Warm Up 510.25.11 What actually causes motion? In

other words, how do objects start, stop, or change direction?

DynamicsDynamics- the study of what causes motion- the study of what causes motion

Newton’s First Law of Motion – “Inertia”

GalileoGalileo

For 2000 years, people believed objects For 2000 years, people believed objects were naturally at rest.were naturally at rest.

Galileo used Galileo used experimentationexperimentation to disprove to disprove 2000 years of accepted belief in motion2000 years of accepted belief in motion

Galileo Galileo (1564- (1564- 1642)1642)

““An object, once An object, once placed in motion, will placed in motion, will move further until move further until another force brings another force brings it to a stop”.it to a stop”.

Implied that the Implied that the natural state of an natural state of an object is constant object is constant motion.motion.

What does this actually mean? ““Implied that the natural state of Implied that the natural state of

objects is continuous motion.”objects is continuous motion.”

What does this actually mean? ““Implied that the natural state of Implied that the natural state of

objects is continuous motion.”objects is continuous motion.”

Things stay in motion unless Things stay in motion unless something stops it.something stops it.

Galileo figured out the identity of that Galileo figured out the identity of that something and called it “Resistance”something and called it “Resistance”

Galileo’s Breakthrough #1Galileo’s Breakthrough #1

He realized that He realized that things slowed down things slowed down due to due to frictionfriction..

Friction opposes all Friction opposes all motion.motion.

Galileo’s Breakthrough #2

In the real world, we have friction. What if there was no friction?

What would happen to motion? Galileo hypothesized and

experimented discovered something entirely new.

In a frictionless world …In a frictionless world …

Objects will never stop, will go foreverObjects will never stop, will go forever Objects needs no force to keep it Objects needs no force to keep it

movingmoving

Why don’t things move on their own on a frictionless surface?

Why don’t things move on their own on a frictionless surface? Something keeps them from moving That “something” must be universal

Ph

oto

gra

ph

by

Be

verly

Jo

ub

ert

In a frictionless world …In a frictionless world …

All objects have “it” even on a All objects have “it” even on a frictionless surface.frictionless surface.

““INERTIA”INERTIA”

Newton incorporated this into his work and called the property “inertia”.

Newton’s First Law of Newton’s First Law of MotionMotion

The Law of InertiaThe Law of Inertia When no external , unbalanced force acts on an object When no external , unbalanced force acts on an object

its velocity remains constant.its velocity remains constant. oror If no net force acts on an object, it maintains its state of If no net force acts on an object, it maintains its state of

rest or its constant speed in a straight linerest or its constant speed in a straight line oror

An object at rest will remain at rest, An object at rest will remain at rest, an object in motion will remain in an object in motion will remain in motion in a straight line unless they motion in a straight line unless they are acted upon by an external force.are acted upon by an external force.

What is Inertia?

Inertia is a property of matter like color and mass.

Inertia is directly related to mass More mass, more inertia

Mass is a measurement of the amount of inertia and matter. mass = matter = inertia

But, what is inertia?

Inertia is the “laziness” of matter Matter does not like to change its

motion The more matter, the lazier it gets

More matter = harder to change

ExampleExample

An elephant at An elephant at rest has a lot of rest has a lot of “laziness” so it “laziness” so it has a greater has a greater tendency to tendency to remain at rest. remain at rest.

(hard to move)(hard to move)

ExampleExample

A runaway train at constant velocity A runaway train at constant velocity has a lot of “laziness” so it has a has a lot of “laziness” so it has a greater tendency to stay in motion. greater tendency to stay in motion.

(hard to stop)(hard to stop)

ImplicationsImplications

External forceExternal force is required to is required to accelerateaccelerate an object. an object. Remember what accelerates mean?Remember what accelerates mean?

External force must be External force must be unbalanced.unbalanced.

NET Force – resultant force.NET Force – resultant force.

Which object will remain at rest, according Which object will remain at rest, according to Newton’s First Law? to Newton’s First Law?

Which object will continue moving with Which object will continue moving with constant velocity, according to Newton’s constant velocity, according to Newton’s First Law?First Law?

5 N

5 N 5 N

5 N

5 N

5 N

8 N

8 N

This car will accelerate.

The cat will move

Look

True or False?True or False?

6 Notions of Force and 6 Notions of Force and MotionMotion

1. If there is motion, there is a force acting to 1. If there is motion, there is a force acting to keep an object in motion.keep an object in motion.

2. There cannot be a force without motion- if 2. There cannot be a force without motion- if there is no motion then there is no force there is no motion then there is no force acting.acting.

3. When an object moving there is a force in 3. When an object moving there is a force in the direction of motion.the direction of motion.

6 Notions of Force and 6 Notions of Force and MotionMotion

4. A moving object has a force within it which 4. A moving object has a force within it which keeps it moving.keeps it moving.

5. A moving object stops when its force is 5. A moving object stops when its force is used up.used up.

6. Motion is proportional to force acting. 6. Motion is proportional to force acting. Therefore a constant speed result from a Therefore a constant speed result from a constant force.constant force.

Answers?Answers?

NOT one of these notions are NOT one of these notions are correct.correct.

These are 6 commonly held These are 6 commonly held misconceptions of force and misconceptions of force and motion, and they all originate motion, and they all originate from Aristotle’s views.from Aristotle’s views.

Difference between Mass and Weight

Differentiating Mass and Differentiating Mass and WeightWeight Mass measures the amount of matter

Mass is a scalar

Does the amount of matter change when an object is moved from the earth into space?

Differentiating Mass and Differentiating Mass and WeightWeight Weight depends on the mass of an

object TIMES the acceleration due to gravity. Weight is a FORCE and a vector.

Compare your mass and your weight Divide your weight in pounds by 2.2.

This is your mass in kg

What is your weight on the moon where gravity is 1/6th the earth’s?

What is your mass on the moon?

Forces

Force

SI unit – Newton; N A force needed to accelerate a 1 kg

mass 1 m/s every second. 1kg*m/s2 = 1N A vector quantity

A force is a push or a pull A force can cause

1. a stationary object to move 2. a moving object to stop 3. an object to accelerate (change speed

or direction)

Net Force - Resultant Force Net force – the combination of all the

forces acting on an object.

The net force changes an object’s state of motion.

Net Force

Balanced force – Net force is equal to zero Object remains at rest

Object maintains its motion

5 N

5 N 5 N

5 N

Net Force

Unbalanced force – Net force is NOT equal to zero The cat will move

The car will accelerate

5 N

5 N

8 N

8 N

Net Force

Objects move in the same direction as the Net Force What is the Net Force on the cat? Which way does

it move?

And the car?

5 N

8 N

8 N

3 N

What are the forces acting on this book?

1. Gravitational Force (Weight)

2. Normal Force (Support Force)The net force on the book is 0N. The forces acting on the book are balanced and the book is in a state of equilibrium.

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Types of Forces

•Normal ForceNormal Force•Magnetic ForceMagnetic Force•Electrical ForceElectrical Force• Gravitational ForceGravitational Force

•Frictional Force - FFrictional Force - Fff

•Tension Force - FTension Force - Ftt

•Normal Force - FNormal Force - Fnn

•Air Resistance Force - FAir Resistance Force - Fairair

•Applied Force - FApplied Force - Fappapp

•Spring Force - FSpring Force - Fspringspring

•Gravitational Force (Weight) Gravitational Force (Weight) – F– Fgg

•Resultant or Net Force - Resultant or Net Force - FFnetnet

Contact Force Field Force

Free body Diagrams

Determine the resultant vector (net force) for these objects.

400 N, Up 200 N, Down 20 N, Left

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Determine the net force for each situation.

0 N – balanced force

5 N, Left

0 N – object in equilibrium

15 N, Up

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Exit Ticket Determine the magnitude of the

unknown forces. www.physicsclassroom.com

Light Bulb MomentLight Bulb Moment

No matter how many applied No matter how many applied forces are acting on an object forces are acting on an object AND the net force is equal to zero, AND the net force is equal to zero, the object will maintain its inertia.the object will maintain its inertia.

Balanced forces on an object, Balanced forces on an object, maintains the state of inertia of an maintains the state of inertia of an object.object.

If the forces acting on an object are If the forces acting on an object are cancelled, the object is said to be in cancelled, the object is said to be in a state a state of equilibriumof equilibrium. Meaning the net force is . Meaning the net force is equal to zero and the object maintains its equal to zero and the object maintains its inertia.inertia.

Weight

Weight

Weight

Air Friction

Air Friction

The object is at rest

Normal Force by the ground

Falling objects will experience terminal velocity

ExampleExample If you are standing, If you are standing,

at rest, the at rest, the balanced forces balanced forces acting on you are:acting on you are:

a.) your a.) your weightweight and andb.) the upward b.) the upward

support force of the support force of the ground called theground called the normal forcenormal force..

QuestionsQuestions

1. Why do professional photographers 1. Why do professional photographers use heavy cameras for their shots?use heavy cameras for their shots?

2. While making a turn to the right, why is 2. While making a turn to the right, why is your body “moving towards the left side of your body “moving towards the left side of the car”?the car”?

3. Why do we use safety seat belts?3. Why do we use safety seat belts? 4. In which location will it be difficult to 4. In which location will it be difficult to

shake a 50 kg rock, on earth or in space?shake a 50 kg rock, on earth or in space?

QuestionsQuestions 5. Which object has more inertia, a 15 kilogram 5. Which object has more inertia, a 15 kilogram

bowling ball or a 15 kilogram bag of pure cotton?bowling ball or a 15 kilogram bag of pure cotton? 6. While standing still what is your inertia with 6. While standing still what is your inertia with

respect to the sun? with respect to the ground?respect to the sun? with respect to the ground? 7. What is the value of the normal force if you are 7. What is the value of the normal force if you are

standing on the table?standing on the table? 8. If you throw a ball horizontally and we neglect 8. If you throw a ball horizontally and we neglect

air friction and the force of gravity, how will you air friction and the force of gravity, how will you describe the motion of the ball?describe the motion of the ball?

Helps explain the behavior of objects Helps explain the behavior of objects in motion- they resist change in in motion- they resist change in motion due to their inertia.motion due to their inertia.