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    Re: Definition: Force vs Power
    From: Trevor Kenchington
    Date: 2003 Apr 24, 10:30 -0300

    David Weilacher wrote:
    > Recently, we were discussing effect of wind and current.  Actually, I believe we still are.
    > It was suggested that Force = mass X velocity.  Also, that Power = velocity 
    squared.  (or is it mass X velocity X velocity?)
    > First, is this stated properly by me.
    With my copy of the CRC Handbook of Chemistry and Physics open in front
    of me (just to make sure that I have not forgotten everything of my
    high-school physics!):
    "Force" is defined as something that changes the motion of matter. A
    boat floating without moving begins to move when a force is applied to
    it. A moving boat slows down, speeds up and/or changes the direction of
    its movement when a force is applied. Wind, prop discharge current, the
    pull of a tow rope, wave drag acting on a moving hull and many, many
    others can supply forces that alter the motion of the boat. (Of course,
    everything is constantly subject to many forces so real-world motions,
    as distinct from physical concepts, appear appear when the balance of
    various forces changes. The forces of buoyancy and gravity, for example,
    are usually far greater than other forces acting on boats but they
    balance one another, at least as averages over reasonably short time
    periods, and thus the motions we see are caused by the smaller forces
    which act horizontally.)
    Quantitatively, "Force" is equal to mass times acceleration, _not_ mass
    times velocity. As Newton realized, a moving object will continue to
    move in a straight line at constant speed unless acted on by some force
    (which is why moving boats hit large static objects when their engines
    fail to go astern at the right time!). In SI units, a force of one
    Newton, if applied to a body with a mass of one kilogram, will
    accelerate that body at a rate of one metre per second per second. That
    is: For each second that the force is applied, the velocity of the body
    will increase by one metre per second (all supposing, of course, that no
    other force acts to resist the effect of the first one, whereas in the
    real world motion generates friction which does directly oppose the
    forces causing the motion).
    When a force acts, energy is converted from some form (such as the
    chemical energy in diesel fuel) into kinetic energy of the accelerated
    object. (In the real world, that kinetic energy usually ends up as heat
    when the moving object eventually gets slowed down again by friction.)
    Again in SI units: When a force of one Newton acts through a distance of
    one metre, one Joule of energy passes into kinetic energy.
    Power is the rate of doing work, meaning the rate at which energy is
    converted from one form to another. A power of one Watt means that one
    Joule per second is being converted, thus a 100 Watt light bulb converts
    100 Joules of electrical energy into heat and light every second.
    Power is energy per second but energy is force times distance, meaning:
    Power is force multiplied by distance and divided by time, while force
    acts to accelerate mass, thus:
    Power is equal to mass multiplied by acceleration and distance, divided
    by time. Unless I am muddling something, that means that:
    Power is equal to mass multiplied by velocity and distance, divided by
    time squared,
    hence to mass multiplied by velocity squared, divided by time.
    Which means that more power will allow a greater mass to reach higher
    speeds in less time -- which matches exactly with the common-sense
    notion of "power" in relation to engines and the movement of boats.
    I hope that makes sense.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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