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!