Course content > Newton's Laws > Kinds of Forces
Prerequisites
From the time we were toddlers in a high chair dropping a spoon thirty times to irritate our mothers -- we learned that unsupported objects fall. In our Newtonian framework, whenever we see an object changing its velocity we look for an interaction with some other object to cause that change. When things fall it's not directly obvious that an interaction with some other object is responsible. Indeed, for thousands of years it was assumed otherwise.
Without the Newtonian framework, there are other ways of interpreting the (apparently) universal fact that unsupported objects fall. Aristotle simple said it was the natural state of all things to seek the central point of the universe -- the center of the earth. Interestingly enough, in Aristotle's view, the earth doesn't cause gravity, gravity -- the attraction of everything towards the center of the universe -- causes the earth to be where it is. For Aristotle, most of the matter of the universe fell to the center, building up to create the earth.
From a modern perspective with Newton's 2nd law, when we see a velocity change this leads us to look for an object causing causing the velocity change. So when we discover that on a round earth, objects on any side of it tend to accelerate toward the center, we can infer a force pointing to the center of the earth, So it's natural to assume that the earth is responsible for that attractive force. But if we look further we can begin to see that other objects such as the sun attract the earth and the earth might not be the center of the universe after all.
One of the difficult things for us to deal with conceptually is that ONLY the earth seems to "do" gravity. BUT Newton showed that the moon, sun, and planets also exert gravitational forces on each other. This allowed us to make sense of the solar system (see Kepler's Laws), but it still seems strange for our everyday experiences. The way Newton made sense of this is to say that EVERYTHING exerts forces of gravity on each other (see Newton's Universal Gravitation), but the force is proportional to the product of the objects' masses AND it falls off like the square of the distance between the centers of the objects. Furthermore, gravity is an extremely weak force. According to modern measurements, it's forty orders of magnitude weaker than electricity. (That's not a factor of 40, it's a factor of 1040!) So the only time we can actually feel gravity as a significant force is when at least one of the objects is of planetary size. Particle physicists, who study interactions between extremely small particles, are fond of saying "gravity is such a weak force we can always ignore it." (Except when they walk to their laboratories!) But animals on the scale of humans or larger ignore the force of gravity exerted by the earth at their peril! Indeed, in a system schema, the earth will interact with all other objects through gravity. Other objects in a system schema (such as block A and B int he system schema intro page) also exert gravitational forces on each other BUT those forces are many, many orders of magnitude smaller than the force of gravity exerted by the earth, and so we will generally ignore them in this introductory physics course. (However, see Romeo and Juliet.)
In this class we will not be doing much planetary astronomy, or studying gravity at positions very high above the surface of the earth. So in all the examples we will be working on we will be able to ignore that the gravitational force exerted by an object decreases quadratically with distance from the center of the earth. The center of the earth is 6,400 km (roughly) from the surface of the earth, so the distance from the center of the earth is almost the same whether you are sitting on a sofa or on an airplane, 10km above the surface. Almost all of the examples we consider will be ones involving distances above the surface of the earth small compared to the radius of the earth.
As a result, we will typically make the approximation that the gravitational force on an object -- its weight -- always points straight down and is independent of the object's position. This is the result we would get if the earth were flat, so we refer to this as the flat-earth gravity approximation. Making sense of gravity is challenging since it's really one of our invisible forces and it is a force that operates even when the two objects do not touch.
Follow-ons
Joe Redish 10/1/11
Wolfgang Losert 9/27/12
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