# 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.

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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… 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

Demos 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

Paired Verbal Fluency Read the biography of Sir Isaac Newton Find a Partner Decide who will go first

Isaac Newton b : 25 Dec 1642 d: 31 March 1727 Principia Mathematica Philosophiae Naturalis

Newton’s Three Laws of Motion

Law #1: “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 is Inertia ? “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

Demo Tennis Ball and Cup Tennis ball has inertia; Newton’s First Law

Demos: 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

Mass 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

The product of MASS and VELOCITY It is inertia in motion Are you afraid of a piece of grass? Momentum

Straw (grass) Through Wood Small mass, great acceleration

Demos: 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 Momentum 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: Newton’s Cradle Conservation of Momentum http://www.youtube.com/watch?v=7_AiV12XBbI&saf ety_mode=true&persist_safety_mode=1&safe=active Swinging Rock Potential and Kinetic energies; Conservation of Momentum

Mass in a Circular Motion When object moves in circular path it has a property called Rotational Inertia AKA: Gyroscopic Inertia

Circular Motion con’t… “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: Bicycle tire Gyroscope Top Frisbee Football Moment of Inertia X-Zylo

Angular Momentum 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 Momentum

Demos : 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-How 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

The following formula is similar to those for Newton’s First Law, but deals with mass in motion in a circular path: mv r = m v r … The mass is not affected by a smaller radius, while velocity is…

What happens in this situation? m v r = ? v r

Demo Spinning on a chair with weights Arms Out? Pulling arms in?

Centripetal Force 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 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 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 nd Law Formulas You’ve seen these before: a = F/m F = m (a) m = F/a

What is the acceleration of these? Acceleration is Inversely Proportional to mass… 5 N Acceleration = 6 kph Acceleration = ?

Solutions Acceleration is Inversely Proportional to mass… 5 N Acceleration = 6 kph Mass is doubled so a/ 2 = 3 kph Mass is tripled so a/3 = 2 kph

Acceleration is Directly Proportional to force… 5 N 10 N = (2 x f) 15 N = (3 x f) Acceleration = 6 kph Acceleration = 12 kph 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 10 N = (2 x f) 15 N = (3 x f) Acceleration = 6 kph

Newton’s Third Law 3 rd 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 Syringe with water Rocket Balloon Beaker of Water on Balance

Common Forces 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 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

Other Physics concepts of interest: ENERGY The quality of an object that enables it to do work Power WORK/ TIME

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

Kinetic Energy The energy of motion Kinetic Energy = 1/2 mv 2

As an object falls, its energy transforms from all Potential to all Kinetic P K P K

At which point will the Potential and Kinetic be equal? P K P K Right here! ½ way

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