Physics 4.2Newton’s 1st Law of Motion

I. Inertia

A. 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 – inertiaex: book and paper in hand…

2. net force – sum of all vector forces acting on an object.

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°

1. Select the coordinate system that applies to the free body diagram.

2. Find x and y components of all vectors.3. Find Net force4. Evaluate answer

Ftable on book = 18 N

Ffriction = 11 N

Fgravity on book = 22 N35°

Answer: Fnet = 2 N

2 N

4.2 Pop Quiz QuestionA 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. a. Find the net external force in the x

directionb. Find the net external force in the y

directionc. Find the magnitude and direction of the

net external force on the crate.a. -33 N b. 329 N c. 3.3 x 102 N at 96° counterclockwise from the positive x-axis

4. Mass is a measurement of inertia…Jumbo Jet 2 mph… orping pong ball 1000 mph

5. Objects in motion…tend to stay in motion

Newton’s BeadsCrash Test DummiesCrash Tests

II. EquilibriumA. Net force must equal zero!