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How can you move a wheel chair sitting on it?

  1. Jun 6, 2014 #1
    As we read earlier that you cant move a body sitting on it,for you and the body make a total system. And from Newton's 3rd law the action you give to the chair is equal and opposite to the reaction the chair gives you back. So the system has no internal net force acting. But when you sit on a wheel chair(a chair having wheels at the legs) you seem to be able to move it with a push totally sitting on it. How is it possible? Or am I missing on some ideas? Thanks
  2. jcsd
  3. Jun 6, 2014 #2
    My idea is that the wheel turns easier when it goes forward, and harder when it goes back. Hope you understand.
  4. Jun 6, 2014 #3


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    Ha ha. Look up Newton's horse and cart puzzle!
  5. Jun 6, 2014 #4
    Thanks UltrafastPED. It helped. But dont you think it is a bit different case? Or should I consider the friction between me and the chair?
  6. Jun 6, 2014 #5
    If you are sitting on a chair and applying force forward or backward for a time.
    The chair will move in the direction of force you applied(not equal or opposite).
    The chair is your friend, not enemy.

    One more answer can be:
    consider that you are sitting on a wheelchair.
    you want to move the chair forward.
    you push your body's lower part(like hips and legs) forward and upper part(like chest) downward & backward .
    At this condition, the forward movement will be more.
    Thus you move forward.
    Last edited: Jun 6, 2014
  7. Jun 6, 2014 #6


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    Friction between wheelchair and ground is the important point - and this provides the net force between wheel chair and ground. If you sit on a perfect frictionless surface, you'll just rotate the wheels without getting forwards.
  8. Jun 6, 2014 #7
    This makes me think about another problem that is sort of interesting. A child on a swing. How does the child get the swing going higher and higher when there is nobody pushing him?
  9. Jun 6, 2014 #8


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    At the top of their arc, the child pulls himself/herself radially inwards by pulling against the chains of the swing. That pull has an upwards component because of the angle of the chain, so by pulling at the top of the arc the child further increases their maxmum height with each swing.

    A fun exercise to see if you can swing past horizontal without someone helping by pushing you.
  10. Jun 6, 2014 #9


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    The internal friction at the axles allows some amount of force to be applied to the ground, and in this case the Newton third law pair of forces involved the ground applying an external force back onto the wheel chair. So a person can somewhat smoothly lean forwards, which applies a backwards force to the ground, coexistant with the ground applying a forwards force onto the wheel chair. This allows the persons center of mass to be accelerated forwards. Then the person has to jerk backwards fast enough to overcome the static friction in the axles so that the wheel chair rolls forwards. This method only works well until the person can't lean forward fast enough to stop the wheel chair relative to the groun in order to transition from static to dynamic (sliding) friction at the axles.

    The child leans backwards or forwards while holding the chain at about shoulder height, well above the seat. The initial reaction doesn't move the center of mass of the child, but it results in the supporting chains angle being offset from vertical, and the horizontal component of tension in the chains result in forwards or backwards acceleration.

    Once sufficient swing is developed, then an alternate method of moving the center of mass inwards and upwards towards the supporting bar at the bottom of the swing and then outwards (downwards) at the peak of the swing results in a net increase in energy because the force at the bottom of the swing opposing both gravity and centripetal acceleration is greater than the force at the peaks of the swing where the opposition to gravity and centripetal acceleration are near zero. The alternate method works better when standing on the swing, squatting and standing. A more extreme example of this can occur on "flying rings", example .wmv video, the guy (Val Dero) goes from a low swing to bar level in 3 or 4 swings (followed by a quadruple back flip):

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