Dr. Joseph W. Howard ©Spring 2008 Motion So Far…. Distance vs. Displacement Speed vs. Velocity...

Dr. Joseph W. Howard

©Spring 2008

Motion So Far….Motion So Far….

• Distance vs. Displacement

• Speed vs. Velocity

• Average Speed

• Average Velocity

• Acceleration– “Speeding Up”– “Slowing Down”

• Distance vs. Displacement

• Speed vs. Velocity

• Average Speed

• Average Velocity

• Acceleration– “Speeding Up”– “Slowing Down”


©Spring 2008

Why do objects move?Why do objects move?


©Spring 2008

Let’s start off by observing some

various situations

Let’s start off by observing some

various situations


©Spring 2008

What can we say?What can we say?

•Net Force to START motion•Net Force to START motion

•Net Force to STOP motion•Net Force to STOP motion

•Net Force to CHANGE motion,not to continue the motion•Net Force to CHANGE motion,not to continue the motion


©Spring 2008

Forces and Newton’s Laws of Motion

Forces and Newton’s Laws of Motion

But what ARE

FORCES?

But what ARE

FORCES?


©Spring 2008

Newton’s First LawNewton’s First Law

• Objects at rest stay at rest unless a “net force” is applied.


• Objects in uniform straight line motion stay in straight line motion unless a “net force” is applied.


• Note: Newton’s First Law Does NOT say…• Note: Newton’s First Law Does NOT say…

• This law does not say that every moving object has a “force” acting on it!• This law does not say that every moving object has a “force” acting on it!

• This law does not say an object at rest has “no force(s)” acting on it!• This law does not say an object at rest has “no force(s)” acting on it!


©Spring 2008

What does a “net Force” cause?What does a “net Force” cause?

What happens when you “force” a change on an object?What happens when you “force” a change on an object?

“change in velocity!”“change in velocity!”

Acceleration!!!Acceleration!!!

?? But what is it proportional

too?

But what is it proportional

too?


©Spring 2008

Forces cause AccelerationForces cause Acceleration

Is it always the same? Same force? Same Acceleration?

Is it always the same? Same force? Same Acceleration?

Objects “resist” a change…….why?Objects “resist” a change…….why?Objects “resist” a change…….why?Objects “resist” a change…….why?

InertiaInertiaHow do we measure

interia?How do we measure

interia?

MassMass


©Spring 2008

MassMass

Mass – The quantity of

matter (nanoscale stuff!) in an object is a measure of an object’s inertia.

Mass – The quantity of

matter (nanoscale stuff!) in an object is a measure of an object’s inertia.


©Spring 2008

Putting it all togetherPutting it all together

Net FORCE to cause a “change” in motion. (stop, start, change direction, etc…. Any change!)

Net FORCE to cause a “change” in motion. (stop, start, change direction, etc…. Any change!)

Net Force causes an ACCELERATIONNet Force causes an ACCELERATION

How much acceleration depends on the object’s MASS

How much acceleration depends on the object’s MASS


©Spring 2008

Newton’s Second LawNewton’s Second Law

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


a = Fnet/ma = Fnet/m

OR

Fnet = maFnet = ma

Units of Force?

Newtons

Units of Force?

Newtons


©Spring 2008

The More Mass Something Has, the Harder it is to Get A Significant Change in Motion.

The More Mass Something Has, the Harder it is to Get A Significant Change in Motion.

Applied ForceApplied Force

Resulting VelocityResulting Velocity

Applied ForceApplied Force

Resulting VelocityResulting Velocity


©Spring 2008

15N15N15N15N

6N6N6N6N 6N6N6N6N

More on “net Forces”More on “net Forces”

2kg2kg 6N6N6N6Na = F/m = (6N)/(2kg) = 3 m/sa = F/m = (6N)/(2kg) = 3 m/s22 a = F/m = (6N)/(2kg) = 3 m/sa = F/m = (6N)/(2kg) = 3 m/s22

to the rightto the rightto the rightto the right

2kg2kga = F/m = (6N – 6N)/(2kg) = 0 m/sa = F/m = (6N – 6N)/(2kg) = 0 m/s22 a = F/m = (6N – 6N)/(2kg) = 0 m/sa = F/m = (6N – 6N)/(2kg) = 0 m/s22

7N 2kg2kg

a = F/m = (15N – 7N)/(2kg) = 4 m/sa = F/m = (15N – 7N)/(2kg) = 4 m/s22 a = F/m = (15N – 7N)/(2kg) = 4 m/sa = F/m = (15N – 7N)/(2kg) = 4 m/s22

to the rightto the rightto the rightto the right


©Spring 2008

If There are More Than Two Forces, They are Added Together as VectorsIf There are More Than Two Forces, They are Added Together as Vectors

Force 1Force 1

Force 2Force 2

Net Force ofForce 1 and Force

2

Net Force ofForce 1 and Force

2

The resultant force is the net force The resultant force is the net force acting on the object.acting on the object.

The resultant force is the net force The resultant force is the net force acting on the object.acting on the object.

MassMass


©Spring 2008

FrictionFriction

Friction is a force! Friction is a force!

It “compels” objects to “change” their motion.

It “compels” objects to “change” their motion.

In fact, FRICTION always opposes motion.

In fact, FRICTION always opposes motion.


©Spring 2008

10 kg 1 m/s

A 10 kg box is being pushed across the floorat a constant velocity of 1 m/s. What is the netforce acting on the box?

A 10 kg box is being pushed across the floorat a constant velocity of 1 m/s. What is the netforce acting on the box?

Conceptual PitfallConceptual Pitfall

Net Force = ZERONet Force = ZERO


©Spring 2008

Conceptual PitfallConceptual PitfallA bus accelerates by the engine applying a force of 2000N East to the tires for 1 minute. Suddenly the bus encounters a strong wind with a frictional air resistance force of 2000N West. Which of the following happens?

A bus accelerates by the engine applying a force of 2000N East to the tires for 1 minute. Suddenly the bus encounters a strong wind with a frictional air resistance force of 2000N West. Which of the following happens?

The bus will continue to gain speed

The bus has no net force, therefore it stops because of the wind

The bus will continue to move with a constant speed

The bus will slow down because there is no acceleration

The bus will continue to gain speed

The bus has no net force, therefore it stops because of the wind

The bus will continue to move with a constant speed

The bus will slow down because there is no acceleration

The bus will continue to move with a constant speedThe bus will continue to move with a constant speed


©Spring 2008

Newton’s Third LawNewton’s Third Law

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

“To every action there is an equal and opposite re-action.”

“To every action there is an equal and opposite re-action.”


©Spring 2008

Newton’s 3rd in Action

Newton’s 3rd in Action

The force the person exerts on the

heavier boulder is equal in magnitude but opposite in direction to the force the boulder exerts on the person.EVEN IF THE BOULDER IS BEING PUSHED UPHILL!!!

The force the person exerts on the

heavier boulder is equal in magnitude but opposite in direction to the force the boulder exerts on the person.EVEN IF THE BOULDER IS BEING PUSHED UPHILL!!!


©Spring 2008

Wait just a moment...

I said Weight!

What is weight?

A measure of the Force of gravity acting on an object .

Weight = (mass)(acceleration of gravity)

Weight = (m)(9.8 m/s2)

Weight = (m)(g)


©Spring 2008

Wait just a moment...

Weight = (mass)(acceleration of gravity)

Weight = (m)(g)

Heavy Object? More force of gravity

Lite Object? less force of gravity

BUT!!! Each has same acceleration of gravity (9.8m/s2 down)

Dr. Joseph W. Howard ©Spring 2008 Motion So Far…. Distance vs. Displacement Speed vs. Velocity...

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