Physics Help Forum Application of Newton's second law, problem with free body diagram

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Oct 20th 2009, 11:51 AM #1 Junior Member   Join Date: Sep 2009 Posts: 24 Application of Newton's second law, problem with free body diagram A window washer pulls herself upward using a bucket-pulley apparatus. (a) How hard must she pull downward to raise herself slowly at constant speed? (b) If she increases the force by 15 % what will her acceleration be? The mass of the person and the bucket is 65 kg. I know that initial acceleration=0, I understand that she and the bucket will be subject to a force of downward pull mg, and that there must be one or several forces adding together in the upwards direction resulting in the same magnitude, but opposite in direction, thus cancelling out with the downward force and giving the constant velocity. So mg=637 N (choosing down as my positive direction)...but how can I find the forces acting upwards? I thought initially that she would just have to pull with the same force, but this is not true... I have tried treating it like a system similar to atwood's machine, but I am a little stumped as to how I could do this... I believe I need help with drawing my free body diagram...please HELP! Last edited by HelenaStage; Oct 20th 2009 at 12:19 PM.
 Oct 20th 2009, 07:04 PM #2 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 Maybe you should draw two free-body diagrams.One for the girl and one for the bucket and the girl together. For the first case, tension of string + normal reaction force= weight of girl For the second case, tension of string= Weight of girl+ weight of bucket __________________ Good results were achieved and the new task is to become a good doctor.

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