3Newton’s First Law of Motion Law of InertiaInertia 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.
4Egg Drop ExperimentWhat will happen to the egg when the pie tin is hit?Why would it do that?
5Newton’s Second Law of Motion Force = mass x accelerationF=maForce 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
6Ball Bearing RaceWhich ball moved farther?Why did it move farther?
7Newton’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!
8Third Law Examples Space Shuttle Launches Jumping, Swimming, Running GravitySwings
9TYPES OF FORCES Fg = Gravity! Fa = Applied Force Ff = Friction FN = Normal ForceFt = Tension ForceFs = Force of a springFair= Air ResistanceFnet = The sum of all forces acting on an object.
10About Forces All forces: Obey the three laws Are measured in Newtons (N)1 Newton = 1 kg * m/s2Note: mass is in kilogramsHave a magnitude and directionThey’re vectors!Are represented with arrows.
11Force of Gravity Always present in a downward direction. Also known as WeightCan be calculated using Newton’s Second Law: F=maFg=m*ag (ag=9.8m/s2)
12Gravity Example Your cat has a mass of 1.6 kg. What is it’s weight? Fg = maFg = 1.6kg * 9.8m/s2Fg = kg*m/s2 = N
13Normal Force Always perpendicular to the applied surface. Equal to the total force applied to that surface
14Normal Force Calculation You headbutt someone with an applied force of 10 N. What is the normal force applied to your head?FN = 10N
15Tension 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 FgFt has an upper limit, if that is exceeded, the cable/string will snap!
16Tension Force Calculation A 4000N block is hanging from a crane. What is the Tension Force applied by the cable?Ft = 4000N
17Friction 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 objectStatic Friction is larger than Kinetic Friction
18Calculating Friction Ff=μFN μ – (pronounced “Mew”) - Coefficient of FrictionDoesn’t have unitsValue is different for different surfacesRough surfaces have a higher μ
19Friction CalculationThe coefficient of friction for a object sliding on a horizontal concrete floor is .62, if the object weighs 20N, what is Ff?Ff=μFNFf= .62 * 20NFf= 12.4N
20Air ResistanceWhen an object moves through air, the air pushes back on the object.Acts in the direction opposite the objects motionFair is normally very small, so we can ignore it, except when an object is falling.
21Free 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 objectRemember that if the object isn’t accelerating, Fnet=0.
22Free Body Diagram Practice A book is sitting on a desk
23Free Body Diagram Practice A book is being pushed along a desk.
24Free Body Diagram Practice A ball is hanging from a string
25Free Body Diagram Practice The same ball, but the string has been cut.