FORCES Write 5 examples of an object accelerating In each example, what causes the object to accelerate?

FORCES Newton’s Three Laws

Newton’s First Law of Motion Law of Inertia Inertia is a property of matter. Inertia is an object’s tendency to resist a change in motion. The law states: An object at rest will stay at rest unless a net force acts upon it. An object in motion will stay in motion unless a net force acts upon it.

Egg Drop Experiment What will happen to the egg when the pie tin is hit? Why would it do that?

Newton’s Second Law of Motion Force = mass x acceleration F=ma Force and acceleration are directly proportional. More Force means more acceleration. Mass and acceleration are inversely related. With the same amount of force, an object with greater mass will have less acceleration

Ball Bearing Race Which ball moved farther? Why did it move farther?

Newton’s Third Law of Motion States that: For every force there is an equal and opposite reaction force. When you push against something, it pushes back!

Third Law Examples Space Shuttle Launches Jumping, Swimming, Running Gravity Swings

TYPES OF FORCES Fg = Gravity! Fa = Applied Force Ff = Friction FN = Normal Force Ft = Tension Force Fs = Force of a spring Fair= Air Resistance Fnet = The sum of all forces acting on an object.

About Forces All forces: Obey the three laws Are measured in Newtons (N) 1 Newton = 1 kg * m/s2 Note: mass is in kilograms Have a magnitude and direction They’re vectors! Are represented with arrows.

Force of Gravity Always present in a downward direction. Also known as Weight Can be calculated using Newton’s Second Law: F=ma Fg=m*ag (ag=9.8m/s2)

Gravity Example Your cat has a mass of 1.6 kg. What is it’s weight? Fg = ma Fg = 1.6kg * 9.8m/s2 Fg = 15.68 kg*m/s2 = 15.68 N

Normal Force Always perpendicular to the applied surface. Equal to the total force applied to that surface

Normal Force Calculation You headbutt someone with an applied force of 10 N. What is the normal force applied to your head? FN = 10N

Tension Force Always in the direction of an object being pulled. Used with ropes, strings, cables, etc. Can either be used when pulling or hanging an object. When hanging, Ft is equal to Fg Ft has an upper limit, if that is exceeded, the cable/string will snap!

Tension Force Calculation A 4000N block is hanging from a crane. What is the Tension Force applied by the cable? Ft = 4000N

Friction Always acts in the opposite direction of the intended motion. Friction must be overcome for an object to accelerate across a surface. Two types of Friction: Static Friction – opposes the start of motion (before an object is moving) Kinetic Friction – opposes the current motion of an object Static Friction is larger than Kinetic Friction

Calculating Friction Ff=μFN μ – (pronounced “Mew”) - Coefficient of Friction Doesn’t have units Value is different for different surfaces Rough surfaces have a higher μ

Friction Calculation The coefficient of friction for a object sliding on a horizontal concrete floor is .62, if the object weighs 20N, what is Ff? Ff=μFN Ff= .62 * 20N Ff= 12.4N

Air Resistance When an object moves through air, the air pushes back on the object. Acts in the direction opposite the objects motion Fair is normally very small, so we can ignore it, except when an object is falling.

Free Body Diagrams Used to show the forces acting on a body. Use a box (or dot) to represent an object. Draw the forces that effect the object Remember that if the object isn’t accelerating, Fnet=0.

Free Body Diagram Practice A book is sitting on a desk

Free Body Diagram Practice A book is being pushed along a desk.

Free Body Diagram Practice A ball is hanging from a string

Free Body Diagram Practice The same ball, but the string has been cut.