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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 …. - PowerPoint PPT Presentation
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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 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