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

12kg

The following box is sitting on a table located on Earth. Determine the missing forces.

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.

5N

2kg

The following box is sitting on a table located on Earth. The box is not moving. Determine the value of the missing forces.

30°

Balanced Force Quiz

• Partner Quiz