Physics 218: Lecture 7 - Dynamics by Alexei Safonov - Prof. Alexei Safonov, Study notes of Mechanics

Download Physics 218: Lecture 7 - Dynamics by Alexei Safonov - Prof. Alexei Safonov and more Study notes Mechanics in PDF only on Docsity!

Physics 218

Alexei Safonov

Lecture 7: Dynamics

Before we begin

-^

Exam 1 Next Monday^ – Only on topics through Chapter 3

-^

Homework and the Mini-practice Exam^ – HW2 was due today^ – HW3 due Monday^ – If you have 70+% on each weekly HW assignment, you can

get bonus points by working on a mini-practice exam^ • Mini-practice exam is now available on MP, it is completely

optional and has to be completed BEFORE the in-class exam inorder to receive points

• To get any bonus points, you need to get 80+% on the mini-

practice exam (so that you will get between 4 ad 5 points)

• If you do it with less than 80%, you won’t get any points.
  • Mini-practice exam is a good way to test yourself before the

exam.^ • Try it as if it were a real in-class test. Set aside about an hour

and a half and try to get it done within that time frame.

Overview of Chapter 4

Where we’re going and why

• Dynamics vs. Kinematics
  • Force• Newton’s Laws of Motion• Mass• Normal Force• Example problems
    • Note: It’s important to be good at 2-

Dimensional motion at this point

Where we’re going and why

Moving from: “How things move”

Kinematics

To: “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?

• Use all the kinematics, vectors and calculus from

Chapters 1-

• Plan: Do the concepts, then do the problems

Force: Our 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”

Newton’s First Law

-^

Example of non-zero net

forces:

• Friction: Makes a moving block 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”

Force to stop a car

You are a car designer. You must develop anew braking system that provides a constantdeceleration. What constant net force isrequired to bring a car of mass

m

to rest from

a speed of

V

within a distance of

D

X

0

= 0

X

F^

= D

V

0

= V

V = 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

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

• She pushes on the

ground and theground PUSHESher forward

• Equal and opposite

force

Ground

on the

Person

Person

on the

Ground

F

F

r

r

=

Free Body Diagrams

Same tricks as in Chapters 1-3: 1. Draw a diagram:

Draw each force on an

object separately! Force diagram!

2. Break each force into the

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

F

P

Θ

A box with mass

m

is pulled along a frictionless

horizontal surface with a force

F

P^

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?