Phy 2053 Announcements I
Exam 1 info
1.
Phy 2053 Announcements II
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Please get there at least 10 minutes early, and preferably 20 minutes
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Material from sections 1.1 – 5.3 of Serway/Vuille, 20 questions, multiple
choice
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Room assignments
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If your last name begins with A through P, you should go to Carleton 100
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If your last name begins with R through Z, you should go to Pugh 170
You be allowed one handwritten formula sheet (both sides), 8 ½” x 11”
paper
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I will be out of town from this afternoon through Feb 13 and next week Feb
17-19. Office hours also cancelled, but I will be reading e-mail
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I will have special office hours Monday Feb 16 10 am -12 pm
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Hooke’s Law gives the force
In any isolated system of objects interacting only
through conservative forces, the total mechanical
energy of the system remains constant.
F=-kx
„
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KEi + PEi = KE f + PE f
If nonconservative forces are present, then the full
Work-Energy Theorem must be used instead of the
equation for Conservation of Energy
Note: Exam 1 will include material that is covered in the HW set
(problems 5.28, 5,40, 5.78, 5.80, 5.85, and 5.92), so you are
strongly encouraged to complete this set before Exam 1 next
week.
Springs: Force and Potential Energy
Conservation of Mechanical Energy
Ei = E f
Problem 48 (box being pulled up an inclined plane) – parts b
and c will not count. The problem is ill-posed.
HW set #5 is not due until Feb 25 (two weeks from
Wednesday),
5.
Prof. Chan out of town from Feb 5 – Feb 14.
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You may not receive any clicker grades this week; if not, don’t
worry, your grades are still being recorded and they will resume
when Prof. Chan returns
HW set #4 is due this Wednesday, midnight
4.
Professors out of town:
2.
H-ITT clicker grades:
3.
Feb 18, 8:20 – 10:10 pm
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k is the ‘spring constant’
Note that F varies with x
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Linear spring is a simple case:
A = ½ B h W = ½ xmax Fappl
= ½kx2
= work done on
spring
Æ potential energy stored in a
spring is
Wnc = (KEf − KEi ) +(PEf − PEi )
Fapplied
PEs = ½ k x2
xmax
Springs and Gravity –
Nonconservative system
Conservation of Energy: Spring + Gravity
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The PE of the spring is added to both sides
of the conservation of energy equation
(KE + PE g + PE s )i = (KE + PE g + PE s )f
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PEg is the gravitational potential energy
PEs is the elastic potential energy associated
with a spring
PE will now be used to denote the total
potential energy of the system
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Wnc = (KEf – KEi) + (PEgf – PEgi) +
(PEsf – PEsi)
„
„
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PEg is the gravitational potential energy
PEs is the elastic potential energy
associated with a spring
PE will now be used to denote the total
potential energy of the system
The same problem-solving strategies apply
1
Example #5-64
A boy starts from rest and slides down a frictionless slide as
shown in the figure. The bottom of the slide is a height h above
the ground. The boy then leaves the slide horizontally and
strikes the ground a distance d away as shown. Determine the
initial height H in terms of h and d.
Spring + Gravity - Conservative System
(KE + PE g + PE s )i = (KE + PE g + PE s )f
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PEg is the gravitational potential
energy
PE g = mgh
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PEs is the spring potential energy
1
PEs = kx 2
2
Example #5-39
The launching mechanism of a
toy gun consists of a spring of
unknown spring constant. If
the spring is compressed by x
= 0.120 m and the gun fired
vertically as shown, the gun
can launch a 20.0 g projectile
from rest to a maximum height
of 20.0 m above the starting
point of the projectile (x=0).
(a) Find the spring constant,
and (b) find the speed of the
projectile as it moves through
the equilibrium position of the
spring.
Power
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The rate at which energy transfer takes
place
Work F Δx
=
= Fv
Δt
Δt
J
kg m 2
SI units are Watts: 1 W = 1 = 1 3
s
s
P =
„
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Motors (US): 1 hp = 550
ft lb
= 746 W
s
2