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Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova
Lecture 12
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A block of mass 20 kg is pushed against a vertical surface as shown. The coefficient of friction between the surface and the block is 0.2. If θ=300, what is the minimum magnitude of P to hold the block still?
θ
P
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A small block, mass 2kg, rests on top of a larger block, mass 20 kg. The coefficient of friction between the blocks is 0.25. If the larger block is on a frictionless table, what is the largest horizontal force that can be applied to it without the small block slipping?
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F
gm1
1N
1NF
1N
gm2
2N
1N
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Quiz (hw Ch 6)
A block of mass m starts at rest at the point x=A, y=B. At time t=0 two forces begin to act on it:
jbitbFjtcicF
2122
211
Here c1, c2, b1 and b2 are known constants. Where will the block be at any subsequent time, t?
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First exam Tuesday, September 23
105 Heldenfels
Section 801 and half of 802 start at 7 pm
Section 803 and half of 802 start at 8:15 pm
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All liquids, gels and aerosols must be in three-ounce or smaller containers.
No calculators and cell phones
BRING YOUR STUDENT ID!
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What is included?
Chapters 1-6
Kinematics, vectors, Newton’s Laws
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Kinematics
dt
Vda
dt
rdV
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In components:
dt
dVa
dt
dVa y
yx
x ;
dt
dyV
dt
dxV yx ;
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If is given, you can find anda
V
r
daVdtaV yyxx ;
dtVydtVx yx ;
ty
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Vectors
jAiAA yx
jBiBB yx
? BAC
jCiCC yx
xxx BAC yyy BAC
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Newton’s 2nd Law
amF
xx maF yy maF
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Newton’s 3rd Law
N
N
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Problem 3 from Test 1 2007
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Problem 4 from Test 1 2007
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Friendly advice
DO NOT…
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DO NOT use Const acceleration case formulas when acceleration is a function of time. You have to integrate or differentiate! What are the Const acceleration case formulas?
)0()0(2
1)( 2 xtvtatx c
)0()( vtatv c
))()((2)()( 1212
22 txtxatvtv c
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DO NOT write the vector sign over a projection!
!,
,FF
scalarsareFF
jFiIf
yx
yx
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DO NOT forget that Fp has two components: Fpcos θ and Fpsin θ
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Do Not Forget to
•Write down what is given and express the answer in terms of what is given
•Box the answer
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Help Session:
When: tonight at 7:15 pm
Where:206 James J. Cain Engineering/Physics Lab Building
Spence St.
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Have a great day!
Hw: Be prepared for the test
Good luck!