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All of us have many times had the experience of setting a body in motion. If we analyze any of these experiences, we readily recall that in each case some force was required to start the object moving. In throwing a ball, moving a piece of furniture, or pulling a sled, the force needed to start the object moving is supplied by one's muscular effort as a push or a pull. In more complex cases, such as setting a car or an airplane in motion, the analysis, although more complicated, will also show that a force is required to start the body moving.
There are many cases in which the force that acts on the body to produce the motion is not directly discernible. It was Newton who first showed that the acceleration of a freely falling body is produced by a force which acts between the earth and the body, called the force of gravitation. We shall encounter other such action-at-a-distance forces in electricity and magnetism, and in molecular and atomic physics.
Once a body has been set in motion by the action of a force, it will not necessarily stop moving when the force is removed. A sled in motion along a level road will continue to move in a straight line along the road, although with diminishing speed. The reduction in speed is due to the force of friction between the runners of the sled and the ground. If there is clean snow on the ground, the force of friction will be very small; if ashes or sand have been dumped on the snow, the force of friction will be greater, and the sled will come to rest much sooner.