physics 218 lecture 7

Physics 218, Lecture VII 1

Physics 218Lecture 7

Dr. David Toback


Before we begin•Exam 1 Next Tuesday – Only on topics through Chapter 4

•Homework and the Mini-practice Exam– When you have 100% on all of them you will have access to a mini-practice exam

– Get a 100% on the mini-practice exam BEFORE the in-class exam, get 5 bonus points

– Mini-practice exam is the right way to study!


Checklist for Today•Things that were due last Thursday:– Chapter 4 reading

•Things that are due yesterday (Monday):– WebCT Prelim, Math Quizzes, Ch.2 HW

•Things that are due today:– Reading for Chapter 5

•For this week and/or due next Monday:– Read Chapter 6 for Thursday– Recitation: Probs from Chap 3&4– Lab 3– All Ch3&4 problems on WebCT due Monday


Chapters 5 & 6• 3 Lectures for Chapters 5 & 6:

–One lecture on Chapter 5–One lecture on Chapter 6–One lecture on problem solving

• Lecture Reading Assignments–Chap 5 due today–Chap 6 due Thursday

Today’s lecture not on the exam


Overview of Chapter 5Where we’re going and why

– Dynamics vs. Kinematics• Force• Newton’s Laws of Motion• Normal Force• Example problems

–Note: It’s important to be good at 2-Dimensional motion at this point


Where we’re going and whyMoving from: “How things move” KinematicsTo: “Why things move that way” Dynamics•Why do you care? Different questions:– Old: What acceleration do you need to go from 0 to 60mi/hr in 6 sec?

– New: How much force does your car engine need to exert to do it?

•Use all the kinematics, vectors and calculus from Chapters 1-4

• Plan: Do the concepts, then do the problems


Newton’s LawsYou shouldn’t memorize them, rather you

need to be able to understand and usethem

• Don’t write them down from the overheads, they’re in your book.

• We’re going to translate them into English• Big picture:

Force


First Concept

What is a Force?• Examples:–Push–Pull–Slap–Gravity–Others?


Newton’s First Law

“Every body continues in it’s state of rest or of uniform

speed in a straight line unless acted on by a non-

zero net force”


Translate that into English: Force

To cause an acceleration (change the velocity) requires

a Forceor

If there is an acceleration, there must be a net ForceForce is a VectorAdd up all the forces (vectors) to

find the Net (or total) force


Newton’s First Law•Example of non-zero net forces:–Friction: Makes a moving block a slow down–Gravity: Makes a ball fall toward the earth

•Example of zero net force–Car just sitting on the pavement•No velocity, no acceleration→no net force

–Rocket ship in outer space•Nothing to slow it down → constant velocity → no net force


Newton’s Second Law

“The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the

acceleration is in the direction of the net force action on the

object”


Translate: Newton’s Second LawThe acceleration is in

the SAME direction as the NET FORCE

→ This is a VECTOR equationIf I have a force, what is my acceleration?

→ More force → more acceleration

→ More mass → less acceleration

gmWWeight

ma F ,ma Fam F

:Equation Vector

yyxxvr

rr

==

==

=

Σ

Σ


Force to stop a carYou are a car designer. You must develop a new braking system that provides a constant deceleration. What constant net force is required to bring a car of mass m to rest from a speed of V within a distance of D?

X0 = 0 XF = D

V0 = V VF = 0


Getting to Newton’s Third Law

How does one apply a force?

•Applying a force requires another object!–A hammer exerts a force on a nail


Newton’s Third Law

“Whenever one object exerts a force on a second object, the second exerts an equal and

opposite force”OR

“To every action there is an equal and opposite reaction”


Skater•Skater pushes on a wall •The wall pushes back– Equal and opposite force

•The push from the wall is a force– Force provides an acceleration

– She flies off with some non-zero speed


Walking

When you walk you push on the groundand the ground PUSHES you forward

• Equal and opposite force

Ground the on PersonPerson the on Ground F- Frr

=


Normal ForceConsider a pen sitting on a table:

– Is the force of gravity acting on it? – Is the pen accelerating?– What is the Force? What is the difference between a force and the Net Force

– What keeps the pen from accelerating?Clearly, there is a second force that keeps it from accelerating

Call this the “normal” force!


Moving towards Problems

Done with the concepts for this chapter

• Overheard…–“I understand the concepts, but I can’t do the problems”

• Sigh… No exam points for “concepts you can’t apply”


Free Body DiagramsSame tricks as in Chapters 1-4:1.Draw a diagram: Draw each force on

an object separately! Force diagram!2.Break each force into its X and Y-

components, THEN sum!!!– Show your TA that you know the

difference between a force, and a component of force

– GREAT way to pick up partial credit


Pulling a box

FP Θ

A box with mass m is pulled along a frictionless horizontal surface with a force FP at angle Θ as given in the figure. Assume it does not leave the surface. a)What is the acceleration of the box? b)What is the normal force?


2 boxes connected with a stringTwo boxes with masses m1 and m2 are placed on a frictionless horizontal surface and pulled with a Force FP. Assume the string between doesn’t stretch and is massless.

a)What is the acceleration of the boxes? b)What is the tension of the strings between the boxes?

M2 M1


Rest of This Week• Recitation:– Problems from Chapters 3 and 4

• Lab: Tensile Strength• Reading for Thursday:–Read Chapter 6: More on force and friction

• HW3&4: Due Monday• Are you ready for Exam 1 next Tuesday?–Use WebCT to study!–Get perfect scores on all Math and HW1-HW4

WebCT mini-practice exam becomes available–Get a 100 on it BEFORE Tuesday’s exam and I’ll give you 5 bonus points on the exam

physics 218 lecture 7

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