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# Physics … Force & Motion - PowerPoint PPT Presentation

Physics … Force & Motion. FORCE …. “Any influence that can cause a body to be accelerated . It is measured in NEWTONS .” A Newton is the force needed to accelerate one kg one meter per second per second A push or pull exerted by an object to/on another object. FORCE cont ….

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Physics… Force & Motion

• “Any influence that can cause a body to be accelerated. It is measured in NEWTONS.”

• A Newton is the force needed to accelerate one kg one meter per second per second

• A push or pull exerted by an object to/on another object

• Always occurs in pairs (Sometimes called Agent & Receiver)

• It can alter the shape or motion of an object

• It is represented by arrows that show direction and size

• Rolling Ball

Shows Acceleration

• Push – Pull; Pushing a closed door; standing on the floor

All show that forces occur in pairs

• Clay ball

Shows how force is required to change shape

• Tennis ball

Change of shape- Elasticity

• Pushing on a table

Forces occur in pairs; Net Force

• Tennis Ball

• Rubber Ball

• Read the biography of Sir Isaac Newton

• Find a Partner

• Decide who will go first

Isaac Newtonb: 25 Dec 1642 d: 31 March 1727

Principia MathematicaPhilosophiae Naturalis

• “Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.”

• Sometimes called the Law of Inertia

What isInertia?

• “The sluggishness or apparent resistance a body offers to changes in its state of motion”

• The larger the mass, the larger its inertia

• A body with less mass is easier to accelerate

• A body with more mass is harder to accelerate

• Tennis Ball and Cup

Tennis ball has inertia; Newton’s First Law

Demos:http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• Ping pong ball

Small inertia; easy to move & stop

• Bowling ball

Larger inertia; hard to move & stop

• Granny

Small inertia; easy to move & stop

• Mack Truck driven by Granny 

Larger inertia; hard to move & stop

Masshttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• Defined as “The quantity of matter in a body”

• It is the measure of Inertia

m = mass; F = force; a = acceleration

m = F/a

F = m(a)

a = F/m

Momentumhttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• The product of MASS and VELOCITY

• It is inertia in motion

• Are you afraid of a piece of grass?

Straw (grass) Through Woodhttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

Small mass, great acceleration

Demos:http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• One Ounce Projectile

A small mass accelerated at high velocity can be produce a lot of force

• Newton’s Jar

Momentum

• Car on a ramp

Momentum

• Eeyore on a car on a ramp

Momentum; inertia in motion

Conservation of Momentumhttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• All energy stored in a moving object is not lost, but only changes form

• This is just like “Energy cannot be created nor can it be destroyed, it can only change form” (Stated by: Albert Einstein; Phillip Morrison, Mr. Watts…)

Demos:http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

Conservation of Momentum

• Swinging Rock

Potential and Kinetic energies; Conservation of Momentum

Mass in a Circular Motion http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• When object moves in circular path it has a property called Rotational Inertia

• AKA: Gyroscopic Inertia

Circular Motion con’t…http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• “An object rotating about an axis tends to remain rotating about that same axis unless it is interfered with by some external force.”

• This definition is similar to Newton’s First Law of Motion

Rotational Inertia Demos:http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• Bicycle tire

• Gyroscope

• Top

• Frisbee

• Football

• Moment of Inertia

• X-Zylo

Angular Momentumhttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• A measure of an object’s rotation about a particular axis

• For an object small compared to the radial distance, it is the product of mass, velocity and radius

• It is the measure of the rotational property of motion

• It is always perpendicular to the centripetal force

Angular Momentumhttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

Demoshttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/:

• Rolling objects

• Washer on a string

• Slingshot (old-fashioned!)

• Bucket with water spun in vertical circles

• Q: What holds the water in the bucket?

• Spin cycle of washing machine

• “Whizzer”

• Brendan on the Scrambler

Carnival Ride Know-Howhttp://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

• It’s better to take the OUTSIDE seat of the car if you’re riding alone

• If you’re riding with someone else, take the INSIDE seat

What happens in this situation? First Law, but deals with mass in motion in a circular path:

m v r = ? v r

Demo First Law, but deals with mass in motion in a circular path:

• Spinning on a chair with weights

• Arms Out?

• Pulling arms in?

Centripetal Force First Law, but deals with mass in motion in a circular path:

• A center-seeking force that causes an object to follow a circular path.

• “Any force that is directed at right angles to the path of the moving body and produces circular motion.”

Centrifugal Force First Law, but deals with mass in motion in a circular path:

• A fictitious outward force due to rotation

• It is a reaction to centripetal force but there is no agent, therefore it is experienced relative to position

• What we are really experiencing is ANGULAR MOMENTUM!

Newton’s Second Law First Law, but deals with mass in motion in a circular path:

Law #2

• The acceleration of a body is directly proportional to the net force acting on the body and inversely proportional to the mass of the body and is in the direction of the NET FORCE

2 First Law, but deals with mass in motion in a circular path:nd Law Formulas

• You’ve seen these before:

• a = F/m

• F = m (a)

• m = F/a

What is the acceleration of these? First Law, but deals with mass in motion in a circular path:Acceleration is Inversely Proportional to mass…

5 N

Acceleration = 6 kph

Acceleration = ?

5 N

5 N

Acceleration = ?

Solutions First Law, but deals with mass in motion in a circular path:Acceleration is Inversely Proportional to mass…

5 N

Acceleration = 6 kph

Mass is doubled so

a/ 2 = 3 kph

5 N

Mass is tripled so

a/3 = 2 kph

5 N

Acceleration is First Law, but deals with mass in motion in a circular path:Directly Proportional to force…

5 N

Acceleration = 6 kph

10 N =

(2 x f)

Acceleration = 12 kph

15 N =

(3 x f)

Acceleration = 18 kph

What if I want the Acceleration to be the same for all these bodies?Acceleration is Directly Proportional to force…

5 N

Acceleration = 6 kph

10 N =

(2 x f)

Acceleration = 6 kph

15 N =

(3 x f)

Acceleration = 6 kph

Newton’s Third Law bodies?

3rd Law

• “Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first”

• For every action, there is an equal and opposite reaction

• You cannot touch without being touched

Demo bodies?

• Syringe with water

• Rocket Balloon

• Beaker of Water on Balance

Common Forces bodies?

• Gravitational – non contact

Every object exerts an attractive force on every other object

• Buoyancy - contact

The tendency to float in a fluid

• Friction - contact

The resistance to motion where two objects touch

Common Forces bodies?

• Magnetic – non contact

The repelling or attraction of a magnet and ferromagnetic material

• Elastic - contact

Ability to spring back to its original size, shape, or position after being stretched, squeezed, flexed, expanded, etc.

• Electrical – non contact

Flow or gathering of a negative charge by electrons

• ENERGY

The quality of an object that enables it to do work

• Power

WORK/ TIME

Potential Energy bodies?

• The stored energy that a body possesses because of its position with respect to other bodies

Kinetic Energy bodies?

• The energy of motion

• Kinetic Energy = 1/2 mv2

As an object falls, its energy transforms bodies?from all Potential to all Kinetic

P

K

K

P

At which point will the bodies?Potential andKineticbe equal?

P

K

Right here! ½ way

K

P