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 ballShows how force is required to change shape

Tennis ballChange of shape- Elasticity

Pushing on a tableForces occur in pairs; Net Force

• Tennis Ball

• http://www.youtube.com/watch?v=sxS-m9CRu34

• Rubber Ball

• http://www.youtube.com/watch?v=N8gxND1kX6k

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

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

• http://www.youtube.com/watch?v=WOvwwO-l4ps

Demos:

Ping pong ballSmall inertia; easy to move & stop

Bowling ballLarger inertia; hard to move & stopGrannySmall inertia; easy to move & stopMack 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/aF = 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

MomentumCar 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 CradleConservation of Momentumhttp://www.youtube.com/watch?v=7_AiV12XBbI&safety_mode=true&persist_safety_mode=1&safe=active

Swinging RockPotential 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 MomentumA measure of an object’s rotation about a

particular axisFor 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:

mvr = mvr… 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 #2The 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

2nd 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

5 N

5 N

Acceleration = 6 kph

Acceleration = ?

Acceleration = ?

SolutionsAcceleration is Inversely Proportional to mass…

5 N

5 N

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

Acceleration = 6 kph

Acceleration = 6 kph

Newton’s Third Law

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

• 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 motionKinetic Energy = 1/2 mv2

As an object falls, its energy

transformsfrom

all Potential to

all Kinetic

P K

P K

At which point will the

Potential and

Kineticbe equal?

P K

P K

Right here! ½ way