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Old Oct 10th 2017, 10:57 PM   #1
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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
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Old Oct 11th 2017, 06:24 AM   #2
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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".
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Old Oct 11th 2017, 09:10 AM   #3
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#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 ...



$m_1g+F_{21}-F_{e1} = m_1a$

Use the equations to determine the magnitude of the forces requested in #2 and #3.
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