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Across 3. The gravitational force between the object and the Earth 5. The rate at which something occurs over a period of time 8. An increase or decrease in the frequency of sound, light, or other waves as the source and observer move towards or away from each other 11. A course along which someone or something moves 12. The rate or a measure of the rate of motion 14. The opposite of acceleration 15. Amplitude is the objective measurement of the degree of change in atmospheric pressure 17. Reduction in the density of something, especially air or a gas 18. The SI unit of force 19. A volume of space that is empty of matter

Down 1. Interrelation of electric currents or fields and magnetic fields 2. The rate at which an object changes its speed or velocity 4. Refers to the mutual attraction of objects with masses 6. The rate something changes position 7. The process by which the working substance in a heat engine 9. Strength or energy as an attribute of physical action or movement 10. A geographic coordinate which specifies the east-west position of a point on the earth's surface 13. The distance between successive crests of a wave 16. The tendency of objects to keep moving in a straight line at constant linear velocity