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Homework Help: Physics Hw Help

  1. Mar 26, 2005 #1


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    I was doing my hw but i am really stuck.
    My teacher ain't that good he doesn't explain things very good.
    If you can help plz help.
    1. a cyclist of mass 60kg riding up a hill which is inclined at 30 degress to the horizontal at a constant speed. the mass of the bicycle is 20kg.
    a)calculate net force.
    it's 0 cause he's goin at constant speed.
    b)total friction=10n, what's the magnitude of the component of the weight of the system that is parrallel to the road surface?
    I have no idea how to do it. i know u have to minus something by 10n.
    in the diagram the weight pulling down is 800N.
    c)claculate dricing force.
    No idea but i think it would the answer in B divided by 100kg.
    d)calc the magnitude of the normal reaction force on the bicycle system
    again i have no idea.
    if you know a site that explains how to do this very well iw would be happy to read it.
    also another question
    2)person skin downhil of 60kg(including skies) is moving down a slope inclined at 30 degrees with increasing speed. she is moving in a straight line down the slope.
    a)direction of the net force on the skier.
    it's down the slope.
    b)Claculate magnitude of the component od the skiers weight that is parrallel to the slop.e
    again i have no idea how to do it.
    c)sum of friction is 8n, claculate net force on skier.

    3)a ball of mass 0.5 kg is thrown vertically upwards.
    a)calc velocity of the ball at the top of it's flight.
    it's 0 cause it stops.
    b)calc acceleration at the top of it's flight
    it's 9.8 ms-1 cause gravity constant.
    c) calculate net force on the ball at the top it's flight?
    i couldn't do this 1, there is a net force cause it's accelerating.
    and last 1
    4)a car of mass 1200kg starats from rest on a horizontal road with a forward thrust of 10 000 N. the resistance to motion due to road and air resistance totals 2500N
    1)calc net force.
    it's 10000-2500
    b)clac acceleration
    it's 7500/1200
    c)clac speed after 5 s.
    this should be easy but i don't get it.
    d)calculate the distance the car has travelled after 5 s
    again it should be easy but it is annoying me.

    If you can help me with these questions it would really help.
  2. jcsd
  3. Mar 26, 2005 #2
    From what it looks like its just an "object on ramp" problem.

    Heres a few things to help you out:

    Component parallel to surface : [tex] mgsin(\theta) [/tex]

    Component normal to surface : [tex] mgcos(\theta) [/tex]

    Force of friction : [tex] \mu_sN = \mu_smgcos(\theta) [/tex]

    For ball problem:

    At the top of the height, is the ball moving?
    What are the forces acting on the ball?
    If you know the acceleration on the ball, and you know the mass, use F = ma to find the net force.

    For car problem:

    [tex] v(t) = v_0 + at [/tex]

    [tex] x(t) = dv/dt = x_0 + v_0t + 1/2 at^2 [/tex]
  4. Mar 26, 2005 #3


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    Thx, I understand most it but
    Force of friction :, i couldn't raed that properly
    I understood the ball one.
    The car one i did not understand, Sorry i'm a bit stupid.
  5. Mar 27, 2005 #4
    Force of friction is proportional to the coefficient of friction x Normal force.
    Coefficient of friction [tex] \mu_s [/tex]
    Normal force [tex] mgcos(\theta) [/tex]

    Velocity of any object under constant linear acceleration is [tex] v = v_0 + at [/tex]
    Position of any object under constant linear acceleration is [tex] x = x_0 + v_0t + 1/2 at^2 [/tex]
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