Engineering Expert Witness Blog

      Have you ever wondered how Earth keeps its steady orbit around its life sustaining sun, or what prevents it from breaking away and flying off willy-nilly into the universe?   It’s more than just simple gravity, it’s the physics behind centripetal force, the topic we’ll be exploring today.

      We’ve been working our way towards a full discussion on gravity in this long blog series, navigating subjects such as the behavior of falling objects, the acceleration of gravity, the masses of Earth and the sun, and the optical measurement of cosmic distances.   We’ve now come full circle from my opening blog on the subject, Gravity and the Mass of the Sun.

      In that blog an equation was introduced as a means to calculate the mass of the sun, and in that equation is the variable we’ll be working towards solving today, F_g, the sun’s gravitational force upon the Earth.   Here again is that equation,

M = (F_g × r²) ÷ (m × G)

      Gravity, mass, and distance all come into play in forming the structure of our universe, and the variables in this equation reflect that:  M, the mass of the sun, r the distance between Earth and the sun, m the Earth’s mass, and G the universal gravitational constant.   With the exception of F_g, all variables in this equation have already been solved for in previous blogs in this series.   For a refresher go to, Calculating the Distance to the Sun, What is Earth’s Mass? and Newton’s Law of Gravitation and the Universal Gravitational Constant.

      As there is no direct means to measure the cosmic quantity, F_g, we’re left to an indirect method for its computation.   The indirect method is based on the phenomenon of centripetal force, F_c something most children become acquainted with when they experience the thrill of twirling an object attached to a string, say a rubber ball, above their heads.   See Figure 1.

Figure 1

      As the ball is twirled, the string becomes taut.   The energy exerted upon it by the child’s hand, coupled with the ball’s mass and traveling speed/velocity, v, make the ball want to move off in a straight trajectory into space, like a launched projectile.   But the string it’s attached to prevents it from doing so, forcing the ball to instead travel a circular path around the center point of rotation.   The taut string and the ball’s circular path are evidence of centripetal force, F_c, at work.

      Next time we’ll employ Isaac Newton’s Second Law of Motion to the centripetal force phenomenon to see how the sun’s gravitational force keeps Earth in a stable circular orbit around the sun.

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