Acclimatization

Acknowledgements 2013 Mark Lesmeister/Pearland ISDThis work is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/3.0/ or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA.Selected graphics and problems from OpenStax College. (2012, June 12). College Physics. Retrieved from the Connexions Web site: http://cnx.org/content/col11406/1.7/Cartoons from Looney Tunes Movie Collection, 2005 Warner Brothers Entertainment. Used under the fair use doctrine for educational purposes.Selected questions from

The Laws of MotionPearland High School Physics2Ask What is the natural state of motion? What is the natural state of a students motion?FORCE Force is a push or pull exerted on some object.Forces cause changes in velocity such as:Start moving, stop moving or change direction.The SI unit for force is the Newton.1 Newton = 1 kg m/s2

Types of ForcesForces can act through contact or at a distance.Contact Force physical contact between two objectsField Force does not involve physical contact between two objects. Example include:electrical forcesmagnetic forcesthe force of gravity

The Law of InertiaPart 1Observation #1An object at rest remains at rest, unless something makes it move.

6Put piece of dry ice on table. (Or have students put an airpuck on the air tables, with no air flowing.) Ask why it doesnt move. Answers might be because it doesnt have any force. This is approximately what most people in the ancient world believed. Aristotle believed that motions were either natural or forced. An object naturally moved to its correct place and then stayed at rest. Heavy things, being composed of earth, naturally moved toward the Earth. An outside agent was needed to move the thing away from its natural position and natural state of rest.Picture a ball on a table in a moving train car.Is rest the natural state of an object?STOPThere are problems with the idea that the natural state of an object is rest, and that an object requires an outside agent to keep moving. The ball is sitting on the table. Is it at rest? Or is it moving? (It is at rest relative to the table. It is moving relative to the Stop sign.) In a train going at a constant speed, the object is moving to someone outside the train. So the idea that rest is the natural state of an object is untenable, since rest depends on what you look at.

7Observation #2An object in motion continues in motion with constant velocity, unless something makes it change its velocity. Constant velocity means constant speed in the same direction.

8Show dry ice again (or have students push on an airpuck with the air on.) Push slowly. As it moves ask if I have to keep pushing it. Will its motion change? When? So, it will stay moving in one direction until made to do something else. So, what is the key idea here? Is something necessary for an object to move, or change its motion? So, something is needed to cause something to speed up or slow down, but not to keep moving.(1)Now push a cart against another cart with magnets out (or have students let puck hit wires). Ask what is happening when it hits the barrier? Did it change its velocity? How?(2)Combining Observations 1 & 2 An object left alone will not change its velocity. Something must cause a change in velocity.A force is something that causes an acceleration or change in velocity by either:Changing speed Changing direction.

9Objects and SystemsAn object is something that has no internal structure, or that we can treat as having no internal structure. Ex: ElectronA system is an object or collection of objects grouped together for study. Ex. AtomsAn electron can be considered an object. A proton is a system, consisting of objects exerting forces on each other. Everything made of atoms is also a system.10External and Internal ForcesAn object cannot exert a force on itself.Internal forces have no effect on the motion of a system as a whole. Only external forces are considered in Newtons Laws.Demonstrate by sitting in a rolling chair and trying to push yourself across the room (pushing yourself, not against another object) that an internal force cannot change motion.) Explain that well find out why we can ignore internal forces later on.11Observation #3An object will not change its velocity unless a net external force acts on it.

12Then ask about tug of war. Ask if force was being applied to the rope. Ask why it didn't move. Elucidate its unbalanced or net forces that matter. (1)Newtons First LawObjects do not change their motion without a cause.

Forces are what cause changes in motion.

It is the net external force acting on an object that determines whether it will change motion.

13Newtons First LawAn object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless the object experiences a net external force. A net external force is required to change velocity.14ForceSI unit of force is the Newton (N).1 N = 0.225 lb1 lb. = 4.448 N

A force is a vector.It has a magnitude, measure in N or lbs.It acts in a particular direction.

15(1)Say that forces are measured in Newtons.

Do demonstration 2 with a long and short piece of rope. Ask why more force had to be applied in one situation. So clearly, direction matters.

Common ForcesThe force of gravity (Fg)or (W) pulls straight down.

The force of friction (Ff) or (f) occurs between two objects that can slide against each other. It opposes the relative motion of the surfaces.Applied force (Fa ) points in the direction of motion.

16Common ForcesThe normal force (FN) or (N) is the support force from a surface.It is called normal because it is always perpendicular to the surface.The tension (FT) or (T) is the force in a rope or string.The tension is the same in every part of a rope.Ask why does the book not fall through the table. Are their forces acting on it? Elucidate that both gravity and the force of the table act on it. Do demo- Normal force.(1)The word normal means perpendicular in mathspeak. Note how we always draw forces going out from the body on which they act, even if they actually point in.(2)

17Free-body diagramFree-body diagrams consider just one object and the forces that act on it.To draw a free body diagramDraw a dot to represent the object.Draw and label vector arrows representing all the forces acting on the object.All the vectors should be shown as acting at a single point.18Assign Worksheet 1 for homeworkSteps for Drawing Free-Body Diagrams : add this to your notesBefore you begin identifying the forces acting in a situation; draw a dot, a circle or a box to represent the isolated object under consideration

Object under consideration: towed carSteps for Drawing Free-Body DiagramsThen draw and label vector arrows representing all external forces acting on the object.Force exerted by towing cable attached to the car.

Gravitational force acting on the car directed towards the center of the Earth

FtFtFg3. The road exerts an upward force on the car

4. Due to the interaction between the road and the back tires, the road also exerts a backward force of friction on the car.

FtFgFnFgFnFtFf Due to the interaction between the road and the back tires, the road also exerts a backward force of friction on the car.

FgFnFtFfObjects on an incline- sketch this in your notes

Working With Newtons 1st LawPart 2InertiaAnother way to say the First Law is to say that objects have inertia. Inertia is the tendency of objects to resist changes in motion.The amount of inertia an object has is determined by its mass.25 Demonstration 3 with physics books and paper.

What is happening?This is the ball in the train car again. The ball appears to start moving for no reason.Does this violate Newtons First Law?Whats happening in (b)? Is Newtons First Law being violated? (No, it is being demonstrated. No outside horizontal forces act on the ball, so it moves a constant rate. The *train car* on the other hand is being accelerated.

26Inertial Reference FrameAn inertial reference frame is one in which Newtons First Law holds.Accelerating reference frames are not inertial.27Inertial reference frameThe Earth is not an inertial reference frame.Large scale motions on the Earth will appear to curve without a force.This is because the Earth is rotating.This includes Large air masses and ocean currents.Long range missiles.

28(1)Why do hurricanes get their motion?(2)

ForcesA force is the interaction of two objects. There are four fundamental interactions, in other words four fundamental forces.

PP2 fmCarbon-14Nitrogene-PP2 fm1 kg4 kgThis slide shows the four fundamental interactions, which the students have studied before. Have them guess what is going to happen in each case. The first interaction shows two protons, indicated by the letter P, separated by a distance of less than 2 fm. Ask the students to predict which way the protons will move. Students should make their predictions publically using hand signals. Then show that they attract, and explain how this related to the existence of nuclei, whose protons would repel according to the electric force.

Next, show the Carbon-14 atom, and explain that another force is responsible for causing a neutron to decay into a proton and an electron. Ask what the nucleus will be then (hint, it has 7 protons now). Then show the beta decay and the formation of Nitrogen.

Then demonstrate the electric force and gravity force, each time asking students to predict the motion before you show it. 29Four fundamental forcesGravitational forcesStrong nuclear forcesWeak nuclear forcesElectromagnetic forces

Agent-Object NotationTo avoid confusion, we will often identify forces with two subscripts.The first subscript indicates what agent is applying the force.The second subscript indicates to what object the force is applied.

FE-M31Weight/Mass Relationship

Do weight/mass proportionality demonstration with a force meter, as set of hooked masses, and preferably a graphing program (like LoggerPro).32Gravitational Mass vs. Inertial MassGravitational mass is the property of an object that determines how much weight it has at a certain place (Earth, Moon, Mars ). It determines the strength of gravitational interaction with other objects, systems or gravitational fields Both weight, i.e. the amount of gravity, and inertia, i.e. resistance to change in motion, seem to depend on the same property of an object, the mass.

There is no reason classically why they should be the same. In Einsteins General Theory of Relativity, the reason for this equivalence is explained.33Gravitational Mass vs. Inertial MassInertial mass is the property of an object that determines its resistance to changes in motion. This property shows how an objects motion changes when it interacts with other objects or systems.Experiments have confirmed that these two properties are the same.

There are two separate experiments that can be used to determine inertial mass and gravitational massEquilibrium modelObjects that are at rest or moving with constant velocity are in equilibrium. According to Newtons First Law, objects in equilibrium have a net external force that equals 0. v = 0, F = 0 F = 0, v = 0

35Equilibrium ExampleA rock-climber who weighs 800 N is held in place by two ropes. One pulls horizontally to the right, and the other pulls up and to the left at an angle of 30o from the horizontal. What is the tension in the ropes?First, draw a FBD for the rock climber and label your forces (drawing your forces against a backdrop of a coordinate system might help with resolving the forces)

Go over this problem, then give a worksheet on equilibrium problems. If you dont get to this example, the students can likely figure them out on their own.36T1T2FG

T1

Equilibrium Example

Model: Equilibrium

The law of action and reactionSection 3

Which has more force?When the boxer hits the bag, which has more force, the boxer on the bag or the bag on the boxer?

Newtons Third LawIf Object A exerts a force on Object B, then B exerts a force on Object A that is equal in magnitude but opposite in direction.2012 OpenSTAX College

Do Newtons 3rd Law lab as brief discovery activity. Then go over above. Ask whats bigger, the force of a bug on the windshield, or the force of the windshield on the bug.40Newtons Third LawThe two forces are called an action-reaction pair.The two forces do not balance each other, since they act on different objects.For every action, there is an equal and opposite reaction2012 OpenSTAX College

Do Newtons 3rd Law lab as brief discovery activity. Then go over above. Ask whats bigger, the force of a bug on the windshield, or the force of the windshield on the bug.41Newtons Third Law

Newtons Third Law

Action-Reaction PairsIdentify all the action-reaction pairs involved in a ball sitting on a table.Action-Reaction PairsIdentify all the action-reaction pairs involved in a ball sitting on a table.