1. Newton’s 1st Law of Motion
Physics 4.2
Newton’s 1st Law of Motion

2. I. Inertia
Newton’s 1st Law – an object at rest remains at rest; an object in motion stays in motion unless another force acts upon it.
1. aka – inertia
ex: book and paper in hand…
2. net force – sum of all vector forces acting on an object.

3. 3. Ex: Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. The free body diagram below shows the forces acting on the book. Find the net external force acting on the book and determine if the book will remain at rest.
Ftable on book = 18 N
Ffriction = 11 N
Fgravity on book = 22 N
35°

4. Select the coordinate system that applies to the free body diagram.
Find x and y components of all vectors.
Find Net force
Evaluate answer
Ftable on book = 18 N
Ffriction = 11 N
Answer: Fnet = 2 N
Fgravity on book = 22 N
35°
2 N

5. 4.2 Pop Quiz Question
A crate is pulled to the right with a force of 82.0 N, to the left with a force of 115 N, upward with a force of 565 N, and a downward force of 236 N.
Find the net external force in the x direction
Find the net external force in the y direction
Find the magnitude and direction of the net external force on the crate.
a. -33 N b N c x 102 N at 96° counterclockwise from the positive x-axis

6. Mass is a measurement of inertia…
Jumbo Jet 2 mph… or
ping pong ball 1000 mph
Objects in motion…tend to stay in motion
Newton’s Beads
Crash Test Dummies
Crash Tests
Equilibrium
A. Net force must equal zero!