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· To visualize force as a vector
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· To find the net force acting on a body and apply Newton's First Law
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· To study mass, acceleration, and their application to Newton's Second Law |
· To calculate weight and compare/contrast it with mass |
· To see action–reaction pairs and study Newton’s Third Law
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Newtonian Mechanics (牛顿力学):
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Newton’s First Law: |
If there is no net force on a body, the body must remain at rest if it is initially at rest, or move in a straight line at constant speed if it is in motion.? For such a body, there are reference frames, called inertial frames (惯性系), from which the body’s acceleration a will be measured as being zero.? |
Newton’s Second Law:  |
 is the net force (净力) or resultant force (合力).
A free-body diagram is helpful in solving problems with the second law: only one body is considered in the diagram.? That body is represented by a dot.? The external forces on the body are drawn as vectors, and a coordinate system is superimposed, oriented so as to simplify the solution.
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define momentum (动量) General form: 
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We refer to the vector sum or resultant as the “sum of forces”? |
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Action force (作用力) and reaction force (反作用力) are equal in magnitude and opposite in direction.? They act on different bodies.? |
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Some particular types of forces |
· The normal force (压力) —When an object rests or pushes on a surface, the surface pushes back. The normal force is always perpendicular to the surface. |
· Frictional force (摩擦力) —the force on a body when the body slides or attempts to slide along a surface. The force is parallel to the surface and directed so as to oppose the motion of the body. A frictionless surface (无摩擦表面，光滑表面) is one where the frictional force is negligible.
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· Tension forces (张力，拉力) —When a force is exerted through a rope or cable, the force is transmitted through that rope or cable as a tension. The force points along the rope and away from the body.
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· Weight (重力) —Gravity’s pull on an object. This force can act from large distances. |
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What are typical sizes for common forces?—Table 4.1 |
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Mass and weight |
Mass characterize the inertial properties of a body. It is reflected in the Newton’s second law.
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Weight is a force exerted on a body by the pull of the Earth. On Earth, g depends on altitude (纬度) and height above the sea level.
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On other planets, gravity will likely have an entirely new value. |
Usually, a spring (弹簧) measures the weight of an object and a balance (天平) determines the mass.
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