4.1 Force -Force is any kind of push or pull on an object. -When you push something, you exert force on it. -Force of Gravity -Object at rest, requires force to start moving -Change in velocity, magnitude, direction, or acceleration requires force. -Spring scale can be used to measure force -force exerted in different direction has a different effect -force is a vector, it had direction and magnitude

4.2 Newton’s First Law of Motion -Aristotle believed that force was need to keep an object moving horizontally -natural state was at rest, force to keep object moving -greater force, greater speed
-Galileo disagreed; Natural for an object to be in motion with a constant velocity just as in rest.
- rough surface, requires force/ smooth surface, requires less force (puck on air hockey table, air reduces friction)
-if no force is applied to a moving object, it will continue to move at a constant speed in a straight line forever. Friction is just push and pull
-forces in opposite direction, net force (vector sum of two forces), equal forces, net force is 0.
-Newton’s first law: Every object continues in its state of rest, or uniform velocity in a straight line, as long as no net force acts on it. -Law of Inertia
-Inertial Reference Frames
-Reference frame, example 4.1, there was no force pushing on the backpacks forward
- in accelerating reference frames, the law of inertia doesn’t apply (noninertial)
-Inertial Reference Frames, the law is valid