Physics â¦ Force &amp; Motion

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# Physics â¦ Force &amp; Motion - PowerPoint PPT Presentation

Physics … Force &amp; 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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Presentation Transcript
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

Tennis Ball
• Rubber Ball
Paired Verbal Fluency
• 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

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 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
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.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/
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

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

Conservation of Momentum

• 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
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…

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
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

Acceleration = 6 kph

Acceleration = ?

5 N

5 N

Acceleration = ?

SolutionsAcceleration 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 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

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 motion
• Kinetic Energy = 1/2 mv2

P

K

K

P