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Net Work

  1. Jul 23, 2006 #1
    How much net work is being done when a car is traveling at a constant speed on a flat road?

    I think there is not net work done.

  2. jcsd
  3. Jul 23, 2006 #2
    Think of Newtons first law and the definition of work.
  4. Jul 23, 2006 #3
    Okay so Newtons First Law states:

    An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

    However, that doesnt say anything about the work being done on the car simply that it stays in motion unless acted upon by another force.
  5. Jul 23, 2006 #4
    What is the definition of work?

    W = Fs x s

    Please point out where the resulting force is acting in this example.
  6. Jul 23, 2006 #5
    I looked up work as well and i think i understand now

    The actual question is:

    In which of the following scenarios is no net work done?
    A car accelerates down a hill.
    A car travels at constant speed on a flat road.
    A car decelerates on a flat road.
    A car decelerates as it travels up a hill.

    I chose B , A car travels at constant speed on a flat road because there is no work upon the car as it travels along a flat road. The force supplied by the car is an upward force and the displacement of the car is a horizontal displacement. As such, the angle between the force and the displacement is 90 degrees. If the work done by the car on the road were to be calculated, then the results would be 0.

    Am i anywhere close?
  7. Jul 23, 2006 #6


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    A car travelling at constant velocity along a flat road does no work against gravity -- however, it still does quite a bit of work against friction.

    - Warren
  8. Jul 23, 2006 #7
    So i am right?
  9. Jul 23, 2006 #8
    As you have learned from Newton's First Law, a body will be still or move with a constant speed in a straigh line if no resulting forces is acting upon it.

    As the care moves in a straight line with constant speed you can deduct that there is no resulting force.

    Without a resulting force, it does indeed not have a net work (see the above given equation).

    Does that help you understand the situation? Remember that there is a difference between getting the answer and understanding it ;)
  10. Jul 23, 2006 #9
    haha..thank you. and yes that does help me understand it much better. It was the part about the resultant where i was getting confused.
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