Academic journal article The Science Teacher

# Why Do We Have a "Law of Gravity," and How Does Sunscreen Work?

Academic journal article The Science Teacher

# Why Do We Have a "Law of Gravity," and How Does Sunscreen Work?

## Article excerpt

Question

Why do we have a "Law of Gravity" but not a "Theory of Gravity?" Is anyone working on one?

Mike Todd, MAT Student, Iowa State University, Ames, Iowa

Before tackling your question let's first get at the distinction between law and theory.

In general terms a law in science is a description of what happens or an understanding of events under similar circumstances. An example is Newton's second law, F = ma, which states that the force acting upon a body is proportional to its mass and its acceleration. Note that the law says nothing about why this is so; it gives no reason or mechanism. Likewise, Boyle's law (P[sub]1[/sub]V[sub]1[/sub] = P[sub]2[/sub]V[sub]2[/sub]) tells us that if you compress a gas sample held at constant temperature, its volume is inversely proportional to the pressure.

A theory is the current interpretation of the body of evidence that has been gathered about a specific event and represents the best explanation of how the mechanisms in the event work. Theories seek to explain to us why relations are as they are. Darwin's theory of evolution is novel not in telling us merely that different species exist in different places and times, but how this came to be: the various conditions (e.g., time, isolation) required for one species to diverge into two. Theories can shift over time, with independent verification by different individuals, refinement, and gained acceptance. Statistical mechanical theory, by examining the molecular basis of gases, gives us insight into why Boyle's plunger pushes back.

Now back to gravity. There are, in fact, both laws describing gravity and theories that attempt to explain it.

Sir Isaac Newton published the "Law of Universal Gravity" in Principia Mathematica on July 5, 1687. Likewise, Johannes Kepler created a set of famous laws describing the motion and trajectories of the planets.

Law and theory go hand in hand. For gravity, Newton's second law becomes W = mg, a law telling us that the weight of an object is its mass times the acceleration due to gravity. Newton's theory of gravitation explains why this could be by describing gravity as a force (invariably a "pull" between masses). …

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