1. Foundations of Physics
Fall 2016
Lecture 7

2. 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

3. 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

4. Today
Work
Springs
Practice quiz
Remind me to post section numbers

5. 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

6. 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

7. 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

8. 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).

9. 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 = J

10. 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

11. 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

12. 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

13. 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.

14. 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

15. 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.

16. 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!

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

18. 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

19. 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

20. 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,.

21. 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

22. 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

23. Physics PHETs
Set g = 0
Set friction to zero to start.
What makes the spring go up and down?