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Homework Help: Force and constant velocity

  1. Jan 1, 2005 #1
    The SI units for force is kg*m/s^2. So, if a object travels a certain distance with no acceleration, because it has a constant velocity. What is the force? Is it correct to find the KE, which units are kg*m^2/s^2 and divide the distance?

    Also, KE is the energy from motion, correct? So, is that energy the same for any distance, if the velocity is constant? KE=.5*m*v^2. If so, how can that be?
  2. jcsd
  3. Jan 1, 2005 #2


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    If acceleration is 0 m/s^2, then the force on the object is 0N (since F=ma).

    Since we have 0 m/s^2 acceleration we know that velocity is constant, and K.E is constant.

    Let's see if our work energy equation gives the same result:

    Work = Force * Distance = (K.Efinal - K.Einitial).

    Force is 0N. So work is 0J. so:
    so K.Efinal=K.Einitial

    So it all works out.
  4. Jan 1, 2005 #3
    Great, thanks. That helps.

    But answer this, if person walks 1 mile it will require a certain amount of kilojoules to "drive" this person to walk this far. So, they in take food to produce these kilojoules. So, how can there be no work? I understand the idea of change of energy or change in acceleration. But work is defined as change in energy and not energy used or created? :uhh:

    Also would there be work if the person walk uphill at a certain angle for a certain distance? Potential?
  5. Jan 1, 2005 #4


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    Note that it's the frictional force that drives a person forward. He exerts a backward force on the ground with his feet, and the friction is the reactional force that pushes him forward.

    We use up energy exerting forces with our muscles, even when those forces don't do any work! :smile: For example a weightlifter can lift up a weight and then hold it motionless. When he's lifting it up, chemical energy is being converted to mechanical energy in his arms, which is then transferred to the weight as kinetic energy and finally gravitational potential energy.

    But while he's holding the weight up in the air, he's not doing any work on it (since it isn't moving). But he's still losing chemical energy, because the muscles need to remain in tension, and keep exerting the upward force on the weight. That energy that he's losing is converted to heat.

    I believe that generally, as long as our muscles are exerting a force, we will be losing energy.
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