Remember!!!!Remember!!!!

Force Vocabulary is due Force Vocabulary is due tomorrowtomorrow

It should be in your notebookIt should be in your notebook The list is onlineThe list is online Use the glossary of the Use the glossary of the

textbooktextbook

FORCES & NEWTON’S FORCES & NEWTON’S LAWSLAWS

ForcesForces (Ch 6)(Ch 6)

Why do things move? What is the Why do things move? What is the cause of this motion we have studied? cause of this motion we have studied?

Explained by Sir Isaac Newton Explained by Sir Isaac Newton 3 Laws of Motion 3 Laws of Motion Explain the way forces influence motion -Explain the way forces influence motion -

what is needed for objects to movewhat is needed for objects to move ““how much” is needed how much” is needed what happens when objects interactwhat happens when objects interact

What is a Force?What is a Force?

Force (F) = push or a pull exerted on Force (F) = push or a pull exerted on an object an object Has magnitude & directionHas magnitude & direction

Contact forces – act only on an object Contact forces – act only on an object by touching itby touching it

Long-range forces – act on an object Long-range forces – act on an object without touching it without touching it

What causes a force? What causes a force?

The agent is the immediate cause of a The agent is the immediate cause of a forceforce

There has to be an agent for a force to There has to be an agent for a force to existexist

What is the agent: What is the agent: A book in your hand? A book in your hand? A book on a desk? A book on a desk? A book falling from the desk? A book falling from the desk?

What kind of forces are What kind of forces are there?there?

Friction (FFriction (Fff) – contact force opposing ) – contact force opposing motion between surfacesmotion between surfaces

Normal (FNormal (FNN) – contact force exerted ) – contact force exerted by a surface on an object by a surface on an object (perpendicular to surface)(perpendicular to surface)

Weight (FWeight (Fgg) – long-range force due to ) – long-range force due to gravitation attraction of 2 objects gravitation attraction of 2 objects (usually Earth & an object)(usually Earth & an object)

What kind of forces are What kind of forces are there?there?

(continued)(continued) Tension (FTension (FTT) – force exerted by a ) – force exerted by a

string, rope or cable when it is string, rope or cable when it is attached to an object & taut attached to an object & taut

Spring (FSpring (Fspsp) – force a spring exerts on ) – force a spring exerts on an objectan object

Newton’s 1Newton’s 1stst Law of Motion Law of Motion

Law of InertiaLaw of Inertia Inertia is an object’s tendency to resist a Inertia is an object’s tendency to resist a

change in its motion change in its motion Law states…Law states…

Objects in motion will stay in motion unless Objects in motion will stay in motion unless a net force acts on thema net force acts on them

Objects at rest will stay at rest unless a net Objects at rest will stay at rest unless a net force acts on themforce acts on them

Newton’s 2Newton’s 2ndnd Law of Motion Law of Motion

Force = mass X accelerationForce = mass X acceleration FFnetnet = ma = ma Force and acceleration are directly Force and acceleration are directly

proportional. The greater the force, the proportional. The greater the force, the greater the acceleration.greater the acceleration.

Mass and acceleration are inversely Mass and acceleration are inversely related. The greater the mass of an object related. The greater the mass of an object the less the acceleration if the same force the less the acceleration if the same force is applied.is applied.

Newton’s 3Newton’s 3rdrd Law of Motion Law of Motion

For every action force there is an For every action force there is an equal and opposite reaction force. equal and opposite reaction force. Two interacting objects become action-Two interacting objects become action-

reaction pairsreaction pairs

How do we measure force? How do we measure force?

One “unit of force” causes a 1 kg One “unit of force” causes a 1 kg mass to accelerate 1 m/smass to accelerate 1 m/s22

Since F = ma,Since F = ma, (1 kg)(1 m/s(1 kg)(1 m/s22) = 1 kg) = 1 kg●m/sm/s22

We call this “unit of force” a Newton (N)We call this “unit of force” a Newton (N)

Friction Friction Force that opposes motion Force that opposes motion

Static Friction – opposes the start of Static Friction – opposes the start of motion motion

Kinetic Friction – opposes motion Kinetic Friction – opposes motion between 2 surfacesbetween 2 surfaces

Coefficient of Friction ( Coefficient of Friction ( μμ)– a ratio of the )– a ratio of the magnitude of the 2 forcesmagnitude of the 2 forces

It’s a constant depending on the 2 surfacesIt’s a constant depending on the 2 surfaces No unit! No unit!

Calculating Friction Calculating Friction FFff = = μμFFNN

What is a Net Force?What is a Net Force?

The net force is the sum of all forces The net force is the sum of all forces acting on a bodyacting on a body

The net force causes the acceleration The net force causes the acceleration of the object of the object Acceleration is always in the direction of Acceleration is always in the direction of

the Fthe Fnetnet

So, 2So, 2ndnd law: F law: Fnetnet = ma = ma When the FWhen the Fnetnet = 0 – system is in = 0 – system is in

equilibriumequilibrium

How do you find FHow do you find Fgg? ?

Recall that F = ma (2Recall that F = ma (2ndnd law) law) So, the force of weight FSo, the force of weight Fgg = mg = mg

g = 9.8 m/sg = 9.8 m/s22 on earth on earth Mass in KILOGRAMS!Mass in KILOGRAMS!

Free-Body DiagramsFree-Body Diagrams

Use a dot (or box) to represent the Use a dot (or box) to represent the objectobject

Represent each force acting on the Represent each force acting on the object with an arrow (vector)object with an arrow (vector) Remember to point it in the right Remember to point it in the right

directiondirection Let’s Practice!Let’s Practice!

Book held in your handBook held in your hand

Book pushed across a desk by Book pushed across a desk by your handyour hand

Book pulled across a desk by a Book pulled across a desk by a stringstring

A ball hanging from a stringA ball hanging from a string

Same ball just after the string Same ball just after the string breaksbreaks

Calculating Lift ProblemsCalculating Lift Problems

Is the object being lifted at a Is the object being lifted at a constant speed? Or is it constant speed? Or is it accelerating? accelerating?

If constant speed, the forces are If constant speed, the forces are balanced. So, Fbalanced. So, Fappiedappied = F = Fgg

Why? Why? If accelerating, FIf accelerating, Fnetnet = ma = ma

Calculating Friction Calculating Friction ProblemsProblems

Is the object moving at a constant Is the object moving at a constant speed? Or is it accelerating speed? Or is it accelerating horizontally? horizontally?

If constant speed, the forces are If constant speed, the forces are balanced. So, Fbalanced. So, Fappliedapplied = F = Fff

Why?Why? If accelerating horizontally, FIf accelerating horizontally, Fnetnet = ma = ma

Let’s Practice # 1 Let’s Practice # 1 A horizontal force of 45 N is needed A horizontal force of 45 N is needed

to keep a 125 N block sliding at a to keep a 125 N block sliding at a constant speed over a horizontal constant speed over a horizontal surface. Calculate the coefficient of surface. Calculate the coefficient of friction. friction.

Let’s Practice # 2 Let’s Practice # 2 A force of 40 N accelerates an 8 kg A force of 40 N accelerates an 8 kg

block 2 m/sblock 2 m/s22 along a horizontal along a horizontal surface. surface. What is the net force?What is the net force?

What is the frictional force? What is the frictional force?

What is the normal force?What is the normal force?

What is the coefficient of friction? What is the coefficient of friction?

What is the net force?What is the net force?

What is the frictional force?What is the frictional force?

What is the normal force?What is the normal force?

What is the coefficient of What is the coefficient of friction?friction?

Let’s Practice # 3 Let’s Practice # 3

A 6000 kg helicopter accelerates A 6000 kg helicopter accelerates upward at 3 m/supward at 3 m/s22. . What is the net force?What is the net force? What is the weight force?What is the weight force? What lift force does the air exert on the What lift force does the air exert on the

propellers ? propellers ?

What is the net force?What is the net force?

What is the weight force?What is the weight force?

What lift force does the air What lift force does the air exert on the propellers ?exert on the propellers ?

What if???What if???

What if the object is moving What if the object is moving horizontally, but the applies force is horizontally, but the applies force is at an angle? at an angle?

Now, we are working with a 2-

D force!

A force includes magnitude & direction, making it a VECTOR.

So, we can break it down!

Resolution of Forces – Breaking down a single vector

into 2 or more vectors(SOH CAH TOA)

Push vs. Pull

A car is pulled with a force of 60N at angle of 37° from the horizontal. Find the vertical and horizontal components of the applied force .                                                                                                                                                              

 

If the cart is rolling with constant velocity, what is the frictional force? Why?

Reminders – Reminders – Don’t forget your kinematic equations Don’t forget your kinematic equations

(motion)(motion) Remember to ID all forces acting on the Remember to ID all forces acting on the

object. Draw the diagram!object. Draw the diagram! If a force is applied at an angle, break it into If a force is applied at an angle, break it into

its x- and y-componentsits x- and y-components Mass needs to be in kg!Mass needs to be in kg! Formulas: Formulas:

FFnetnet = ma = sum of all forces acting on the object = ma = sum of all forces acting on the object FFgg = mg = mg FFff = = μμFFNN