Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 13.
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Transcript of Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 13.
Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova
Lecture 13
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
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?
1. A crate of mass m is on the flat bed of a pick up truck. The coefficient of friction between the crate and the truck is . The truck is traveling at the constant velocity of magnitude V1. Draw the free body diagram for the crate, if the crate does not slip.
2. The truck starts to accelerate with an acceleration ac. Draw the free body diagram for the crate, if the crate does not slip.
First exam Tuesday, September 29
105 Heldenfels
Section 801 and half of 802 start at 7 pm
Section 803 and half of 802 start at 8:10 pm
All liquids, gels and aerosols must be in three-ounce or smaller containers.
No calculators and cell phones
BRING YOUR STUDENT ID!
What is included?
Chapters 1-6
Kinematics, vectors, Newton’s Laws
Kinematics
dt
Vda
dt
rdV
In components:
dt
dVa
dt
dVa y
yx
x ;
dt
dyV
dt
dxV yx ;
If is given, you can find anda
V
r
daVdtaV yyxx ;
dtVydtVx yx ;
ty
Vectors
jAiAA yx
jBiBB yx
? BAC
jCiCC yx
xxx BAC yyy BAC
Newton’s 2nd Law
amF
xx maF yy maF
Newton’s 3rd Law
N
N
Problem 2 from Test 1 2008
Problem 3 from Test 1 2008
Problem 1 from Test 1 2008
Friendly advice
DO NOT…
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
DO NOT write the vector sign over a projection!
!,
,FF
scalarsareFF
jFiIf
yx
yx
DO NOT forget that Fp has two components: Fpcos θ and Fpsin θ
Do Not Forget to
•Write down what is given and express the answer in terms of what is given
•Box the answer
•Indicate the origin and positive x and y
Help Session:
When: tonight at 7:10 pm
Where:206 James J. Cain Engineering/Physics Lab Building
Spence St.
Have a great day!
Hw: Be prepared for the test
Good luck!