P H Y S I C S   

Chapter 3: Newton’s Laws of Motion

Section 3A: Forces and Free-Body Diagrams

Force

Action that changes an object’s state of motion

Slow down Speed up acceleration Change direction

Two Types

1) Contact Force: through contact push/pull

2) Field Force: at a distance gravity/electromagnetism

Free Body Diagram

Shows all forces (vector) acting on an object Unit for force = N (Newton)

FN = 500 N

Fg = 500 N

Ff = 200 N Fp = 1000 N

Newton

The amount of force it takes to accelerate a 1kg mass at 1 m/s2

1N = 1kg*m/s2

Common Forces Fg = weight downward

FN = normal perpendicular to surface

Ff = friction (includes air) opposite to motion

Fp = push/pull

FT = tension in stringFN

FgFRICTION

FRICTION

Net Force

Vector sum of all forces on an object

ΣF = (summation) net force

x and y components:

F(x) = F·cos θ

F(y) = F·sin θ

Fp

Fp(x)

Fp(y)

θ

In-Class Problem #1

A 9 N book sits at rest on a table.

ΣF = 0 N

In-Class Problem #2

A sky diver that weighs 600 N falls towards the earth and experiences a wind resistance of 250 N.

ΣF = 350 N