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Homework Help: Vector Motion and Perpendicular

  1. Aug 11, 2016 #1
    I'm currently studying Projectile Motion and Motion in Two Dimensions now in Physics class, and I happened to quickly jot down what my teacher said onto my computer. I came across the note and it says "Vector forces cannot be perpendicular to each other." I'm a little unsure of what is meant and I cannot ask my teacher, he's not here for a while. Please explain.
  2. jcsd
  3. Aug 11, 2016 #2


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    It sounds like what you wrote down was either misheard, or is omitting some important context. There is no general rule that forces cannot be perpendicular. Every time a horse pulls a cart, the cart is subject to two perpendicular forces: gravity pointing down and the horse pulling forward.
  4. Aug 11, 2016 #3
    you must give the context of that statement ....on the face of it the statement does not carry any sense as there are no scalar forces and all forces can be designated by a vector .....more over the force can act in any direction so perpendicular forces can exist...say you are pushing a block on a table the force of push and the reaction of table surface are perpendicular and are physically independent.
  5. Aug 11, 2016 #4
    Ohh ok. I guess I misheard him or something. Sorry for the misunderstanding. That makes perfect sense, though, how vector forces can be in any direction, and of course they can perpendicular just like you horse drawn cart example. But it what cases would the vector quantities not be able to be perpendicular to each other? I'm little confused now on why that would be the case. I mean, a vector quantity is merely just a force or concept (such as displacement) who's positive and negative numbers flux depending on the direction of its motion, etc. So why would there be a case where they cannot physically intercept at 90 degrees or so? Sorry, I'm a little confused now.
  6. Aug 11, 2016 #5
    As the context is a projectile motion and the force on the body is always acting vertically downward and its effect on horizontal motion is zero as the vector component of the force in horizontal direction is will be zero......so the perpendicular component of a vector force must be zero...this is just a guess....
  7. Aug 11, 2016 #6
    Ohh I get it. So because gravitational forces act downwards, it only affects the Vertical Component of a projectile object, thus causing it to accelerate. However, when excluding air resistance, there are no forces acting upon the horizontal direction and so its horizontal motion is zero, or at least its acceleration is zero as the object will be in a constant state of motion. So, when cutting off air resistance for a moment, the horizontal motion will be zero, as no force is acting upon the horizontal component, and the force of gravity is acting downwards, but because the other component is zero, they cannot be perpendicular to each other. Is this right sorry?
  8. Aug 11, 2016 #7
    actually the motion can be seen in horizontal and vertical direction separately and combined displacement is the actual path.
    one should not think that a force is necessary for motion .....force only changes the momentum of motion(velocity)
    so if no force is there in horizontal direction the initial velocity will be maintained.....whereas the vertical component of velocity will be first decelerated and after reaching max. height it will again accelerated in the fall back.
  9. Aug 11, 2016 #8
    Ok, makes sense. I understand that something will remain in constant motion until a force disturbs it and its state (Newton's First Law), and that force, of course, changes object velocity, but in reference to that "one should not think that a force is necessary for motion", I'm a little unsure of what you mean. Do you mean that an external force does not need to be applied in order to set an object in motion? If so, then how would an object be able to move in the first place if something has not disturbed its state of rest and set it into motion? Thank you for helping me too by the way. I've been referring to this type of image. upload_2016-8-11_21-9-33.png

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  10. Aug 11, 2016 #9


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    To "change" the state of motion, force is required. Hence, to set an object in motion from rest or to bring a moving object to rest, force is needed. If a body is moving with a constant velocity, no "net" force is acting on it.
  11. Aug 11, 2016 #10
    Ok, Thankyou for your help! I really appreciate it.
  12. Aug 15, 2016 #11
    The term deceleration is not defined in physics.
    We could say the acceleration is constant at about -9.81 m/s2.

    By convention, downward pointing vectors are assigned a negative directional sense.
    The magnitude of an acceleration vector is always positive, no matter what.
  13. Aug 16, 2016 #12
    Ok thankyou! I understand now.
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