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Since the particle has size, it takes time for the force to propagate at the speed of light. A similar analogy may be used to demonstrate the principle. A model car, 1/2 light second in length, has a controller module in the back and a motor connected to drive wheels in the front. The motor is designed to receive a velocity command and issue an "acknowledge". The controller will issue a new velocity command after it has received each "acknowledge" from the motor. Each new velocity command is interpreted by the motor to be an instruction to increase velocity by the amount received. In this way a constant velocity value transmitted by the controller results in a steady increase in the car's velocity (i.e. it accelerates). It is clear that the car's length will directly determine its acceleration. In a similar way, the resistance matter has to the application of a force is caused by the time required to convey the information throughout the particle. Obviously, the description becomes more complicated with atoms as there is great distance between the nucleons and the electrons and there is elastic interaction between them.