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Homework Help: Kinematics of a slingshot

  1. Nov 7, 2005 #1

    Aka

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    i have a test tomorow and I am having trouble with these questions can some one pls check over my answers?
    1. a slingshop shoots a 12g stone at 36m/s. It goes in an arc and at the top of the arc it is going at 14m/s. How high is it at this point?
    12g=0.012kg
    36m/s=v1
    14m/s=v2
    Ek=1/2mv2
    =1/2(0.012kg)(36m/s)2
    =0.216J
    Eg=mgh
    0.216=0.012kg(9.8m/s)h
    0.216J
    0.1176
    =1.8m
    the hight is 1.8m
    2. A child of 27kg climbs to the top of a 5.0m high playground slid
    A) what is her potentian energy?
    27kg=m
    5.0m=h
    Eg=mgh
    =27kg(9.8m/s)(5.0m)
    =1323J
    the potential energy is 1323J
    B)Her actual speed at the bottom is only 3m/s. According to conservation of energy how fast should se be going?
    Ek=1/2mv2
    =1/2(27kg)v2
    1323/13.5=v2
    98=v2
    9.9=v
    she should be going 9.9m/s
    C) the slid is 8.0m long. determine the force of friction acing
    d=8.0m
    Ff=?
    d=(v2+v1/2)t
    t=1.6s
    a=v2-v1/t
    a=9.9m/s-0m/s/1.6s
    a=6.2m/s2
    F=ma
    27kg(6.2m/s2)
    F=167.4N
    the force of friction is 167.4N
    3. A 72kg passenger is in a van moving at 15m/s when it colides with a wall. The frount part of the van collapses 0.50m during the crash. determine the work done on the passenger and the force the seatbelt exerts during the acident.
    72kg=m
    15m/s=v1
    0.50m=d
    w=?
    Ff=?
    Ek=1/2mv2
    =1/2(72kg)(15m/s)2
    =8100J
    ...now what??????
    4/ A roler coaster has a height of 94.5m above the ground at its highest point. deteremine the maximum speed of the cars as they come down from this high point. the actual speed is 41.1m/s. Calcuate the work done by friction.
    Eg=mgh
    =1kg(9.8m/s2)(94.5m)
    =926.1J
    Ek=1/2mv2
    926.1=1/2(1kg)v2
    926.1/0.5=v2
    1852.2=v2
    43m/s=v
    the maximun speed it 43m/s
    i dont know how to calculate the force of friction
     
    Last edited: Nov 7, 2005
  2. jcsd
  3. Nov 7, 2005 #2
    For the first question you forgot to take into account the fact that the projectile still has kinetic energy at the top so [itex]PE_{top} = KE_{bottom} - KE_{top}[/tex]
     
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