Quantifying Force

[Bernard NebelKeymaster]

 Hold a pound weight in your hand. Its push down is a force of one pound. Now, that same amount of force can be exerted in any direction. Thus, a pound is more than a certain weight; it can be taken as a unit of force. Unfortunately, weight is dependent on gravity and gravity differs with location. It becomes less at higher elevations and especially as one goes into space. Therefore, physicists needed a more precise unit of measurement for force.

Have kids reflect on an experience such as pushing someone on a bicycle on a smooth, level surface. As they pushed with a certain force the person was accelerated, i.e., went faster and faster (up to the pushers ability to keep pushing with the same force).

Have kids reflect further: Pushing with the same force, a light person can be accelerated (gotten up to speed) more quickly than a heavy person. Or, the heavy person has to be pushed harder to accomplish the same acceleration. Thus, there is a relationship between force, mass, and acceleration: More force on the same mass will cause greater acceleration; the same force on greater mass will cause lesser acceleration, and so on. (force = mass x acceleration; mass = force/acceleration; acceleration = force/mass)  If desired, here is a sheet of problems for kids to solve: http://jh.gatesvilleisd.org/ourpages/auto/2015/3/10/54832281/wkst-newtons_second_law_problems.pdf

Physicists decided to determine the exact force needed to accelerate a one kilogram mass at a rate of one meter per second per second (neglecting friction) and call that force one Newton. (1N = 1kg x 1m/s²) On earth at sea level: the force of one pound equals about 4.5 Newtons. Or, one Newton equals  0.225 pounds. (For any such conversion, type into your browser: Newton pound conversion) (The “meters per second per second” may be confusing. Note that meters per second would be a constant velocity. The second per second is to say that that the object gains that velocity each second, i.e., accelerates.

Again, whether or not and how much an object accelerates can be calculated based on its mass (kg) and Newtons (forces) applied (including friction locking it in place).

Have kids reflect again that there is no energy expressed or implied in such calculations. To be sure, a person may exert and expend a great amount of physical energy in pushing on something. But that energy is a separate issue from the pounds or Newtons of force applied. If the object doesn’t move all of that energy simply goes off as waste heat and sound (grunting and groaning). As the the object moves some of that energy is transferred into the object’s movement (movement energy). Still, that energy is a separate issue form the forces applied. The following video may help:

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