Question O’ The Day
• What is a force?
– Take 1 minute and 37 seconds and talk about
it with the people sitting around you. Be AS
SPECIFIC as possible. (Bonus points for
using the words vector or scalar in your
explanation).
Writing Assignment O’ The Day
• On your whiteboard
• Write down absolutely anything
you know (or think you know)
about forces.
• It doesn’t matter if you’re right or wrong I
just want to know what you think.
Force is…
• A force is a push or a pull resulting from
an interaction between objects.
• Since you can’t push or pull without doing
so in a direction…force is a vector.
• The unit for force is….ehh actually we’ll
wait a couple of days for that one.
Gravitational Force
• The force between two objects that have mass.
• ALL objects that have mass are gravitationally attracted to each
other.
• http://www.youtube.com/watch?v=KDp1tiUsZw8
Applied Force
• A push or pull by a person.
• http://www.youtube.com/watch?v=1FqpDmUO0y0
Normal Force
• The supportive force when one thing rests on another thing.
• Squishy force.
• Pillow force.
Frictional Force
• The force that opposes motion when moving or still.
• Different surfaces cause different amounts of frictional forces.
• Friction causes heat.
Air Resistance Force
• The force that occurs when particles of air hit a moving
object, making it harder to move.
• http://www.youtube.com/watch?v=AYJ1YBL39lo
Spring Force
• The force applied to a stretched or compressed spring
wanting to return to it’s regular position.
• http://www.youtube.com/watch?v=zG22qQydPVQ
Tension Force
• A pulling force in a string, rope, chain, or other similar
material.
• Bridges have lots of tension forces.
Word Sort
• Gravitational Force
• Frictional Force
• Applied Force
• Tension Force
• Normal Force
• Air Resistance Force
• Spring Force
Contact Forces
•
•
•
•
•
•
Applied
Air Resistance
Frictional
Normal (Pillow force)
Spring
Tension
Action at a Distance Forces
• Gravitational
• Electric
• Magnetic
Research!
Forces Examples
Hover Disc Demos
Free Body Diagrams
• Free body diagrams are used to represent
the forces acting on any given object.
• Let’s do some (actually…a whole bunch
of) practice problems.
• Come on up and grab whiteboards.
• On one whiteboard sketch out the system
schema.
• On another whiteboard sketch out the
Free Body Diagram
Free Body Diagrams
• A book is at rest on a tabletop. Diagram
the forces acting on the book.
Free Body Diagrams
• A person is suspended motionless from
the ceiling by two ropes. Diagram the
forces acting on the person.
Free Body Diagrams
• An egg is free-falling from a nest in a tree.
Neglect air resistance. Diagram the forces
acting on the egg as it is falling.
Free Body Diagrams
• A flying squirrel is falling vertically from a
tree to the ground at constant velocity.
Consider air resistance. Diagram the
forces acting on the squirrel.
Free Body Diagrams
• A rightward force is applied to a book in
order to move it across a desk at constant
velocity. Consider frictional forces. Neglect
air resistance. Diagram the forces acting
on the book.
Free Body Diagrams
• A rightward force is applied to a book in
order to move it across a desk with a
rightward acceleration. Consider frictional
forces. Neglect air resistance. Diagram the
forces acting on the book.
Free Body Diagrams
• A college student rests a backpack upon
his shoulder. The pack is suspended
motionless by one strap from one
shoulder. Diagram the vertical forces
acting on the backpack.
Free Body Diagrams
• A skydiver is descending with a constant
velocity. Consider air resistance. Diagram
the forces acting upon the skydiver.
Free Body Diagrams
• A force is applied to the right to drag a sled
across loosely packed snow with a
rightward acceleration. Diagram the forces
acting upon the sled.
Free Body Diagrams
• A football is moving upwards towards its
peak after having been booted by the
punter. Diagram the forces acting upon the
football as it rises upward towards its
peak. Neglect air resistance.
Free Body Diagrams
• A car is coasting to the right and slowing
down. Diagram the forces acting upon the
car. Neglect air resistance.
Elevator Activity: 1.24.14
• Elevator Activity
• http://www.youtube.com/watch?v=8BODmDUdh-c
• Big Ideas:
• Unbalanced forces cause acceleration (speed up or slow down).
• Balanced forces mean things are moving with a steady velocity.
Things About Stuff
• Stuff is lazy.
• Stuff likes to do whatever it’s doing unless
something makes it do something
different.
Law of Laziness
• Mr. Heffernan’s Law of Laziness– If something is not moving it will keep not
moving unless something makes it move.
– If something is moving it will keep moving in
the same way unless something makes it
change how its moving.
Newton’s First Law
• Newton’s 1st Law– An object moves with a constant velocity
unless a force causes it to accelerate.
Inertia
• Inertia- The resistance of an object to a
change in it’s motion.
• Inertia = Laziness
• The inertia of an object is directly
proportional to its mass.
• Mo’ Mass, Mo’ Inertia
Question
Question
Inertia
• How would playing with the hover disc be different if it was more
massive?
Answer Questions
Gravitational Force
Mass vs. Weight
• Mass – Mass never changes
– How much stuff is present in something.
• Weight = Gravity Force= Fgrav
– How much that stuff is pulled toward another object due
to gravity.
Gravity Force a minute…
• In our lab we saw that there is a
relationship between the mass of an object
and the gravitational force on that object
• Weight = Gravity * Mass
• Newtons = m/s2 * kg
Sample Problem
Stop
• Answer Questions
The following box is sitting on a table located on Earth.
Determine the missing forces.
12kg
The normal force is “smart”.
Question
• A 5 kg wooden box is being pushed to the
right along a wooden table with a constant
velocity of 4 m/s. What’s the normal force
acting on the box? (Always start with a
Free Body Diagram)
STOP
Question O’ The Day
• My Goal in Life.
• Friction Lab
Post Lab Discussion
Coefficient of Friction
• Coefficient of friction (μ) is basically a
value of how “frictiony” two pressed
together surfaces are.
• The higher μ is…the more friction there
will be.
• μ has no units
Static Friction
• Our lab took a look at static friction Ffs.
• As applied force was increased…static
frictional force increased to a maximum
value.
Kinetic Friction
• In order to take a look at kinetic
friction…let’s do a gedanken experiment
•(Say experiment).
• It is generally harder to get a heavy object
moving than it is to keep it moving.
• Because of this… μs is generally greater
than μk.
Our Equation
• A steel box is being slid to the right with a
constant speed along a steel surface. The
normal force on the box is 700N.
Determine the force of friction acting on
the box.
• A rightward force is being applied to a
wooden block sitting on a wooden table.
The normal force of the box is 500N.
What is the maximum force I can apply to
the box before it begins to move?
• Partner Quiz: 1D Balanced Forces
Springs Post Lab
• Springs Lab Graphs
Da Spring Konstant
Spring Force
Spring
Constant
How far the
spring
stretches or
compresses
Da Spring Konstant
• The spring constant is a measure of how
difficult it is to stretch or compress a
spring.
• As the spring constant goes up…spring
forces goes ______ for the same
displacement?
• As displacement goes up….the spring
force goes _______ for the same spring?
• The spring constant ONLY depends on
what kind of spring you have.
Balanced Forces Practice
• Springs and 1D Balanced Forces Practice
• Back in high school, my buddy used to drive this truck.
• Naturally, his truck broke down and we had to push it back to his
house.
• One of my friends decided to try and pull the truck instead of push it.
• Since that truck is long gone, lets model this with pulling a 2kg mass.
The following box is sitting on a table located on Earth. The
box is not moving. Determine the value of the missing forces.
2kg
30°
Balanced Force Quiz
• Partner Quiz