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= m.
| (2) |

**Note: **

- is inversely proportional to
*m*. This means that, for the same force, a smaller mass will have a larger acceleration. - Newton's second law is a vector equation which contains three scalar
equations (in three dimensions):
*F*_{x}=*ma*_{x},*F*_{y}=*ma*_{y},*F*_{z}=*ma*_{z}. - The first law is a special case of the second law.
- The SI unit of force is the
**Newton**(N).**Definition**:**1 Newton**is the force that produces an acceleration of 1 m/s^{2}when acting on a 1 kg mass. In the cgs system: 1 dyne = 1 g cm/*s*^{ 2}= 10^{- 5}N. In the British engineering system: 1 pound (lb) = 4.448 N.

**Definition**: **Weight** ( ) is the force exerted on an
object by
a gravitational field. From Newton's second law,

w = mg.
| (3) |

- Weight is a vector with direction towards the earth's center, or perpendicular to the earth's surface.
- The weight of an object is different on the earth and on the moon since
the strength of the gravitational field is different (
*g*_{earth}*g*_{moon}). - The value of
*g*varies with distance from the center of the earth (more on this in chapter 7). As a consequence:- Since the earth isn't a perfect sphere, the weight of an object varies slightly from place to place on the earth's surface.
- The weight of an object varies slightly with altitude above the earth's surface.

- In comparison,
**mass**is a scalar with a value independent of location. Notice however that, in the approximation that*g*is constant, mass is proportional to the magnitude of the weight and the two quantities can be used interchangeably. This is called the equivalence principle.

10/9/1997