Foundations of Physics Fall 2016 Lecture 7

Announcements Midterm next Friday! Midterm will cover everything up through the FNTs due in the lab before the exam. NEW ideas covered in lab Study session next Thursday night (Victoria hosting) Location and time TBA You Need to bring: Calculator (PHONES ARE NOT ALLOWED!) pencil (NO NOTES!) I will supply: Scratch paper Equations Clipboards Midterms

Announcements Last Week’s quiz: Average: 87% overall You will get these back at the end of lab Remind your lab instructor! Rubric and solutions will be posted by noon today. Grades will be posted on canvas sometime next week. No “calculate your quiz score” assignment this time Exception: People who missed first quiz

Today Work Springs Practice quiz Remind me to post section numbers

Review ΣΔE = Q + W The sum of all the changes in energies in a system is equal to heat plus work leaving or entering the system. Total mechanical

RECALL: Heat Transfer (Q) Heat Transfer can only happen if there is a Temperature difference. Heat is a transfer of energy (a process) that takes place from a hot object to a cold one because the objects are at different temperatures. But what is work? Low temp High temp Energy leaves hot objects in the form of heat Energy enters cold objects in the form of heat

WORK WORK is just another way to transfer energy into a system (just like Heat). Work has the SAME units as energy and heat: Joules ΣΔE = Q + W When work is done in “Mechanical systems,” it involves a FORCE (push or pull) that causes the object in the system to move some distance

Whenever ENERGY enters a system due to a push or pull, WORK is done. Work is a transfer of energy (process) that takes place from a physical system to another physical system due to an interaction that involves “Force”. Whenever ENERGY enters a system due to a push or pull, WORK is done. Situation: A pitcher winds up a pitch and throws a baseball. If the start of the Wind up is at height “h” and the release height is also at height “h”, draw an Energy System Diagram that represents the work the pitcher does on the baseball. System: Baseball KE Speed Work The pitcher’s hand “pushed” the baseball. The pitcher’s hand exerted force on the baseball. As a result, the baseball started moving (its KE increased).

Getting Quantitative: A pitcher throws a 0 Getting Quantitative: A pitcher throws a 0.3kg baseball 44m/s (100mph) how much energy is transferred from the pitcher’s hand in the form of work? System: Baseball Initial: Ball at rest in pitcher’s hand Final: Ball just leaves the pitcher’s hand KE Speed Work During what interval is the pitcher doing work? Physics way of saying , pitcher pitched a ball at 100mph, The pitcher’s hand exerted 290 N amount of force on the baseball over a distance of 1m parallel to the ball’s velocity. Thus, the pitcher 290Joule amount of work on the ball, which is equal to the increase in the KE of the baseball. 100mph=160km/60min=160,000m/3600sec=44m/sec 290Joule ∆KE = Work KEfinal - KEinitial =1/2(m)(vf2) – 0 = W (0.5)(0.3kg)(44m/s)2 = 290.4 J

Work can only be done when Force is applied To be more precise, we need the concept of “Force” : “Push” or “Pull” An overall push (or pull!) in the direction the object is travelling has the effect of speeding it up. Consider a block being pushed by you on a level surface with no friction: Block is already moving, you push in same direction: direction of Force Force is what causes mass to accelerate Force as an agent of interaction of two objects, we are more interested in the interaction itself. Energy was transferred into the KE system of the block in forms of work KE Speed Work direction of travel W=ΔKE=FΔd

Block is already moving, you push in same direction: Consider a block being pushed by you on a level surface with no friction: Block is already moving, you push in same direction: direction of Force KE Speed Work direction of travel W=ΔKE=FΔd What does this d mean? The distance the block travels The distance the force is exerted over

Block is already moving, you push in same direction: Consider a block being pushed by you on a level surface with no friction: Block is already moving, you push in same direction: direction of Force KE Speed Work direction of travel W=ΔKE=FΔd What does this d mean? The distance the block travels The distance the force is exerted over

If I push as hard as I can on the wall, and the wall doesn’t move am I doing “work”? Yes, you are doing work on the wall. No, the wall is doing work on you. Yes, you and the wall are both doing work on each other. No, there is no work being done.

If I push as hard as I can on the wall, and the wall doesn’t move am I doing “work”? Yes, you are doing work on the wall. No, the wall is doing work on you. Yes, you and the wall are both doing work on each other. No, there is no work being done. Remember! W = F*Δd

In order for Work to be done: There needs to be a force acting on the object That force is causing the object move in the direction the force is acting* *For NET work (total work) to be done on an object, the speed also needs to be CHANGING.

Gravity is a force, therefore you can model a book falling as an open system! Ignoring Friction, find the final speed of a book just before it hits the floor after it falls from a height of 10 meters to the floor. Ignoring Friction, find the amount of work done by gravity on the book as it falls from a height of 10 meters to the floor. Work KEtrans KEtrans PEgravity Speed because even though this is true, it messes wih the convention…. If the system is the ball, then you must write that the potential energy indicator is changing…. Weird to define system… but I wouldn’t mark you wrong for this, it’s just easier to stick with the convention Speed Height + + ΔKE = W ΔKE +ΔPE= 0 2a convention… …but this works too!

Still using conservation of energy & energy interaction diagrams Energy and Springs Still using conservation of energy & energy interaction diagrams

Potential Energy: Gravitational and springs The indicator is the change in vertical distance that the object moved (I.e. change in the distance between the center of the Earth and the object) ∆PEgrav = h The indicator is how much the spring is stretched or compressed, x, from its equilibrium position. ΔPEspring = (1/2) kΔx2

Potential Energy: Springs Springs contain energy when you stretch or compress them. We will use them a lot in Physics 2a. The indicator is how much the spring is stretched or compressed, x, from its equilibrium position. k is a measure of the “stiffness” of the spring. x: Much easier to stretch a spring a little bit than a lot! ΔPEspring = (1/2) kΔx2

ΔPEmass- spring = (1/2) kΔx2 Mass-Spring Systems ΔPEmass- spring = (1/2) kΔx2 k is a property of the spring only PEmass-spring does not depend on mass PE = 0 arbitrary Proof for mass=spring not caring about change in gravitational PE? x x x ΔPEs = ½ k Δx2 = ½ k(xf2 - xi2) Therefore, the positive and negative values don’t matter, just absolute magnitude of distance from equilibrium,.

Mass-Spring Systems PEmass-spring KE Speed ∆x System: mass-spring Initial: mass at rest at 2cm Final: mass at x=0 initial X = 2cm final (x

Mass-Spring Systems PEmass-spring KE Speed ∆y ΔPEspring = ½ kΔx2 System: mass-spring Initial: mass at equilibrium & moving Final: mass at rest at x=-2cm In the proocess of springing, Initial equilibrium final at certain distance REALLY IMPORTANT TO DEFINE INITIAL AND FINAL ΔPEspring = ½ kΔx2 = ½ k (x2-x2) initial final Δx = -2cm

Physics PHETs http://phet.colorado.edu/sims/mass-spring-lab/mass-spring-lab_en.html Set g = 0 Set friction to zero to start. What makes the spring go up and down?