A Acceleration If an object held near the surface of the earth is released, it will fall and accelerate, or pick up speed, as it descends. This acceleration is caused by gravity, the force of attraction between the object and the earth. The force of gravity on an object is also called the object's weight. This force depends on the object's mass, or the amount of matter in the object. The weight of an object is equal to the mass of the object multiplied by the acceleration due to gravity.
A bowling ball that weighs 16 lb is actually being pulled toward the earth with a force of 16 lb. In the metric system, the bowling ball is pulled toward the earth with a force of 71 newtons (a metric unit of force abbreviated N). The bowling ball also pulls on the earth with a force of 16 lb (71 N), but the earth is so massive that it does not move appreciably. In order to hold the bowling ball up and keep it from falling, a person must exert an upward force of 16 lb (71 N) on the ball. This upward force acts to oppose the 16 lb (71 N) downward weight force, leaving a total force of zero. The total force on an object determines the object's acceleration.
If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb (71 N) bowling ball and a 500 lb (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m/sec (32 ft/sec), resulting in an acceleration of 9.8 m/sec/sec (32 ft/sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air exerts a significant upward force on the falling feather and makes it fall more slowly.
The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the object's weight, will change because the strength of the earth's gravitational pull is not the same everywhere. The earth's pull and the acceleration due to gravity decrease as an object moves farther away from the center of the earth. At an altitude of 4000 miles (6400 km) above the earth's surface, for instance, the bowling ball would weigh only about 4 lb (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of grav