Forces of Nature Weblecture
Faraday begins with an apology, since he had been sick and the initial lecture was postponed, but then he turns to his purpose in holding all of his lectures: to instill a sense of wonder in his audience at the workings of nature. What he means here by "wonder" is both the feeling of awe and amazement at how nature works, but also "wondering about" how it works. He wants the members of his audience, both young and old, to learn to look at the world around them with deliberate inquiry, and to begin to investigate natural phenomena, starting with the force of gravity and working through a series of demonstrations:
Many scientists are very defensive of their position and methods for looking at the universe (although even within the scientific community, there is a disagreement about the scope of science and what these methods entail), and tend to oversimplify the relationship between science and other ways of looking at the univers, including religion.
Often the defense a science takes the form of presenting historical events with a particular agenda. This is especially true when scientists and historians discuss the position of Galileo in the church in the 17th century. Here are a couple of arguments to examine closely when reading histories of science:
This is not meant to be an exhaustive history of the reception of the Copernican heliocentric, but only a small attempt to point out that science and religion (which often means religious institutions) in the 16th and 17th centuries had a complex relationship. One cannot infer from a single event (the condemnation of Galileo) that all Europeans agreed with the Roman Curia in its actions, or that even all members and officials of the Catholic Church were in sympathy with the condemnation (they were not). Despite Galileo's condemnation, the 17th century saw many scientific achievements, including Harvey's discovery of the closed circulator system (which depended on Galileo's work in hydraulic mechanics), Hooke's discovery of the cell (which depended on an "inverted" telescope — the microscope), Nicholas Steno's work on sedimentation (which would lead to far more controversy on the age of the Earth), and Roemer's measurement of the speed of light, using observations of the four moons of Jupiter that Galileo discovered.
Physicists use two kinds of values to talk about certain concepts: scalars and vectors. Scalar values, like distance and speed, have magnitude. Vector values have magnitude and direction.
Distance is a measure of space. Displacement is a measure of position. If in one hour you walk northwards for one mile, eastwards for one mile, and southwards for one mile, you have covered a distance of three miles, but your displacement is only one mile (eastward) of your origin. Your speed is distance/time or 3 miles/1 hour, but your velocity is displacement/hour, or 1 mile/hour.
Acceleration is change in velocity/time, so it is always a vector, with direction. This can be in the same direction as velocity, in which case your velocity increases. If acceleration is in the direction opposite to velocity, then the change in velocity is negative: it decreases. (Some people might call this deceleration).
We can talk about average velocity or average acceleration, the change in displacement over time, or the change in velocity over time. Another key idea is instantaneous velocity: the velocity (speed and direction) that an object has at a given moment. Newton invented his methods of calculus invented largely to help determine instantaneous velocity for situations where velocity is constantly changing or undergoing acceleration.
Aristotle, Galileo, and Newton all defined the concept of inertia, or the tendency of a body to remain in a particular state of motion or change its motion.
Another way to express this idea is to say that matter resists changes to its state of motion.
The corollary to the idea that a body requires an outside force to change its state of motion and accelerate is the realization that where change in velocity or acceleration is observed, a net force must be present. Further, the amount of change depends directly on the force and inversely on the mass involved:
Δv/Δt = a = F/m.
This doesn't mean that there are no forces present where we observe lack of motion (a stationary object), or lack of change in motion (constant velocity). Rather, in those cases, the sum of all the forces acting on the object cancel each other out, and the object in in static equilibrium. The art and science of building structures is a study in the proper alignment of forces to achieve static equilibrium.
Newton's third law recognizes that all forces are interactions between two masses. The Earth's gravitational field pulls down a book lying on a table; the table pushes back on the book with equal but opposing force. If the forces were not equal and opposite, there would be a net force and the book would not only be moving but accelerating. The two forces acting on the book are equal and opposite, but arise from different sources.
In the example of the book, the book is not free to move through the table: the equal and opposite forces result in a static situation. What happens when one object is free to move, for example, when you stand in tennies on the grass, and push (throw) a ball away from you? What would happen if instead you were on ice?
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