經典力學導論（簡體書）

This book grew out of Harvard Univers it's honors freshman mechanics course. It is essentially two books in one. Roughly half of each chapter follows the form of a normal textbook， consisting of text， along with exercises suitable for homework assignments. The other half takes the form of a "problem book，" with all sorts of problems （and solutions） of varying degrees of difficulty. I've always thought
that doing problems is the best way to learn， so if you've been searching for a supply to puzzle over， I think this will keep you busy for a while.

Preface
1 Strategies for solving problems
1.1 General strategies
1.2 Units，dimensional analysis
1.3 Approximations，limiting cases
1.4 Solving differential equations numerically
1.5 Problems
1.6 Exercises
1.7 Solutions
2 Statics
2.1 Balancing forces
2.2 Balancing torques
2.3 Problems
2.4 Exercises
2.5 Solutions
3 UsingF=ma
3.1 Newton’S laws
3.2 Free—body diagrams
3.3 Solving differential equations
3.4 Proiectile motion
3.5 Motion in a plane，polar coordinates
3.6 Problems
3.7 Exercises
3.8 Solutions
4 0scillations
4.1 Linear differential equations
4.2 Simple harmonic motion
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Tension
Tension is the general name for a force that a rope， stick， etc.， exerts when it is pulled on. Every piece of the rope feels a tension force in both directions， except the end points， which feel a tension on one side and a force on the other side from whatever object is attached to the end. In some cases， the tension may vary along the rope. The "Rope wrapped around a pole" example at the end of this section is a good illustration of this.　In other cases， the tension must be the same everywhere. For example， in a hanging massless rope， or in a massless rope hanging over a frictionless pulley， the tension must be the same at all points， because otherwise there would be a net force on at least some part of the rope， and then F = ma would yield an infinite acceleration for this （massless） piece.
Normal force
This is the force perpendicular to a surface that the surface applies to an object.
The total force applied by a surface is usually a combination ofthe normal force and the friction force （see below）. But for frictionless surfaces such as greasy ones or ice， only the normal force exists. The n'ormal force comes about because the surface actually compresses a tiny bit and acts like a very rigid spring. The surface gets squashed until the restoring force equals the force necessary to keep the object from squashing in any more.
For the most part， the only difference between a "tension" and a "normal force" is the direction of the force. Both situations can be modeled by a spring.
In the case of a tension， the spring （a rope， a stick， or whatever） is stretched， and the force on the given object is directed toward the spring. In the case of a normal force， the spring is compressed， and the force on the given object is directed away from the spring， Things like sticks can provide both normal forces and tensions. But a rope， for example， has a hard time providing a normal force.
In practice， in the case of elongated objects such as sticks， a compressive force is usually called a "compressive tension，" or a "negative tension，" instead of a normal force. So by these definitions， a tension can point either way. At any rate， it's just semantics. If you use any of these descriptions for a compressed stick， people will know what you mean.
Friction
Friction is the force parallel to a surface that a surface applies to an object.Some surfaces， such as sandpaper， have a great deal of friction. Some， such as greasy ones， have essentially no friction. There are two types of friction， called "kinetic" friction and "static" friction.
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