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Force and tension

  • Thread starter BriannaUND
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  • #1
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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!
 

Answers and Replies

  • #2
lightgrav
Homework Helper
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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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