Newton's Laws

PHYS-TIKTOK newtons laws and human body video

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Newton's 3 Laws


  1. A body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force.

  2. The acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system, and inversely proportional to its mass.

∑ F = ma

  1. Whenever one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts.

a. Assume that it is necessary that the pull of the erector spinae muscle results in a force of 55 N that will cause the back (spinal column) to rotate. Determine the tension (or force) produced by the erector spinae muscle.

b. How much force is causing compression of the spinal column? Limit your analysis to the two forces shown in the sketch.

Tik-Tok Problem 4

In the diagram, Fe represents the force of the erector spinae muscle and W represents the weight (90 lb) of the trunk of the body. There is also a line (with no arrows) that represents the axis of the spinal column. The person is bending over so that the spinal column makes an angle of 30o with the horizontal. The erector spinae muscle makes an angle of 12o with the axis of the spinal column.