Force:Force:

An action exerted on an object An action exerted on an object

(a push or a pull) (a push or a pull) in order to change the state of rest or in order to change the state of rest or motion of an object. motion of an object.

Measured in Newtons (N)Measured in Newtons (N)

Free Body DiagramsFree Body Diagrams

•Identifies Identifies allall of the Forces acting on an object of the Forces acting on an object

•Each Diagram has a Each Diagram has a Vector ArrowVector Arrow::• Size of arrow: Shows the magnitude of the force

• Direction of arrow: Shows direction of force

2 Categories of Forces2 Categories of Forces

• Contact Forces: things touching

• Each other

• At-a-distance Forces:

things that don’t touch each other

Force of Gravity = FForce of Gravity = Fgg

• The Earth pulls you downward

• Any object near Earth is under it’s pull

Force of Tension (FForce of Tension (FTT))• Contact force acting through a string,

chain, etc…

Force of Friction = FForce of Friction = FFF

• Always goes in the opposite direction of motion (or the opposite direction an object would move)

Normal Force = FNormal Force = FNN

• The thing that pushes stuff up

Normal ForceNormal Force• Surface Force that acts perpendicular to object

• Consider the following: 1.0 kg mass on a table

– We know from Newton’s 3rd law the table has to….• Push upward with the same force

– How does the block know?

– What if it didn’t push hard enough?– What if it pushed too hard?

Normal Force (FNormal Force (FNN))

• Reactive Force

Types of ForcesTypes of Forces

• PUT THESE IN YOUR BOOKY BOOK

• Fg = Force of Gravity

• FA = Force Applied

• FT = Force of Tension

• Ff = Force of Friction

• FN = Normal Force (“Support”)

Example: Book on TableExample: Book on Table

Fg force on book by the Earth

Support force on book by table

Fg

FN Draw a circle around your object

Example: ContinuedExample: Continued

2) Block Moving1) Push Block 3) Book stops

A cat is at rest on a table top. The free-body A cat is at rest on a table top. The free-body diagram for this situation is shown below: diagram for this situation is shown below:

A rightward force is applied to a Piano in order to A rightward force is applied to a Piano in order to move it across a desk at constant velocity. move it across a desk at constant velocity.

Consider frictional forces. Consider frictional forces.

Force:Force:•An action exerted on an object An action exerted on an object (a push or a pull) (a push or a pull) in order to change the state of rest or motion of an object.in order to change the state of rest or motion of an object.

•Measured in Newtons (N)Measured in Newtons (N)

Net Force:Net Force:• Combination of all the forces acting on an object.

(add them up)

Balanced ForcesBalanced Forces

• If all of the pushes and pulls are even on an object, then the object “keeps on doing what it was doing”

• Tug-o-war• Two teams that are evenly matched play tug-

o-war• NOTHING HAPPENS

Unbalanced ForcesUnbalanced Forces

• Unbalanced forces cause objects to change velocities

• Tug-o-war– Two teams that are unevenly matched play

tug-o-war– One team will win

Unbalanced Force Unbalanced Force

• What happens when an object has an unbalanced force acted upon it?

• Unbalance forces cause acceleration!!

If the forces are…If the forces are…

• Balanced –– Nothing happens

– Ex: A book sitting on a shelf

• Unbalanced – – The object changes what it is doing

– Ex: pick something that is moving

A rightward force is applied to a plate in order to A rightward force is applied to a plate in order to move it across a desk with a rightward move it across a desk with a rightward

acceleration. Consider frictional forces. Neglect acceleration. Consider frictional forces. Neglect air resistance. air resistance.

A rightward force is applied to a box in order to A rightward force is applied to a box in order to move it across a desk at constant velocity. move it across a desk at constant velocity.

Consider frictional forces. Neglect air resistance Consider frictional forces. Neglect air resistance

A force is applied to the right to drag a sled A force is applied to the right to drag a sled across loosely-packed snow with a rightward across loosely-packed snow with a rightward

acceleration. acceleration.

What is wrong with the What is wrong with the following?following?

1. An object at rest has no forces acting on it.

2. If an object is moving, the object must have a force acting on it or else it would stop.

3. If an object is traveling at a constant speed it needs to have a constant force applied.

• Mass is the measure of the inertia of an object– The more mass an object has, the

harder it is to move it (or stop it)

• Mass ≠ Weight• Weight changes due to gravity

– You can be “weight-less” in space...but you’re not mass-less!

MassMass

• Mass is a PROPERTY– Measured in kilograms (kg)

• Weight is a FORCE– Measured in newtons (N)

Mass, ContinuedMass, Continued

A force, in simple terms, is either a push or a pull

Newton’s 2Newton’s 2ndnd Law Law

The acceleration of an object is directly proportional to the net force acting on the object and is inversely proportional to the mass of the object.

In other words:

F = mass x acceleration

F = m a