Base motion

In the sermon Chain Reactions Pastor Steven Furtick of Elevation Church, Charlotte, NC, described the importance of perspective. He showed how our perspective (state of mind) is a product of our connections (how we relate our surroundings to our perceptions).

Pastor Furtick used the experiences of Paul and Silas in Philipi to show how a proper perspective enables us to be used by God. As Paul and Silas preached the good news of Jesus, they were stripped, beaten, and thrown in prison. However, Acts 16:22 tells us what happened about midnight of the same day: “Suddenly there was such a violent earthquake that the foundations of the prison were shaken. At once all the prison doors flew open, and everyone’s chains came loose.” God released them from their chains by shaking the foundation beneath them.

This sermon inspired the engineer in me. There is a direct analogy between this foundation-shaking and the concept of  base motion. In mechanical vibrations, base motion occurs when a mass (m) is excited through an elastic support (spring, k, and damper, c) by the base displacement. An example is the motion of an automobile’s chassis in response to a wavy road surface. The figure shows base motion, y (the road surface), and the mass motion, x (the chassis). These are related through the displacement transmissibility equation, which is the ratio of x to y.


When the displacement transmissibility is low, the base motion (y) is only weakly transmitted to the mass (x). When it is high, the mass motion is similar to the base displacement. Consider the difference between a Cadillac and a Porsche. Their suspensions are tuned very differently. The Cadillac is designed for a smooth ride, so it has low transmissibility — you don’t feel the road. The Porsche suspension, on the other hand, offers high transmissibility and “tighter handling” – you directly feel the road.

Back to the sermon… if we (we’re the x in the figure) want to be affected by God’s foundation shaking (this is the y), if we want to be released from our chains by God’s earthquake, we need to be tightly connected to Him. We require high displacement transmissibility (like the Porsche). We increase our transmissibility when we ask God to bless our perspective, when we connect to Him through His word. As Pastor Steven said, “Being blessed is a state of mind, not a state of affairs.”



About Tony Schmitz

Tony Schmitz received his BS in Mechanical Engineering from Temple University in 1993, his MS in Mechanical Engineering from the University of Florida in 1996, and his PhD in Mechanical Engineering from the University of Florida in 1999. He is a mechanical engineering professor at the University of North Carolina at Charlotte.
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