Transcript Chapter 5 - AstroStop

Chapter 5
Newton's Third Law
of Motion
1. FORCES AND INTERACTIONS
 Force
is not a thing but is an
interaction between one thing and
another.
 Examples: 2 people pushing each
other on skate boards, driving a nail,
punching a bag
 No single isolated force
 Demo - Bathroom Scales
2. NEWTON'S THIRD LAW
OF MOTION
 Law
3 - Whenever one object exerts a
force on a second object, the second
object exerts an equal and opposite
force on the first.
 Action-reaction
same object.
pairs never act on
Defining Your System

Two objects define a system for a Newton’s
third law interaction.

We are not considering (necessarily) the net
force acting on an object.

An object cannot exert a force on itself to
cause an acceleration.
Action and Reaction on Different Masses
Consider you and the earth
Action: earth pulls on you
Reaction: you pull on earth
Recoil
F
m =a
F=a
m
Video – Scooter Propulsion
Cannot touch without being touched
Reaction: road pushes on tire
Action: tire pushes on road
Reaction: gases push on rocket
Action: rocket pushes on gases
3.

SUMMARY OF NEWTON'S
THREE LAWS
Law 1 (Law of Inertia) Every object
continues in its state of rest, or of uniform
motion in a straight line, unless it is
compelled to change that state by forces
impressed upon it.


 F  ma

Law 2

Law 3 - Whenever one object exerts a force
on a second object, the second object exerts
an equal and opposite force on the first.
4. VECTORS

Imagine that you have a map that leads you to
a buried treasure.

This map has instructions such
as 15 paces to the north
of the skull.

The 15 paces is
a distance and
north is a direction.
N
 Quantities
that require a magnitude and
direction for specification are called
vectors.
 Those
quantities that have no direction
are called scalars.
 Examples
of scalars in physics are
mass
time
distance
density
work
energy

Examples of vectors in physics are
displacement
velocity
acceleration
force
momentum
angular momentum
 The
math associated with scalars is
familiar to everyone.
 The
math associated with vectors is
more involved.
 Let’s
explore the graphical addition of
vectors.
 Let’s
use a treasure map again as an
example of the addition of vectors.
 Let’s
imagine the instructions tell you to
go 4 miles east then 3 miles north.
5 miles
3 miles
36.90
4 miles

In this case you could have gone 3
miles north first and then 4 miles east
next and still end up at the same
location.

Your final position is 5 miles at 36.90
north of east.

It would have saved time if that had
been the one distance and one direction
traveled in the first place.

We say that the 5 miles at 36.90 north of
east is the vector sum of the 4 miles east
vector and the 3 miles north vector.

The order of the addition does not matter.

Examples of addition of vectors follows.
The method used will be the head-to-tail.
Force Vectors
What is the resultant force?
3 Newtons north
5 Newtons @
370 north of east
4 Newtons east
Velocity Vectors
What is the actual velocity?
80 km/h north
wrt the wind
100 km/h @
530 north of east
Wind at 60 km/h east
Components of Vectors

Consider the vector R
y

R

Ry

Rx
x
Velocity Components in Projectile Motion
Chapter 5 Review Questions
A bug and a car collide. Which experiences
the greater force?
(a) bug
(b) car
(c) neither, they both experience the same
magnitude of force
Consider hitting a baseball with a bat. If we
call the force applied to the ball by the bat the
action force, identify the reaction force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing