Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 12.

Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova

Lecture 12

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

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?

F

gm1

1N

1NF

1N

gm2

2N

1N

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?

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

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 3 from Test 1 2007

Problem 4 from Test 1 2007

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

Help Session:

When: tonight at 7:15 pm

Where:206 James J. Cain Engineering/Physics Lab Building

Spence St.

Have a great day!

Hw: Be prepared for the test

Good luck!