Dynamics
Why things move
Accelerations caused by Forces
Forces
Non Contact
Gravitational
Electromagnetic
Contact
Nuclear
Pushes
Pulls
W
Electric
Magnetic
Normal
M
N
perpendicular to surface
Friction
f
Tension
T
Labeling Forces
What forces are acting
on the yellow ball?
F
Labeling Forces
What forces are acting
on the yellow ball?
Fby, on
Labeling Forces
What forces are acting
on the yellow ball?
Wby, on
Labeling Forces
What forces are acting
on the yellow ball?
WEarth, Ball
Labeling Forces
Fby, on
ANYTHING LESS IS AN INCOMPLETE ANSWER!!!
Sir Isaac
Newton
(1642-1727)
Newton’s Laws of Motion
1st Law:
• no net force  no acceleration
• no acceleration  no change in motion
• Objects at rest will remain at rest and
objects in motion will remain in motion
at constant speed, in a straight line,
unless acted on by a net force
Newton’s Laws of Motion
1st Law:
• no net force  no acceleration
• no acceleration  no change in motion
• Objects at rest will remain at rest and
objects in motion will remain in motion
at constant speed, in a straight line,
unless acted on by a net force
Mass
Measure of how much ‘stuff’
Measure of how hard it is to speed up or
slow down the object
Mass and weight are very different
POP!
More net force - more acceleration
(constant mass)
a  Fnet
F
a
F
a
More mass - less acceleration
(constant net force)
1
a  M
F
a
F
a
Putting it all together
Fnet
a  M
Fnet
a  Constant * M
Newton is defined such that constant = 1.
Fnet  M a
Newton’s
2nd Law
Newton’s Laws
1st
Objects at rest will stay at rest and objects in motion
will stay in motion, at a constant speed, in a straight
line, unless acted on by a net force.
2nd
Objects with a non-zero net force accelerate in the
direction of the net force via Fnet = ma
THE BRICK OF DEATH!!!
I
Physics
•What THING is
pushing on the
rocket?
•Is this a contact
or non-contact
force?
Poor Gary:
Gary has a major problem. After studying Newton’s
Third Law in his physics class, he has convinced
himself of its validity. Unfortunately, he has convinced
himself while sitting in a closed room with a single
door. He reasons to himself: “If I push on the door to
go out, Newton’s 3rd Law tells me that the door is
going to push back with an equal force. That force is
going to cancel the force that I push with. Even if I
push as hard as I can, that door is going to push back
just as hard! Oh no! I’m stuck in this room! Why did
I ever take physics!”
The famous bug problem:
• A bug splats on the windshield of a car
moving at 100 mph. Which feels the
greater force, the car or the bug?
• Which has the worst day?
Newton’s 3rd Law and the Scale
Apparent
Weight
NFS
NSP
person
True
Weight
WEP
scale
WES
NPS
Apparent weight
• measured as the reading of the scale
• equal to the normal force of the scale on
your feet
• equal to weight for a=0
Elevator
v = constant
Apparent
Weight
NSP
a=0
True
Weight
WEP
Elevator
a is upward
Apparent
Weight
NSP
a
True
Weight
WEP
Elevator
a is downward
Apparent
Weight
NSP
a
True
Weight
WEP
Elevator
a =9.8 m/s/s down
How large should the
normal force be?
NSP
g
WEP
Elevator
a =9.8 m/s/s down
g
How large should the
normal force be?
Apparently
Weightless??
WEP
The weightlessness of space!
What’s his mass?
80 kg
What’s his weight?
648 N !
What ?
Apollo 17
Earth as seen from the Moon
What is the correct orbit of the space shuttle?
What is the correct orbit of the space shuttle?
a = 8.1 m/s2
a = 8.1 m/s2
a = 8.1 m/s2
a = 8.1 m/s2
a = 8.1 m/s2
a = 8.1 m/s2
Orbit
“…there is an art to flying, or rather a
knack. The knack lies in learning how to
throw yourself at the ground and miss.”
– Douglas Adams
Life, The Universe, and Everything
Circular motion
Which direction is the
instantaneous velocity?
Uniform Circular motion
Which direction is the
instantaneous velocity?
Which direction is the
acceleration?
Uniform Circular motion
Which direction is the
instantaneous velocity?
Which direction is the
acceleration?
TS,B
Freebody
WE,B
Uniform Circular motion
Which direction is the
instantaneous velocity?
Which direction is the
acceleration?
TS,B
Freebody
WE,B
Uniform Circular motion
Which direction is the
instantaneous velocity?
Which direction is the
acceleration?
Ty
Freebody
Tx
WE,B
Aren’t we missing something?
THE CENTRIFUGAL FORCE!
Centrifugal. adj. Moving or directed away from
the center.
Ty
Tx
WE,B
THE CENTRIFUGAL
CENSORED FORCE!
CENSORED adj. Moving or directed away from
Centrifugal.
the center.
Ty
Tx
WE,B
Forces
Non Contact
Gravitational
Electromagnetic
Contact
Nuclear
Pushes
Pulls
W
Electric
Magnetic
Normal
M
N
perpendicular to surface
Friction
f
Tension
T
THE CENTRIFUGAL
CENSORED FORCE!
CENSORED adj. Moving or directed away from
Centrifugal.
the center.
The centri*&%#l force DOES NOT EXIST!
For uniform circular motion the net force
T
must be towards the center of the circle. y
Therefore: The acceleration is ‘center
Tseeking’
x
WE,B
THE CENTRIFUGAL
CENSORED FORCE!
CENSORED adj. Moving or directed away from
Centrifugal.
the center.
The centri*&%#l force DOES NOT EXIST!
For uniform circular motion the net force
T
must be towards the center of the circle. y
Therefore: The acceleration is centripetal
Tx
WE,B
The swinging bucket of
death
C
I
Physics
Draw freebody
diagrams for the water
at points A, B, and C.
B
A
Centripetal Acceleration
For U.C.M. there must be a centripetal net
force
Centripetal Acceleration
For U.C.M. there must be a centripetal net
force  centripetal acceleration