Newton’s First Law
“Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by a force impressed on it.”
Same as Galileo’s law of inertia.
- If a body moves with constant velocity in a straight line, then there is NO net Force acting on the body.
- If the body is moving in any other way (i.e. accelerating), then there MUST be a Force acting on the body.
Galilean Relativity revisited:
- “Rest” and “Uniform Motion” really are the same! No net force on the object
- As Galileo argued, no experiment in a steadily moving ship will show that is is moving. Only by looking outside can you detect relative motion.