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333 PHYSICS - UNIT 2
FORCES
Explanations as to why things
occur versus just observation.
Copernicus & the moving Earth
What idea did Copernicus
propose that revolutionize
science?
What view was held prior
to Copernicus proposal?
In the 1500’s, Galileo reasoned…
In the year 1642, Galileo died
and Issac Newton was born.
He was a genius who
essentially wrote the book of
physics in 18months.
He furthered Galileo’s idea
and generated Newton’s Laws
of Motion.
NEWTON’S LAWS OF MOTION
Law of INERTIA OR 1ST LAW:
INERTIA
Mass
• Mass is
• Mass is measured
Mass is not weight.
You can change your weight without changing
your mass. How?
LOCATION
MASS
WEIGHT
EARTH
50kg
500 Nwtns
MOON
?
83 Nwtns
JUPITER
?
1250 Nwtns
Question: A top view of a circular pie tin with a
quarter section cut out. Which path would a
marble follow after exiting the circular track?
Explain the purpose of seatbelts using the
concept of inertia.
An argument against a
moving Earth (Copernicus)
was the following:
Consider a bird sitting at
rest at the top of a tall tree.
If the earth is really moving
around the sun at 30km/s
and also rotating at
1000mph, then how does a
bird dive down to catch a
worm?
Why doesn’t classroom wall hit us if we jump up in air?
Why do all objects fall
at the same rate? (g=-10m/s2)
Aristotle felt heavy things fell
faster than lighter ones.
However, without air
resistance, a light object falls
with the same acceleration
as a heavy object… Explain
using weight and inertia.
3rd LAW or Action/Reaction
Walking or Jumping
Swimming
Use 3rd law to describe
the motion of:
A Rocket taking off
A helicopter hovering
over the ground
A school bus collides with a bug and kills it.
QUESTION: Which body receives the greater
force of impact, bug or bus? WHY?
Who pulls harder on the rope?
Who wins the tug of war?
NET FORCE (FNET )
Net =
Net Force =
NET FORCE
Newton’s 2nd Law
Fnet  ma
If net force
then acc
If net force
then acc
If mass
then acc
If mass
then acc
Fnet  ma
When FNET = 0, acceleration is 0….,why?
When FNET = 0, velocity is constant…,why?
Net Force causes Acceleration
Mass Resists Acceleration
TYPES OF FORCES
Normal force = N
Friction force = f
Tension force = T
Weight force = W
1 kg = 2.2lbs
Force diagrams
FBD’s with 2nd law (constant acc)
Example 1: A 7.5kg box sits on a table. It is then pulled
from rest to the right by rope with a constant force of
50N. There is a resistive force of 13N on the box.
a)Draw a force diagram on the box.
b) Write the 2nd law force equation with specific forces.
c) Determine the acceleration of the box.
d)Determine the speed reached by the box after 3.5sec
of being pulled.
Example 2: An empty elevator of mass 2700 kg is pulled
upward by a cable. The elevator accelerates up at 1.3m/s2.
a) Draw a force diagram for the elevator.
b) Write the 2nd law force equation with
specific forces.
c) Determine weight of elevator.
d) Determine the tension in the cable.
e) Determine the tension in the cable if it was
moving with a constant velocity.
Draw the forces acting
on the skydiver.
Fnet  ma
When a skydiver reaches
terminal speed, they are
moving _________________
What is the value of the acceleration of the
skydiver when they reach terminal speed?
_________________
What is true about the forces acting on the skydiver
when they reach terminal speed?___________
Which direction does skydiver
move (relative to ground) when
they open up the parachute?
What does the parachute change
about the skydiver?
What happens to FORCE of air
resistance due to the parachute? What
happens to the skydiver’s weight?
What is the direction of the NET
FORCE on the skydiver right after
opening parachute?
What happens to speed of skydiver after parachute has
been open for awhile? Forces on skydiver?
FRICTION FORCE
Two (2) types of friction:
1)Static:
2)Sliding:
Static Friction is present
when man pushes on box,
until box moves. Also
present on his feet.
Sliding Friction between player and dirt
STATIC FRICTION BEHAVIOR
Normal force
Friction force
Coefficient of
friction
Friction Example: The coefficient of sliding friction is
0.40 for a box being pulled to the right across a table.
The mass of the box is 2kg.
a) Draw a force diagram for the box.
b) Determine the weight of the box.
c) If the normal force equals the weight force on the
box, determine the sliding friction between the box
and table surface.
d) If the box is being pulled by a force of 12N, determine the net
force on the box.
e) Determine the acceleration of the box.
Equilibrium
• EQUILIBRIUM IS A CONDITION WHERE FORCES
CANCEL OUT IN A PARTICULAR DIRECTION.
• FNET = 0
• BALANCED…ZERO ACCELERATION
Assuming no air
resistance, EXPLAIN
the following picture.