Force

Chapter 5

Aristotle and Galileo

Aristotle

-all objects require a continual force to keep moving (a rolling ball slows down over time)

Galileo

-Realizes the existence of friction

-In space (no friction) a moving object continues forever.

Isaac Newton (1642 to 1727)

• born the year Galileo died, 1642.

• Invented calculus (while living in seclusion to avoid the plague)

• Three laws of motion• Principia

Mathematica

http://en.wikipedia.org/wiki/Image:GodfreyKneller-IsaacNewton-1689.jpg

Newton’s Three Law’s of Motion

1. All objects remain at rest or in uniform, straight-line motion unless acted upon by an outside force. (inertia)

2. Force = mass X acceleration

3. Every action (force) has an equal and opposite reaction (force).

The First Law

• All objects remain at rest or in uniform, straight-line motion unless acted upon by an outside force. (inertia) – Inertia –tendency of an object to remain at rest or in

constant motion.– mass - measure of inertia.– Mass and inertia are directly proportional

The First Law

Ball in jar example:

Does it take less force to push the elephant (ignore friction) on earth or on the moon?

Does it take less force to move the elephant if he is “weightless” in space?

Inertial Reference Frames

• Non-accelerating (constant velocity) reference frame

• All laws of physics are identical in all Inertial Reference Frames

• Cannot tell if you are moving in an Inertial Reference Frame

Free Body Diagrams

• Only show forces NOT resolved components

Draw a free body diagram for a box sitting on a table.

Draw a free body diagram for a box sitting on a ramp

Draw a free body diagram for a box being pushed up a ramp

A hockey puck slides at constant velocity across the ice. Which of the following is the correct free-body diagram?

Free Body Diagrams: Ex. 2