# qod what is force? what is force?. forces & the laws of motion physics i mr. padilla

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• QODWhat is force?

• Forces & the Laws of MotionPhysics IMr. Padilla

• Aristotle (~400 B.C.)Divided motion into 2 typesNaturalViolentNatural Straight up or straight down.Objects seek their natural resting placeClouds UpRocks DownHeavens Perfect Circles around EarthViolent imposed motionA result of forces that pushed or pulled. Had an external cause.These ideas held for about 2000 years.

• Copernicus (1473-1534)Nicholas Copernicus was the first to reason that the Earth & planets moved around the sun.Heavily persecuted for his ideas and worked in secret to hide them.His work was published for the first time on the day he died.

• GalileoBelieved Copernicus and so was placed under house arrestForce any push or pullFriction any force that acts between materials as they move past each otherOnly when friction is present is a force needed to keep an object in motionDiscredited Aristotles ideas of motion

• Changes in MotionForces can act through contact or at a distanceContact forces result from the physical contact between 2 objects.Ex: Jordan pushes KevinField forces does not involve physical contact.Ex: Gravity, electric fields Field forces were developed to explain how an object can exert force on another without touching.

• NewtonNewtons 1st Law of MotionEvery 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.An object in motion stays in motion, an object at rest stays at rest.AKA law of inertia

• Inertia in ActionInertia: the reluctance of any body to a change in the state of motion.Book on a tableAir hockey tableTable cloth & dishesWhere would they go?Ball thrown in outer spaceCar without a driving forcePlanets without the suns gravity

• Mass vs. WeightThe inertia of an object depends on its massHow much material is in the objectThe more mass = more inertiaMass is a measure of the inertia of an object.Mass is not weight.Weight force of gravity of an objectWeight can change in different environments: mass, and therefore inertia, remain constant.Mass & Weight are proportional.

• WeightWeight can be calculated by taking an objects mass times gravityW = mgThe SI unit for weight is kg (x) m/s2 = kgm/s2 or Newtons (N)A mass of 1kg has a weight of 9.8N, or 2.2lbs

• Mass ExampleFelicia, the ballet dancer, has a mass of 45.0 kg. a) What is Felicias weight on Earth? b) What is Felicias mass on Jupiter, where acceleration due to gravity is 25.0 m/s2? c) What is Felicias weight on Jupiter?

• Net forceForce is a vector quantity.Has magnitude and direction.Net force When considering Newtons 1st Law, you must think of net forces not just forces.Net Force combination of all forces acting on an object.All individual forces can be added and treated as a single (net) force

• FBDA free body diagram can be drawn to represent the forces acting on an object.A FBD shows vectors for forces acting on a single object

• EquilibriumWhat forces act on a book at rest on a table?At least 2 weight and a support (normal) forceIf the net force acting on an object is zero, it is in equilibrium.No change in state of motion.Normal force always acts at a right angle to the surface.

• QODDetermine your weight, in Newtons, on the other planets in our solar system.Mercury 3.59 m/s2Venus 8.87 m/s2Mars 3.77 m/s2Jupiter 25.0 m/s2Saturn 11.08 m/s2Uranus 10.67 m/s2Neptune 14.07 m/s2

• AccelerationSo far: a = V/tThis is the definition of aThe cause of acceleration is force.Applying a force (or net force) to an object causes it to accelerateMore force causes more acceleration (direct)Mass resists the accelerationmore mass causes less acceleration (inversely)

• 2nd LawAcceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.Ora ~ (net force)/massa = F/mBetter yet Fnet = ma

• ExamplesA force of 30.N is applied to a 12kg mass, how fast does it accelerate?How much force is needed to accelerate a 700.kg vehicle at a rate of 4.0m/s2?F = ma30.N = 12kg(a)a = 30N/12kga = 2.5 m/s2F = maF = (700.kg)(4.0m/s2)F = 2800N

• More Great ExamplesBen and Sam push on opposite sides of Chris. Ben pushes with 400.N of force, Sam with 200.N. What is the net force on Chris?If Chris has a mass of 75kg, what will his acceleration be?A constant force exerted on an object, will result in what kind of motion?Fnet = F acting on ChrisFnet = 400.N 200.NFnet = 200.N

F = ma200N = (75kg)aa = 200N/75kga = 2.7 m/s2Constant force produces constant acceleration.

• Free FallinIn a vacuum all things fall at the same rate due to gravity. In reality, air resistance provides friction and causes some things to slow down. All objects reach a point where the force of gravity pulling them down is equal to the force of air resistance pushing back on them.This is called terminal velocity.Humans = 150-200 km/h in free fall15-20 km/h with a parachute.

• ForcesForces are not a thing by themselves, but part of an interaction.Forces always occur in pairs.One force is called the action force, the other is called the reaction force.

• Newtons Third LawWhenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.To every action, there is an equal and opposite reaction.

• Action/Reaction PairsWhen a hammer hits a nail, which is the action and which is the reaction force?Identify the action/reaction forces when you walk.Action: Object A exerts force on object BReaction: Object B exerts force on object A

• Different MassesForces objects exert on each other are always equal in strength and in opposite directions.Newtons second law (F=ma) still applies.If the masses are greatly different, then the object with the smaller mass will undergo greater acceleration.

• ExamplesA gun with a mass of 20kg shoots out a .04kg bullet. The gun kicks back at .2m/s2, at what rate does the bullet accelerate?F = Fma = ma(20kg)(.2m/s2) = (.04kg)a4N = (.04kg)a100m/s2 = aCalculate the kick of a 50kg cannon that shoots a 3kg cannonball out with an acceleration of 12m/s2.

• Action - ReactionA car accelerates along a road. Strictly speaking, what is the force that moves the car?We know that Earth pulls on the moon. Does the moon also pull on the Earth? If so, which pull is stronger?Can you identify the action and reaction forces of an object falling in the vacuum of outer space?

• CancellationDo action/reaction forces cancel each other out? Create a system to determine net force on an object.Ex: Boy on a skateboard, pulling against a girl standing on the ground.Draw systems, identify action reaction forces.

• FrictionFriction is a force that affects motion.It acts when materials are in contact with one another and always opposes motion.Friction is due to the irregularities in the two surfaces.Constant speed = no net force so friction = driving forceDifferent surfaces have different amounts of friction between them ()Friction is not restricted to solids , it also occurs in fluidsThis can be felt even at slow speedsAir resistance is friction acting on something moving through air.

• FrictionTwo types of FrictionStatic friction (Fs)Restrictive force when object is not in motionFs = -FaKinetic friction (Fk)Restrictive force when object is in motionNet external force acting on an object is equal to the difference between applied force and force of kinetic frictionF - FkKinetic friction is less than static friction.

• FrictionThe force of friction is proportional to the normal force.Easier to push a chair across the floor at a constant speed than to push a heavy desk at the same speed.

• Coefficient of Friction is a Ratio of ForcesDefined as the ratio between the normal force and the force of friction between two surfaces.Coefficient of Kinetic Friction k = Fk FnCoefficient of Static Friction s = Fs,maxFn

• If the value of and the normal force on the object are known, then the magnitude of the force of friction can be calculated directly.Ff = Fn

• Coefficients of FrictionMaterial s k steel on steel0.740.57Aluminum on steel0.610.47Rubber on dry concrete1.00.8Rubber on wet concrete-----0.5Wood on wood0.40.2Glass on glass0.90.4Waxed wood on wet snow0.140.1Waxed wood on dry snow-----0.04Metal on metal (lubricated)0.150.06Ice on ice0.10.03Teflon on teflon0.040.04Synovial joints in humans0.010.003

• Air Resistance is a form on frictionWhenever an object moves through a fluid medium (air or water), that fluid provides a resistance to the objects motion.Car at low speed = FR is roughly proportional to the cars speed in the opposite directionCar at higher speed = FR is roughly proportional to the square of the cars speed in the opposite directionWhen the magnitude of FR equals the magnitude of the force moving the car forward, the net force is zero and the car moves at a constant speed.

• ExamplesA 24 kg crate initially at rest on a horizontal floor requires 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.A student moves a box of books by attaching a rope to the box and pulling with a force of 90.0 N at an angle of 30.0o. The box of books has a mass of 20.0 kg, and the coefficient of kinetic friction between the bottom of the box and the sidewalk is 0.50. Find the acceleration of the box.