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Homework Help: Help! lesson 3 physics!

  1. Feb 23, 2005 #1
    greetings...i need some help please and someone to check if i did the problems correctly.

    1.) How much kinetic energy will a 5kg ball have after rolling down a 1m high incline?
    My Answer: 4.43 J
    If Potential Energy=mgh, then P=(5kg)(9.81m/s^2)(1m) so P=49.05 J. If this is true, then Kinetic energy = 49.05 J = 1/2mv^2. 49.05=1/2(5)v^2. Then i divided 49.05 by 2.5 (49.05/2.5) which gave me v^2=19.62. I then took the square root of 19.62 which is approximately 4.43 J.

    2.) Why are no collisions perfectly inelastic?
    My Answer: Almost all collisions involve some rebounding.

    3.) A lever is used to raise a 300N rock a distance of 0.6 meters. You must use a force of 75N to accomplish this task. What work is done?
    My Answer: 2.45 J
    If work can equal mass times gravity times height, then work equals 300N times 0.6 meters, which equals 180. I then did 180 divided by 75N (the amount of force needed), coming up with the solution: 2.45 J

    thank you very much for checking my work!

  2. jcsd
  3. Feb 23, 2005 #2
    1) After 1m, all potential energy will have been converted to kinetic energy. So your calculation should read:
    Ek=49.05 J
    You went the whole way and calculated the velocity, which is measured in m.s^-1. It looks as though you got confused between kinetic energy and velocity.

    3) This is quite clearly incorrect. You certainly used more than 2.45 J of energy to lift a 30kg rock from the ground! Read up on levers and you should be able to get this one.
  4. Feb 23, 2005 #3
    EDIT! Not correct, read scholar's post. Your solution would be correct if
    the question was: Calculate the velocity.

    I don't know, but your answer sounds good.

    That doesn't seem correct. First of all I can see that's not correct because
    of the units:
    You divided 180Nm=180J by 75N which gives you 2.4m and NOT 2.45J
    ALWAYS check the units, it gives you a first hint if your calculation is correct.
    Last edited: Feb 23, 2005
  5. Feb 23, 2005 #4
    so if Work= Force Times Distance, then Work= (75N)(0.6m) which equals 45 Joules...where exactly does the 300N rock come ino the equation?

    the problem im facing is that 45 J is not one of the choices i have to choose from

    oh and i really thank you guys for the tip/help..im going to look up more information about levers now.
  6. Feb 23, 2005 #5

    I just send you a private message. I think it's just 300N times 0.6m.
    The 75N is the force you need if you use a lever.

    Sorry, my fault.
  7. Feb 23, 2005 #6
    oh, ok. thank you edgardo.

    One more question.

    What power is required to accelerate a 500kg car from zero to 18 m/s in one minute?

    First i found the kinetic energy

    Ek=250(324)=81000 J

    Power= Joules per second
    Power= 81000 J / 60
    Power= 1350 W

    It takes 1350 W to accelerate a 500kg car from zero to 18 m/s in one minute.
  8. Feb 23, 2005 #7
    Hello Mark, that seems to be correct.


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