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Plase any one crrect my answer

  1. Jun 17, 2007 #1
    Hi
    Can any body correct my answers ?



    1- when the speed of a moving object is doubled, its
    a- Acceleration is doubled
    b- Weight is doubled
    c- Kinetic energy is double
    d- Kinetic energy is increase four times

    My answer is d


    2- an iron sphere of mass 40 kg has the same diameter as an aluminium sphere whose mass is 10 kg. the sphere are dropped at the same time from a tower. When they are 115 m from the ground, they have the same:
    a- Kinetic energy
    b- Potential energy
    c- Momentum
    d- Impulse
    e- Acceleration

    My answer is a


    3- how much work is done by a force of 10N in moving an object through a distance of 2 m in the direction of the force?
    Work= force* distance
    = 10 * 2 = 20 J



    4- A bag of rice weighs 150 kg. to what height should it be raised so that its potential energy is 10 J ?
    PE= mgh
    10 = 150 * 9.8 * h
    h = 10/(150 * 9.8) = 0.0068 m




    5- calculates the work done in lifting 250 kg of water through a vertical height of 8 meters.

    PE = mgh
    = 250 * 9.8 * 8 = 19600 J
    Work = PE
    So work = 19600 J



    6- A truck weighing 10000N and traveling at 35m/s stops at a distance of 40 m decelerating uniformly. What is the force exerted on it by the brakes? What is the work done by the brakes?
    V2 = u2 + 2aS
    u2 = -2 aS because the final speed is zero
    1225 = - 2 * 40 * a
    a = 1225/-80 = -15.31 m/s2
    for the time
    v = u + at
    0 = 35 + (-15.31)* t
    - 35 = ( -15.31 ) * t
    t = -35/-15.31 = 2.28 s
    force = mass * acceleration
    10000 = mass * 9.8
    mass = 10000/9.8 = 1020.4 kg

    force exerted = mass * acceleration
    = 1020.4 * - 15.31 = - 15622.32N
    work done by the brakes:
    work = force * distance
    = 10000 * 40 = 400000 Joules

    7- A man drops a 15 kg rock from a top of 20 m building. What is its speed just before it hits the ground? What is its kinetic energy when it reaches the ground?
    PE= 15 * 9.8 * 20 = 2940 Joules
    KE = PE
    KE = ½ mv2
    2940 = ½ * 15 * v2
    2940 = 7.5 * v2
    v2 = 2940/7.5
    v = square root of ( 2940/7.5)
    v = 19.79 m/s
    KE= PE
    So KE =2940 Joules
     
  2. jcsd
  3. Jun 17, 2007 #2
    2) I don't think so, Kinetic E.=(1/2)mv^2, don't they different masses? What forcce acts on both of they equally?
     
  4. Jun 17, 2007 #3
    ok,, i think the answer in number 2 should be E,, because the 2 spheres have same diameter,, so same effect of air resistance,, they were dropped at the same time and have same effect of g,, so acceleration is the same...:cool:

    i got the same answers on everything but my velocity in number 7 is 19.80m/s
    anyway,, the KE i got is the same...:grumpy:

    that's all!
     
  5. Jun 17, 2007 #4

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    Problem 2 should state to ignore air resistance, since none of the answers are correct if it is included.
     
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