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Homework Help: Need help with a few questions

  1. Nov 15, 2004 #1
    hi guys im a college junior...taking physics for the first time in my life...its fun...but still tuff...just can i get help with these couple questions..i mean it would be very gracious of any of u to help me out...i know im new and all, but this place...i was just browsing around and it rocked sooo....and

    wow i never new this place existed....i might come here much more often

    question 1: a 3.0 kg body is initially moving northward at 15m/s. Then a force of 15N, towards the east acts on it for 4.0 s (a) At the end of the 4.0 s, whats the body's final velocity? (b) what is the change in momentum during the 4.0 s?

    i know the answers...can you just show me how to work out part A??? i know b is 60 but i keep getting 25 for part a which is not right

    i would really appreciate this...thank you...illpost the other 3 questions as well ? :smile:
    Last edited: Nov 15, 2004
  2. jcsd
  3. Nov 15, 2004 #2
    The position of three particles written as (x,y) coordinates are, particle 1 (mass 4.0 kg) at (4.0m, 0m); particle 2 (mass 6.0kg) at (2.0m, 4.0m); particle 3 (mass 3.0 kg) at (-1.0m, -2.0) what is the location of the center of mass????
  4. Nov 15, 2004 #3
    A projectile of mass 2.0 Kg approaches a stationary target body at 8.0 m/s. The projectile is deflected through an angle of 90.0 degrees and its speed after the collision is 6.0 m/s. WHat is the speed of the target body after the collision is perfectly elastic????

    any help on how to do this one guyzz??
  5. Nov 15, 2004 #4

    solved thsi one, thanks
  6. Nov 15, 2004 #5

    is this one like this???

    .2(8) + .2(6)= 2.8 m/s???
  7. Nov 15, 2004 #6
    For the first part, mind your direction is all the thing I can advised on kinematic question.
    For third one, split the case into 2 parts which are y-component and x-component.
    Do these help?
  8. Nov 15, 2004 #7
    thanks for responding...what do u mean mind the direction?? meaning the 90 degrees? but how?

    and for the second one i think u mean i did split it into the x,y and it worked...

    now the third one, is thatright???
    Last edited: Nov 15, 2004
  9. Nov 15, 2004 #8
    Since the force oppose the moving direction, you should get deceleration.
    My x- and y- component is for the third one since you already know how to deal with the second one. Btw, are you given the mass of the target body?
  10. Nov 15, 2004 #9
    i mean the answer is supposed to be 2.8 m/s....so im confused to what im doingwrong...why does the deflection even matter? i mean isnt it just initial and final speeds???

    and any idea on the first one....
  11. Nov 15, 2004 #10
    of course it matter, i think your third question has missing information. Can you recheck? How can we solve for the target body without any information given on it at all.

    Sorry for the first part, I mistook the direction of force as south. Since the force is acting eastwards, take the initial velocity of the object in the east direction to be 0 and calculate its final velocity. Since east and north are perpendicular to each other, the force will not impose any effect on the northward movement. with one velocity in the north direction and the other in the east direction, you can calculate the final velocity.
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