Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Can u help me ?

  1. Feb 9, 2010 #1
    Hello People :
    I'm a new member and a new physics student too.
    I need help in these points please

    1- Its Known that when a ball falls on your head you are harmed but if the ball bounces off ur head the harm is greater bec the impulse in this case is doubled why??

    2- How far is infinity coming out from a positive point charge?

    http://img36.imageshack.us/img36/7075/inclinedsurface.jpg [Broken]
    3- Why do we say that the body is moving under the effect of gravity when the angle of inclination is smaller than 30 degrees?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 9, 2010 #2
    Hi there,

    1. The variation in impulse is [itex]2\mathbf{p}[/itex] because the initial momentum is
    [tex]\mathbf{p}[/tex] and the final is [tex]-\mathbf{p}[/tex] so [tex]\Delta \mathbf{p}=\mathbf{p}-(-\mathbf{p})=2\mathbf{p}[/tex].
    In order to visualize the signs I chose for p you can imagine a ball hitting the wall. When the ball hits the wall, it's direction will be the opposite of what it had inittialy.

    2. What do you mean by how far is infinity?

    3. The body would move under the effect of gravity if the angle was bigger than 30 degrees too, so the question might be what are the conditions that a system might have in order to make that afirmation. It would be something like:

    A body moves under the effect of gravity if gravity is the only force responsible for it's motion.
  4. Feb 10, 2010 #3
    but why do you assume that the linear momentum (-p) is equal in magnitude and opposite in direction? Its not a condition
    it must be opposite in direction but it may be equal in magnitude and may be not.

    Acually i should have explained more
    the aim of this experment is calculating the free fall acceleration
    it is written "don't make the angle of inclination greater than thirty to avoid sliding of the body by excess of acceleration and let the body move under the effect of gravoty"

    so why?
    That is my question

  5. Feb 10, 2010 #4
    Because the ball bounces off I assumed it to be a perfectly elastic colision, and adding to that the fact that there are no external forces acting on the system, it is perfectly safe to say that the moment of the ball is conserved, so it only changes direction, not magnitude.

    I may be interpreting your question wrong, but if i'm not, you just have to calculate the projection of the force of gravity that acts on the body.
  6. Feb 11, 2010 #5
    Oh yeah i got it now Thanks
    O how?

    Thanks again
  7. Feb 12, 2010 #6
  8. Feb 12, 2010 #7
    Draw a fbd for the body sliding down the ramp. You have only two forces, Fg and N. With x being directed down the ramp the force of gravity perpendicular to the ramp is equal to the Normal force and the force of gravity along the ramp is what causes the acceleration. Find out what that force is equal to and you'll see why the larger the angle the faster it falls.
  9. Feb 13, 2010 #8
    this is what i mean i can't understand this passage

    What do u mean by fdb ?

    Thanks for all of u
  10. Feb 13, 2010 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Neither do I. At best, it uses confusing wording; at worst, it is just wrong.

    It means a free body diagram , i.e. a drawing that indicates every force acting on the object of interest. Both your teacher and your physics text book should have shown you examples of this.
  11. Feb 14, 2010 #10
    I think i need to buy another book .

    Do u mean this ?
    http://img535.imageshack.us/img535/1381/inclined.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  12. Feb 15, 2010 #11
    Fg sin theta is responsible for sliding the body downwards
    if theta is 30 degrees then Fg sin 30 = 1/2 Fg = 1/2 mg

    and since the mass is always constant
    therefore the body is falling by acceleration equals to 9.8/2 = 4.9 m /s^2
    if theta = 90
    therefore the body is falling freely because Fg sin 90 = Fg
    where the acceleration is 9.8

    therefore Fg sin theta increases by increasing the angle and consequently the acceleration while Fg cos theta decreases which is the reaction of the surface

    so Am I right ? Is that all?
  13. Feb 16, 2010 #12
    hello there ?
  14. Feb 16, 2010 #13
    Yes, sin theta increases as the angle increases, therefore the acceleration increases as the angle increases.
  15. Feb 17, 2010 #14
    OK ,Thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook