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    akemper_21's Avatar
    akemper_21 Posts: 1, Reputation: 1
    New Member

    Mar 25, 2009, 08:05 PM
    Force, Work how to determine force without accelration
    I know we're not supposed to post up the full question but I figured it would just be easier to explain it. A 350 kg piano slides 3.8 m down a 31 degrees incline and is kept from accelerating by a man who is pushing back on it parallel to the incline. The effective coefficient of kinetic friction is 0.30.

    And I'm trying to figure out the force exerted by the man. Now what I got is F = m * a... so 350 kg is the mass and I guess gravity 9.8. So the force is 3234 N... but is that right? I just need to be led in the right direction.
    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
    Ultra Member

    Mar 25, 2009, 09:08 PM

    Start by separating the problem into two components -- vertical and horizontal. Calculate the force in both directions. In the vertical direction, it is gravity + the vertical component of the force + frictional forces. In the x direction, it's the horizontal component of the applied force + the frictional force. Gravity works only on the vertical compoents. Once you get that, you can calculate the resultant vector.
    sarnian's Avatar
    sarnian Posts: 462, Reputation: 9

    Mar 26, 2009, 03:00 AM
    Hello akemper

    Your data is : A 350 kg piano slides 3.8 m down a 31 degrees incline.
    Without the slide the piano would drop vertical (gravity = 9.80665 m / sē).
    Calculate the vertical force.
    Draw a triangle with a hypothenusa of 3,8 meter and a 31 degree corner opposite the vertical drop line.
    Calculate the total horizontal force.
    Deduct the friction.
    What is left over is the force that has to be pushed by the man.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
    Uber Member

    Mar 26, 2009, 03:08 AM

    Hum, sarnian, your answer is a little to understand than that of Perito... I would have broken down the forces into components, as he said. Because the resulant force is in fact reduced by the slope and the acceleration towards the bottom is thus another value than 9.81m/s.

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