12B crossword puzzle

1. Newton's 2nd law of motion.
3. without this, rocketry would be impossible, as would all other kinds of motion.
4. a pulling force that is transmitted through a rope, chain, or similar object.
10. an object in motion continues in that motion unless acted upon by an external force.
13. he called the tendency of matter to resist change in its motion inertia.
14. concept that was "invented" by physicists as a measure of inertia.
15. Newton's 1st law of motion.
16. merged Galileo's principle of inertia into a more general statement of physics that included his own discoveries about the actions of forces.
17. letter used in Newton's 2nd law equation to represent the net external force on an object, in newtons.
18. the tendency of matter to resist change in motion.
19. first person to have tested ideas about motion.

1. Newton's 3rd law of motion.
2. required to act on any change in an objects motion.
5. force that acts in a direction perpendicular to the surfaces where two objects make contact.
6. for every action force, there is an equal and opposite reaction force.
7. a title often given to Newton's third law of motion.
8. the acceleration of an object is directly proportional to the net force and is inversely proportional to the object's mass.
9. forces must be unbalanced.
11. an object at rest remains at rest.
12. wrote book mathematical principles of natural philosophy, principia.