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 Oct 10th 2017, 11:57 PM #1 Junior Member   Join Date: Oct 2017 Posts: 1 Physics 100 hw help This unit is Dynamics and covers Newton's first and third law. Attached is a three part problem where we have to apply those laws to find the forces on different blocks. How wpuld i go about solving this problem if F=ma? Anything helps. Thanks Attached Thumbnails
 Oct 11th 2017, 07:24 AM #2 Senior Member   Join Date: Aug 2010 Posts: 242 First, because "F= ma" you can ignore the "speed v= 7 m/s". Force depends on acceleration, not speed. You are told that the acceleration is -3 m/s^2, negative because it is downward. If the elevator were standing still the "force" on each brick would be its weight, mg, where g= -9.81 m/s^2 the "acceleration due to gravity". While accelerating downward at -3 m/s^2, F= ma= m(9.81- 3)= m(6.81). The mass, m, is given in kg while the acceleration is given in m/s^2 so the force, ma, is given in "kg m/s^2" or "Newtons".
 Oct 11th 2017, 10:10 AM #3 Senior Member     Join Date: Aug 2008 Posts: 110 #1 is a gimme ... $F_{net}=m_3a$ See attached force diagram for the following system of scalar equations. Note the downward direction is treated as positive ... $m_3g-F_{23}=m_3a$ $m_2g+F_{32}-F_{12}=m_2a$ $m_1g+F_{21}-F_{e1} = m_1a$ Use the equations to determine the magnitude of the forces requested in #2 and #3. Attached Thumbnails

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