other options don't work out. Since I reasoned that if the forces are the same the brick and the window wouldn't do anything, they would cancel out. Like normal force and gravity, objects at rest, ect. I reasoned that if the brick were to break the glass, it could not, therefore push on the glass with the same amount of force that the glass pushed back on it. Apparantly i'm wrong. I don't know why though? I'm thinking maybe the wording has led me to think about the wrong things here. I'm going on the teacher's demonstration in class Ie: finger pushes block right, block pushes finger left.

    Hope these forums are quick, i've got 11 days to learn most all the material on the exam... :eek:
    Last edited: Nov 30, 2004
  2. jcsd
  3. Nov 30, 2004 #2
    I am quite sure that the answer is B. I think the forces are always equal and opposite in direction(one of newtons laws I think): Even if a brick breaks through the window, or if the window were to stop the brick, the force would be equal and opposite on one another. I think of it like this: The force that the brick exerts(right word?) on the window will be the same, regardless of whether or not the window breaks, and vice-versa.
  4. Nov 30, 2004 #3

    Andrew Mason

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    Homework Helper

    This is an application of Newton's Third law. Forces always come in equal and opposite pairs. At all times when the brick is applying a force to the glass, the glass is 'pushing back' with an equal and opposite force.

    The fact that the brick goes through the glass results from the fact that the brick's energy exceeded the energy required to break the glass. While the molecules were being pulled apart by the brick prior to breaking, the total force exerted on the brick by those molecules was exactly equal to and opposite to the force being applied by the brick to those molecules.

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