Forces and Newton’s Laws of Motion
Topics:
• Motion and Newton’s first law
•
•
•
•
•
What is a force?
Identifying forces
Newton’s second law Free‐body diagrams
N t ’ thi d l
Newton’s third law
Sample
p q
question:
These ice boats sail across the ice at great speeds. What gets
the boats moving in the first place? What keeps them from going
even faster?
Slide 4-1
What Causes Motion?
In the absence of any forces acting on it, an object will continue
moving forever. Motion needs no “cause.”
Slide 4-8
Seat Belts: An Application of Newton’s First Law
Slide 4-9
What Is a Force?
A force...
force
... is a push or pull.
... is a vector.
... acts on an
object
object.
... requires an
agent
agent.
... is a contact force or a
long-range force.
Slide 4-10
Force Vectors
Slide 4-11
→
A Short Catalog of Forces: Weight w
→
Slide 4-12
→
Spring Force Fsp
Slide 4-13
Tension Force T
Slide 4-14
Normal Force n
Slide 4-15
→
Friction fk and fs
Slide 4-16
Drag D and Thrust Fthrust
Slide 4-17
Identifying Forces – The free body diagram
Slide 4-18
Example
A block is dragged uphill by a rope
rope. Identify all
forces acting on the block.
Slide 4-19
Example
Block A hangs from the ceiling by a rope
rope. Another block B hangs
from A. Identify the forces acting on A.
Slide 4-20
Exercise
A ball
ball, hanging from the ceiling by a string
string, is pulled
back and released. Identify the forces acting on it just
after its release.
Slide 4-21
Newton’s Second Law
Slide 4-22
Example
An elevator,
elevator lifted by a cable,
cable is going up at a steady speed
speed.
• Identify the forces acting on the elevator.
• Is T greater than, equal to, or less than w? Or is there not
enough
h iinformation
f
ti tto ttell?
ll?
Slide 4-23
Free-Body Diagrams
Slide 4-24
Newton’s Third Law
Slide 4-25
Identifying Forces for Interacting Objects
Slide 4-26
Checking Understanding
10-year-old
10
year old Sarah stands on a skateboard
skateboard. Her older brother
Jack starts pushing her backward and she starts speeding up.
The force of Jack on Sarah is
A. greater than the force of Sarah on Jack.
B. equal to than the force of Sarah on Jack.
C less
C.
l
th
than th
the fforce off Sarah
S h on Jack.
J k
Slide 4-27
Answer
10-year-old
10
year old Sarah stands on a skateboard
skateboard. Her older brother
Jack starts pushing her backward and she starts speeding up.
The force of Jack on Sarah is
B. equal to than the force of Sarah on Jack.
Slide 4-28
Quiz
1 What is a “net
1.
net force?
force?”
Slide 4-2
Quiz
2 List at least three of the steps used to identify the forces
2.
acting on an object.
A. Identify “the system” and “the environment
A
environment.””
B. Draw a picture of the situation.
C. Draw a closed curve around the system.
D L
D.
Locate
t every point
i t on th
the b
boundary
d
off thi
this curve where
h
th
the
environment touches the system.
E. Name and label each contact force acting on the object.
F Name
F.
N
and
d llabel
b l each
h llong-range force
f
acting
ti on the
th object.
bj t
Slide 4-3
Quiz
3 Which of these is not a force discussed?
3.
A. The tension force.
C. The orthogonal force.
B. The normal force.
D. The thrust force.
Slide 4-4
Reading Quiz
4 An action/reaction pair of forces
4.
A. point in the same direction.
B. act on the same object.
C are always long
C.
long-range
range forces.
forces
D. act on two different objects.
Slide 4-6