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Homework Help: Force and tension

  1. Sep 19, 2005 #1
    I am having some trouble with two problems- thanks for any help!
    71) A .2kg ball is dropped from 10 m above the beach, leaving an impression in the sand 5.0 cm deep. What is the average force acting on the ball by the sand?
    So far I have converted 5.0cm to .05m. I calculated Fg= .2 kg (9.8m/s2) = 1.96 N. Now I am confused as to how I should calculate the normal force of the sand.
    73) On an inclined plane (with angle of 37 degrees) a pulley is holding block m1 on the incline and block two is hanging on the other side. The weights of the blocks are m1= 3.0kg, m2= 2.5kg. What is the tension in the string if the acceleration is 1.2 m/s2?
    I calculated T= m2a = (2.5kg)(1.2m/s2) = 3 N but the answer in the back of the book says it should be 21 N?
    Thanks again for the help- I really appreciate it!
     
  2. jcsd
  3. Sep 19, 2005 #2

    lightgrav

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    Homework Helper

    73: your 3N is the TOTAL (net) Force.
    gravity contributes about 25N
    and the string has to cancel all but 3N of it.

    72: it falls 10m from rest : ½ at^2 to find t,
    then get v = a t just before hitting the sand.
    The Force by the sand has to cangel gravity
    and have enough left over to "decelerate" it.
    Do you know energy? otherwise, v^2 = 2ax.
     
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