Aristotle on Motion Force causes motion Natural motion and unnatural motion Natural motion—not caused by forces Smoke and boulders—up, down Planets and the heavens—circular Unnatural motion— requires force, push or pull People push or pull a cart Thought the earth did not move 4th century BC

Copernicus and the Moving Earth Changed the view of the world Earth and other planets revolve around the sun Avoided persecution by hiding his discoveries Close to death he released his ideas for print De Revolutionibus 1473-1543

Galileo on Motion Championed Copernicus’ ideas, was persecuted and put on trial Proved through experimentation force is not needed to keep an object moving Explained friction (a force that opposes motion) Every object resists change to its state of motion—Inertia! 1564-1642

Galileo on Motion Galileo showed that experiments are better than logic in testing knowledge

Speed vs Velocity Speed is how fast Speed = distance/time Velocity is how fast and in what direction (a vector quantity) Constant speed means steady speed. Something with constant speed doesn’t speed up or slow down. Constant velocity means both constant speed and constant direction. (Motion in a straight line at constant speed) Unless stated otherwise, all speeds are relative to the surface of the Earth. Motion is relative. Example: When you sit on a chair, your speed is zero relative to the Earth but 30 km/s relative to the sun.

Acceleration The rate at which the velocity is changing Applies to decreases as well as increases in speed Change may be in magnitude, direction, or both Acceleration = change of velocity/time interval

Chapter 4: Newton’s First Law of Motion--Inertia

A Vector Quantity Vector: a quantity with magnitude (size) and direction Some Examples: Displacement Velocity Acceleration Force An arrow is used to represent the magnitude and direction of a vector quantity. Length of arrow: magnitude Direction of the arrow: direction

Newton’s Law of Inertia Newton’s First Law—Every object continues in a state of rest, or of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it. Things tend to keep doing what they are already doing. 1643-1727

Examples A hockey puck on the surface of slippery ice moves a long way until ice and air friction finally stops it. If you toss an object in a vacuum of outer space (no friction) it will move forever in a straight-line path.

Mass—A Measure of Inertia Mass is the amount of material in an object, or a measure of its inertia The more mass..the greater inertia and the more force it takes to change its state of motion Mass is not volume Volume—a measure of space, cubic cm, cubic m, and liters Mass is measured in kilograms Which is more difficult to set into motion?

Mass—A Measure of Inertia Mass is not weight Weight is a measure of the gravitational force acting on an object Depends on location Mass and weight are proportional Weight on Mars 40% of that on Earth. Jump 2.5 times higher! Talk about hang time! On Mars On Earth

Mass—A Measure of Inertia One Kilogram weighs 9.8 Newtons The unit of force is the Newton—N Learn how to convert kg to N and vice versa Weight = mass x acceleration due to gravity or weight = mg Weight = mg

Net Force Net force—The combination of all forces acting on an object

50 N 30 N 20 N 15 N 25 N 45 N 300 N Net force=352

Net force—The combination of all forces acting on an object 300 190 Net force=355 200 N 200 N

760 N Fnet = 2776 N 2670 N 140 N 60 N 80 N 100 N 180 N 2670 N 2670 N 300 190 Net force=355 200 N 200 N

Pythagorean Theorem a2 + b2 = c2 Paddling = 650 N Current = 400 N Wind = 200 N

Current = 400 N 650N-400N =250N Paddling = 250 N Fnet = 320 N Wind = 200 N Pythagorean Theorem a2 + b2 = c2

Bull blast = 1050 N Horizontal Force = 325 N Gravity = 535 N

Bull blast = 1050 N Fnet = 609 N Bull blast - gravity = 515 N Horizontal Force = 325 N Gravity = 535 N

525 N 15,250 N 9,875 N

14,725 N Fnet= 17,730 N 9,875 N

Equilibrium Support force—normal force: at right angles to Equilibrium is when net force is equal to zero—the object is in a state of rest Compression

Equilibrium How about tension? Tension is shared equally among vertical supports 2 N 2 N 4 N

Vector Addition of Forces Equilibrium is when net force is equal to zero—the object is in a state of rest Tension increases as the angle of the supports increases