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Forces PresentationAcademic PhysicsForces Unit

Table of ContentsIntroduction to ForcesAtwoods MachineNewtons LawsFrictionDynamicsFree Body DiagramsNet ForceComplex Force ProbsInclined PlanesMass and Weight

Introduction to ForcesA force is a push or pull, an agent that can affect the state motion of an object.Force is measured in Newtons (symbol N)Forces can be created from direct contact or be remote.There are also special forces that occur within atoms.

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Introduction to Forces 2These forces involve pushing and pulling objects directly or through some object(s).Examples include:Direct TouchUsing strings, chains, or ropes.BuoyancyThese Forces do not require direct contact.Examples include:GravitationElectrostatic ForcesMagnetic ForcesTable of ContentsNext SlideDirect Contact ForcesRemote ForcesTable of ContentsNext SlideTable of ContentsPrevious Slide

Introduction to Forces 3There are also some special additional forces that exist in physics. These include:Strong Nuclear Force: Force that allows atomic nuclei to stay in tact.Weak Nuclear Force: Force that is involved in radioactive nuclear decay.Next SlideTable of ContentsPrevious Slide

Introduction to Forces 4Kinematics is the study of motion involving how objects move, including the following quantities.Position, Distance, DisplacementSpeed, VelocityAccelerationDynamics is the study of relating forces to kinematics, involving why objects move.Table of ContentsNext SlidePrevious Slide

Mass and WeightMass is a measure of the amount of matter in an object, a measurement that is constant. The unit for mass is the kilogram (kg)Weight is the force on an object due to presence of gravity, a variable quantity dependent upon location. The unit for weight is the Newton (N)As always the acceleration due to gravity is The equation involving weight is shown below.Table of ContentsNext SlidePrevious Slide

Mass and Weight 2Below are 3 sample calculations involving weight.Calculate the weight of a 15kg lawn mower.

Calculate the mass of a frog weighing 4N.

Calculate the gravity on Mars if a 4.0kg object weighs 15.2NTable of ContentsNext SlidePrevious Slide

Newtons Laws of MotionSir Isaac Newton was a British scientist that did extensive research and devised theories involving forces and mechanics.Newton devised 3 laws that describe relationships between forces and motion.Table of ContentsNext SlideTable of ContentsTable of ContentsNext SlideTable of ContentsPrevious Slide

Newtons Laws of Motion 2Newtons first law is known as the Law of Inertia. It is stated as follows:Every object continues in its state of motion, being at rest or at a constant velocity in a straight line, unless acted upon by an outside forceIn short, objects only experience acceleration unless a force is involved.Consider some realistic situations in which forces and acceleration are paired.

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Newtons Laws of Motion 3Newtons Second Law deals with the relationship between force, mass, and acceleration. It is stated as follows:The acceleration of an object is directly proportional to net force, inversely proportional to mass, and in the same direction as the net force.It also can be written in equation form, which is shown below.Table of ContentsNext SlideTable of ContentsNext SlideTable of ContentsPrevious Slide

Newtons Laws of Motion 4Here are 2 sample calculations involving Newtons 2nd Law.How much force is required to accelerate a 0.2kg hockey puck with an acceleration of ?

What is the acceleration on the puck if the force is 10N?Next SlideTable of ContentsNext SlideTable of ContentsPrevious Slide

Newtons Laws of Motion 5Newtons third law deals with the effects of forces on multiple objects during an interaction. It is written as follows:Every action has an equal and opposite reaction.Each force is paired with another forceConsider several examples:A Hammer and NailA Cannon Firing a Cannonball.

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Free Body DiagramsFree Body Diagrams (FBDs) show all the forces acting on a single object.The object is represented by a dot.Forces are represented by labeled arrows.Only forces are shown on the diagram, not other values such as mass, acceleration, and velocity.Table of ContentsNext SlidePrevious Slide

Free Body Diagrams 2There are several special forces involved in FBDs, which are listed below.Weight (W): Force due to presence of gravityNormal Force : Supporting force of surfaces; always perpendicular to surface.Tension (T): Force found in strings and chains.Buoyancy (B): Supporting force of floatation.Friction (Ff): The force acting against motion.Air Resistance (Fair): Oppositional force of air on moving objects

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Free Body Diagrams 3Here are some sample FBDs of objects.Please note: the diagrams are superimposed over the pictures. You are never required to draw a picture, only the FBD.Table of ContentsNext SlidePrevious SlideFalling RockBoy Pulling WagonSled On Hill

Net ForceNet force is the vector sum of all forces acting on an object.If , then an object will not accelerate. The situation is called equilibrium.If , then an object will accelerate. This situation is called nonequilibrium.Table of ContentsNext SlidePrevious Slide

Net Force 2Net Force can be found by performing graphical or analytical vector addition.If the forces are along the same directional line, they can be simply added or subtracted (Right +, Left -).An sample net force calculation is shown below.For the given FBD of a tug of war, find FNet.Table of ContentsNext SlidePrevious Slide

DynamicsDynamics basically involves combining force problems with kinematics problems. In a given situation with forces, Newtons 2nd Law serves as a bridge between forces and motion.Table of ContentsForcesF = maKinematicsNext SlidePrevious Slide

FrictionFriction occurs between two surfaces in contact, always opposing motion (180 opposite)Static friction occurs between surfaces at relative rest between each other.Sliding friction (also known as kinetic friction) occurs between surfaces in relative motion with each other.For any two given surfaces in contact, the static friction will be greater than the sliding friction.Table of ContentsNext SlideTable of ContentsNext SlidePrevious Slide

Friction 2Friction can be quantified using a coefficient m (a Greek letter mu)An equation using the coefficient is shown here.

The coefficient depends on the two surface types involved. It is also unitless.The equation shows that frictional force also depends on the normal force .Table of ContentsNext SlidePrevious Slide

Friction 3A sample friction problem is shown below.The coefficient of friction between 2 wood surfaces is 0.4. A wood block (m = 1.3kg) is pushed across this flat surface with a force of 21N. a) Draw an FBD for the problem.

b) What is the weight of the block?

c) What is the normal force acting on the block?Table of ContentsNext SlideNext SlideTable of ContentsNext SlidePrevious Slide*On a flat surface, the normal force is equal to the weight.

Friction 4The sample friction problem is continued below.The coefficient of friction between 2 wood surfaces is 0.4. A wood block (m = 1.3kg) is pushed across this flat surface with a force of 21N. d) What is the force of friction acting on the block?

e) What is the net force acting on the block?

f) What is the acceleration of the block?

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Friction 4There are other oppositional forces that behave similarly to friction.Air resistance opposes motion of objects through air. Consider the wind felt by riding in a car.Terminal velocity is the velocity at which a falling object will reach equilibrium and fall at a constant velocity. This is dependent upon the shape, size, and density of the object.Table of ContentsNext SlidePrevious Slide

Friction 5The picture below shows a falling object during different stages of its fall.Table of ContentsNext SlidePrevious SlideReleaseSome Time After ReleaseTerminal VelocityDrop

Atwoods MachineA machine called an Atwoods Machine involves two masses connected by a string and hung from a pulley.These machines serve as great devices for developing an understanding of force and acceleration concepts.Atwoods Machine AppletTable of ContentsNext SlidePrevious Slide

Atwoods Machine 2In order to solve Atwoods machine problems, a FBD must be drawn for each mass.

Next, the net force equations must be set up for each mass (Align with direction of motion as the positive direction).Table of ContentsNext SlidePrevious SlideMass 1Mass 2

Atwoods Machine 3Calculate the acceleration using Newtons 2nd Law.

The equation can also be modified into:

This is a generalized equation for any Atwoods machine.Table of ContentsNext SlidePrevious Slide*Total mass must be used.

Atwoods Machine A sample Atwoods machine problem is shown below.Table of ContentsNext SlidePrevious Slide

Complex Force ProblemsThese problems involve multiple objects, each with their own forces involved.A FBD must be drawn for each object.Table of ContentsNext SlidePrevious SlideBlockHanging Mass

Inclined PlanesInclined planes are slopes in which the effects of gravity affect the movement of objects.The pitch of the slope is gauged by an angle formed at the base called q.The weight