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Homework Help: Center of mass & linear momentum help please

  1. Apr 15, 2005 #1
    I have been missing for class 3days and i was get lost because my daughter gets sick in the past weeks :cry: :( until todate. The problems i need everyone helps below are just a sample exam. I have contact my professor about my family problems and he was nice, gave me the sample exam which it's similar to the exam this Monday. This chapter is the hardest chapter I ever seen :( Please walk me through this

    1) A dump truck is being filled with sand. The sand falls straight downward from rest from a height of 2.00 m above the truck bed, and the mass of sand that hits the truck per second is 55 kg/s. The truck is parked on the platform of a weight scale. By how much does the scale reading exceed the weight of the truck and sand?

    2) When jumping straight down, you can be seriously injured if you land stiff-legged. One way to avoid injury is to bed your knees upon landing to reduce the fore of the impact. A 75 kg man just before contact with the ground has a speed of 6.4m/s
    a. In a stiff-legged landing he comes to a halt in 2 ms. Find the average net force that acts on him during this time.
    b. When he bends his knees, he comes to a halt in 0.1 s. Find the average net force.
    c. During the landing, the froce of the ground on the man points upward, while the force due to gravity points downward. The average net froce acting on the man included both of these roces. Taking into account the directions of teh frocesw, find the force of teh ground on the man in parts a & b

    3) A golf ball strikes a hard, smooth floor at an angle of 30 degree and as the drawing shows, rebounds at the same angle. The mass of the ball is 0.047kg, and its speed is 45m/s just beforece and after stirking the floor. What's the magnitude of the impulse applied to the golf ball by the floor?

    All helps are appreciated.
  2. jcsd
  3. Apr 15, 2005 #2


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    Let's take 3) first:
    How is impulse related to change in momentum?
  4. Apr 15, 2005 #3
    Here is how I've done to #3

    J = 2(0.047kg)(45m/s)cos30 = 3.66m/s
  5. Apr 15, 2005 #4
    Want me to do all of this for you, and email it to you? When do you need it by?
  6. Apr 15, 2005 #5


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    It looks like you've got the basic idea right, however:
    1) Your unit for impulse should be kg*m/s (you've written m/s in the last expression)
    2) The ball struck at 30 degrees angle TO THE FLOOR.
    Therefore, you must use sin(30)=1/2, rather than cos(30) (do you see why?).
  7. Apr 15, 2005 #6


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    No, she doesn't want you to do it all for her! She wants you and us to give her some hints and help so she can learn to do it herself.
  8. Apr 15, 2005 #7
    Awwwwww my daughter got fever again :( I'm going to die, i don't know if i can make it on Monday

    I'm so frustrating now

    If i still can't figure it out by tonight I'll just ask for the answer then because i don't have time on the weekend :(

    Here's my email: lonelygal_2@yahoo.com

    Thank you guys
  9. Apr 15, 2005 #8
    I was only trying to be helpful. Forget it. :bugeye:
  10. Apr 18, 2005 #9
    :cry: help me out please..............
  11. Apr 18, 2005 #10


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    1) You cannot know what the force looks like for each grain of sand that hits the truck. Fortunately, you don't have to know. You just need to find the averge force over a period of time while the sand is falling. For some amount of mass of sand [tex] \Delta M [/tex]. you can find the velocity with which it hits the truck, and hence its momentum. All that momentum gets lost in the collision. The change in momentum is equal to the implulse, which is the average force applied times the duration of that force, [tex] \Delta t [/tex]. The ratio [tex]\frac{ \Delta M}{ \Delta t} [/tex] is a given of the problem.

    That should give you a good start.
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