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Some problems no clue what to do

  1. Sep 24, 2005 #1
    ok even if this seem simple to you please help me i failed the test and ihave to do corrections so could someone please help me out

    1. A 14 kg bucket of water is raised from a well by a rope. If the upward acceleration of the bucket is 2.2 m/s2, find the force exerted by the rope on the bucket of water.
    in Newtons


    2. A boat moves through the water with two forces acting on it. One is a 6.5 103 N forward push by the motor, and the other is a 0.9 103 N resistive force due to the water.

    (a) What is the acceleration of the 1200 kg boat?
    m/s2
    (b) If it starts from rest, how far will it move in 9 s? in
    m
    (c) What will its velocity be at the end of this time interval? in
    m/s



    3. [SFHS99 4.P.27.] A 2 kg ball is dropped from the roof of a building 230 m high. While the ball is falling to Earth, a horizontal wind exerts a constant force of 12.0 N on it.

    (a) How far from the building does the ball hit the ground?
    m
    (b) How long does it take to hit the ground?
    s
    (c) What is its speed when it hits the ground?
    m/s



    4. A loaf of bread weighs 4.1 N on Earth.

    (a) What should it weigh, in newtons, on the moon, where the acceleration due to gravity is one-sixth that on Earth?
    N
    (b) What should it weigh on Jupiter, where g is 2.64 times Earth's gravity?
    N



    5. [SFHS99 4.P.47.] A 4.66 kg briefcase is sitting at rest on a level floor.

    (a) What is its acceleration?
    m/s2
    (b) What is its weight in newtons?
    N


    6. [SFHS99 4.P.52.] A 92 kg clock initially at rest on a horizontal floor requires a 656 N horizontal force to set it in motion. After the clock is in motion, a horizontal force of 556 N keeps it moving with a constant speed.



    7. [SFHS99 4.P.56.] A car is traveling at 50.1 km/h on a flat highway.

    (a) If the coefficient of friction between the road and the tires on a rainy day is 0.100, what is the minimum distance needed for the car to stop?
    m
    (b) What is the stopping distance when the surface is dry and the coefficient of friction is 0.600?
    m



    8. [SFHS99 4.P.64.] A hockey puck is hit on a frozen lake and starts moving with a speed of 13 m/s. Exactly 5.00 s later, its speed is 5.9 m/s.

    (a) What is its average acceleration?
    m/s2
    (b) What is the average value of the coefficient of kinetic friction between puck and ice?

    (c) How far does the puck travel during this 5.00 s interval?
    m


    9. [SFHS99 4.P.59.] A girl coasts down a hill on a sled, reaching the bottom with a speed of 8.6 m/s. The coefficient of friction between the sled's runners and the snow is 0.050, and the girl and sled together weigh 675 N. How far does the sled travel on the surface before coming to rest?
    m


    thanks
     
  2. jcsd
  3. Sep 24, 2005 #2

    EnumaElish

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    Can you at least post the answers that need to be corrected? So people can see what exactly was wrong and where?
     
  4. Sep 25, 2005 #3

    Lisa!

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    AS you know first you need to find the external forces acting on system and then use :

    [tex]\sum \vec{F} = m \vec{a}[/tex]

    I'll do #1 for you (other questions are almost like that). Then tell us what you've done for the rest, and then someone will help you.

    We have 2 forces here:
    1. The weight of the bucket which is downward: [tex]m \vec{g}[/tex]
    2. The force by the rope which should be upward in orderto overcome the weight and gives the bucket an upward acceleation: [tex] \vec {T}[/tex]

    Now we have: [tex]T -m g = ma[/tex]
    You have m, g, a. So you can find T.


     
    Last edited: Sep 25, 2005
  5. Sep 25, 2005 #4
    sorry about that the answers where on a different sheet we had to do them all over from scratch to try and get half credit back i didnt know i had to use t-mg =ma that helped alot i was using ma =fa-fk i just need that forumla thanks alot though sorry about that.
     
  6. Sep 25, 2005 #5

    Lisa!

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    What about the rest? Can you do them now?
     
  7. Sep 25, 2005 #6
    not all of them im kinda stuck i dont know how to solve for the problems involving coefficients of friction i dont know how to us the Fk =uK-Fn to find the distance travel and the acceleration for instance number 9 and number 3 i dont know hwo to do either
     
  8. Sep 25, 2005 #7

    EnumaElish

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    Aren't you supposed to use F = m a = m g ?
    (a) What is the acceleration acting on any object on Earth without which all would be adrift in space?
    (b) F = m a = m g
     
  9. Sep 25, 2005 #8
    i already had those right i cant get 3 or 9 was what i was saying
     
    Last edited: Sep 25, 2005
  10. Sep 26, 2005 #9

    EnumaElish

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    For 3.a, draw a diagram of a ball falling and being pushed by a wind. It will be a triangle. The units of the triangle should either be meters or m/s but not both. You need to calculate the base of the triangle (the distance from the building). I think you'll need to use a trig function to calculate the triangle's base.
     
  11. Sep 26, 2005 #10
    ok, i got all of them except 8b 3 and 7. i have no clue for 7 and 8b because i need a wieght i figure out how fast it will slow down and i cant find Fk without the normal force 3 im not sure of but i could figure it out if i take my time
     
    Last edited: Sep 26, 2005
  12. Sep 26, 2005 #11

    EnumaElish

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    I am not a physicist; can you write the definition for coef. of friction?
     
  13. Sep 26, 2005 #12
    force of kinetic friction = coeficient of kinectic friction *normal force

    normal force = ma
     
  14. Sep 26, 2005 #13

    Päällikkö

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    You do not need to know the weight. I always use the symbols to solve the equation, and only into the final one will I place the values. I recommend this approach.

    m will cancel out, just keep the equations rolling.
     
  15. Sep 26, 2005 #14
    oik thanks ill give it a shot



    ok i have no clue how does the m cancel out i dont even know how to use force in the equation i only have an initial velocity and a coeficient of kinetic friction
     
    Last edited: Sep 26, 2005
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