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Math Help - Mechanics - Energy, work and power

  1. #1
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    Mechanics - Energy, work and power

    The driving force exerted by D's engine is 4 times as great when D is at the top of the hill as it is when D is at the bottom. Find the ratio of the power developed by D's engine at the top of the hill to the power developed at the bottom.

    Thanks in advance!!
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    A Plied Mathematician
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    You're more likely to get more accurate help if you post this question on the physics help forum, our sister site. I say that for your benefit.
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    MHF Contributor ebaines's Avatar
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    Since power is defined as in work per unit time, and work is force times distance, then power = force time velocity. You tell us the engine develps 4 times as much force at the top of the hill as at the bottom, but you don't tell us about the velocity - specifically whether the velocity of D is the same at the top of the hill as at the botttom. Without that information it's impossible to answer.
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  4. #4
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    Opps, sorry, I forgot the previous parts

    A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal. The driving force exerted by C's engine has magnitude 1800N. Another car D, also of mass 1200kg, climbs the same hill with increasing speed. The speed at the bottom is 8m/s and the speed at the top is 20m/s. It is assumed that the resistance to the motion of D is constant and has magnitude 700N. The driving force exerted by D's engine is 4 times as great when D is at the top of the hill as it is when D is at the bottom. Find the ratio of the power developed by D's engine at the top of the hill to the power developed at the bottom.

    Thanks in advance!!
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  5. #5
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    Quote Originally Posted by Cathelyn13 View Post
    Opps, sorry, I forgot the previous parts

    A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal. The driving force exerted by C's engine has magnitude 1800N. Another car D, also of mass 1200kg, climbs the same hill with increasing speed. The speed at the bottom is 8m/s and the speed at the top is 20m/s. It is assumed that the resistance to the motion of D is constant and has magnitude 700N. The driving force exerted by D's engine is 4 times as great when D is at the top of the hill as it is when D is at the bottom. Find the ratio of the power developed by D's engine at the top of the hill to the power developed at the bottom.

    Thanks in advance!!
    Find what ratio of power? Thrust to weight ratio?
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    Re: Mechanics - Energy, work and power

    Net work done, W by the engine = 1800x500 = 9x10^5 J
    W =change in potential energy of car, W1 + work done against resistance, W2
    W1 = 1200x9.8x500x sin 6 = 6.1463x10^5 J
    W2 = 2.8537x10^5 J is the answer to th first problem.
    In the second problem, we have to find out the net external work done, WE, by the engine.. It has three components. Change in kinetic energy of car, KE, change in potential energy, PE and work done against resistance.

    WE = 0.5x1200(20^2-8^2) + 6.1463x10^5 + 700x500
    WE = 2.016x10^5+6.1463x10^5+3.5x10^5 = 1.1623x10^6 J is the second answer.

    lol, its been 2 years since you asked this question :P anyways..
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    Re: Mechanics - Energy, work and power

    A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal.

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    MHF Contributor ebaines's Avatar
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    Re: Mechanics - Energy, work and power

    Quote Originally Posted by LanellePalmer View Post
    A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal.
    Do you have a question?
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    Forum Admin topsquark's Avatar
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    Re: Mechanics - Energy, work and power

    Quote Originally Posted by Ackbeet View Post
    You're more likely to get more accurate help if you post this question on the physics help forum, our sister site. I say that for your benefit.
    Point 1. Yes, this thread is two years old.

    Point 2. LanellePalmer is a spammer.

    Point 3. PhysicsHelpForum no longer exists.

    -Dan
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  10. #10
    MHF Contributor ebaines's Avatar
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    Re: Mechanics - Energy, work and power

    Quote Originally Posted by topsquark View Post
    Point 3. PhysicsHelpForum no longer exists.

    -Dan
    PhysicsHelpForum is alive and kicking, although traffic is a bit slow:

    Physics Help Forum
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    Forum Admin topsquark's Avatar
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    Re: Mechanics - Energy, work and power

    Quote Originally Posted by ebaines View Post
    physicshelpforum is alive and kicking, although traffic is a bit slow:

    physics help forum
    HOLY CRAP!! I just figured that the Physics forum fell under the "mash hammer" and disappeared.

    Gee, I'm a mod on that forum too. Maybe I ought to visit it once in a while.

    Thanks for the info!

    -Dan
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