N t ’ L
Newton’s
Laws off M
Motion
ti
Isaac Newton developed his 3 Laws
of Motion (by the time he was 23!)
to describe motion in the natural
world.
According to Newton
Newton, why did
the chicken cross the road?
Chickens at rest tend to stay
at rest and chickens in motion
tend to cross the road
N t ’ 1stt Law
Newton’s
L
off Motion
M ti
Newton’s 1stt Law off Motion is a
restatement of Galileo’s concept of
inertia:
“Every object
“E
bj
continues
i
iin iits state
of rest, or a uniform speed in a
straight line,
line unless acted on by a
nonzero force.”
N t ’ 1stt Law
Newton’s
L
off Motion
M ti
Old tablecloth
bl l h trick
i k
http://www.youtube.com/watch?v=cuG8sIi
V8iQ
The brief friction between the dishes and
the fast-moving tablecloth starts the
dishes moving but then the friction
b t
between
th
the dishes
di h and
d the
th table
t bl stops
t
the dishes before they slide very far.
N t ’ 1stt Law
Newton’s
L
off Motion
M ti
Force Block Diagrams
{
{
Used to show the different forces acting on
j
an object.
Helpful for finding the net force
What forces are acting
g on the green
g
box if it is still?
Normal force
Weight
(Gravity)
N t ’ 1stt Law
Newton’s
L
off Motion
M ti
Force Block Diagrams
What forces are acting on the green
b
box
if someone is
i pushing
hi
it?
N
Normal force
Pushing Force
Friction
Weight
eg t
(Gravity)
F
P
W
N t ’ 1stt Law
Newton’s
L
off Motion
M ti
Example: What is the
force on the dog?
When will
the dog
move?
Wh t is
What
i Acceleration?
A
l ti ?
Non-accelerated Motion
Occurs when an object is moving at
a constant velocity
Problem 1
1m
5m
The ball reaches the wall with a speed
p
of
1 m/s and takes a time of 5 seconds
Wh t is
What
i Acceleration?
A
l ti ?
Non-accelerated Motion
Animal
Distance
Time
Speed
Cheetah
75 m
3s
25 m/s
Greyhound
160 m
10 s
Gazelle
1 km
Turtle
Speed = Distance / Time
100 km/h
/
30 s
1 cm/sec
Wh t is
What
i Acceleration?
A
l ti ?
What about Accelerated Motion?
Acceleration occurs when the object is
not moving at a constant speed
Time
Acceleration
Velocity
v = at
Distance
d = ½ at2
0s
2 m/s2
0 m/s
0m
1s
2 m/s
/ 2
2s
2 m/s2
3s
2 m/s2
4s
2 m/s2
5s
2 m/s2
N t ’ 2ndd Law
Newton’s
L
off Motion
M ti
Newton was the first scientist to
recognize the connection between
force and mass in producing
p
g
acceleration.
Newton’s 2nd Law of Motion states:
“The acceleration produced by a net
force on an object is directly proportional
to the net force, is in the same direction as
the net force, and is inversely proportional
to the
t e mass
ass o
of the
t e object.”
object
N t ’ 2ndd Law
Newton’s
L
off Motion
M ti
A simpler way to state Newton’s 2ndd
Law is:
acceleration = net force
mass
N t ’ 2ndd Law
Newton’s
L
off Motion
M ti
Question:
Before we defined acceleration as the
change
h
in speed
d over time and
d now
we are saying that acceleration is
the ratio of force to mass – which is
it?
a = v/t or
a = f/m
Acceleration is defined as that change
in velocity over time but is caused
by force applied to the mass
CHECK YOUR NEIGHBOR
Suppose in a high-flying
f
airplane the
captain announces over the cabin
public address system that the
plane is flying at a constant 900
km/h and the thrust of the engines
km/h,
is a constant 80,000 newtons.
What is the acceleration of the
airplane?
N t ’ 2ndd Law
Newton’s
L
off Motion
M ti
Sample Problem
Suppose you apply the same force (1
N)) to two d
different
ff
carts, one with
ha
mass of 1 kg and the other with a
mass of 2 kg.
kg Which cart will
accelerate more?
1N
1 Kg
a= ?
1N
2 Kg
a=?
How are you feeling about this
concept?
t?
{
Reflection
f
How well do you understand Newton’s
1st Law? How about his 2nd Law?
What can you do to improve your
understanding?