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Sky divers air resistance balances with their weight

  1. Jan 9, 2005 #1
    When a car is travlling at constant speed, we say that the forward force is equal to the frictional force. How is that possible? Shouldnt the car be stationary?
    Samething with sky divers, when the air resistance balances with their weight, how is it possible that they still move downwards? Shouldnt the forward force be greater than the frictional force? :confused:
  2. jcsd
  3. Jan 9, 2005 #2
    When a skydiver jumps out of a plane, he accelerates until the friction force on him becomes as great as the gravity's force, then he stops accelerating.

    "Stationnary" is only a particuliar value (0) for a constant speed, such 4 or -15, or (5,-2,4).
    Last edited by a moderator: Jan 9, 2005
  4. Jan 9, 2005 #3
    So, what about the car then?
  5. Jan 9, 2005 #4


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    When a car travels with constant speed (they mean velocity!) there's no acceleration and thus no net force acting on the car. That's why the forward force and backwards force (friction here) must cancel.

    No force on an object doesn't mean the object is stationary. It means that the object doesn't accelerate. It will move with constant velocity.

    In the case of the skydiver, when the gravitational force is cancelled against the force from air resistance the skydiver will fall with constant velocity.
  6. Jan 9, 2005 #5


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    Zero acceleration doesn't automaticaly mean zero velocity.It means constant velocity in general,and in a very particular case,that constant in zero.For the car,the velocity is obviously different from zero.It can go to zero,if the car decelarates,which means that the friction force would be smaller that the traction force exerted by the engine.

  7. Jan 9, 2005 #6
    If these two forces are equal in magnitude but opposite in direction, the second law of Newton states that Ma = 0. Zero acceleration means constant velocity. So if you had any initial velocity , it will remain the same.

    Net force =0 does not automatically imply that the object stands still. It means the the velocity is CONSTANT. It can be zero but it is not necessary. It depends on the initial conditions

  8. Jan 9, 2005 #7
    If you put a moving car in neutral (inhibit the forward force), friction takes over and you will deccelerate until friction dissappears. You need to give it gas to equalize friction.

    Note though that from the driver's perspective, the car is stationnary. In physics, it is thus more useful to consider "moving at constant speed" vs "accelerating", rather than "moving" vs "stationnary".
  9. Jan 11, 2005 #8
    Both of this situations are in that of equilibrium, where the sum of the forces is zero. Terminal velocity is reached when the force of drag, or air friction is equal to that of gravity. Forces cause acceleration, so if there is no acceleration there is no net force or there are forces but they cancel eachother out so to speak.

    Ff=frictional force between the car and the ground
    Fe=force exerted by the engine of the car in the forward direction
    note both models assume that air resistance is proportional to the sqaure of the instantaneous velocity.

    Car: Fe-Ff=0=ma therefore there is no change in velocity and the car continues at the same speed.
    Or you could modify it to include the air resistance as well...
    Fe-Ff-kv^2=0=ma again there is no change in speed

    Skydiver mg-kv^2=0=ma no change in velocity the skydiver falls at the same speed.
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