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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 ….

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  • “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
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
  • 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 newton b 25 dec 1642 d 31 march 1727
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 is 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

  • 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

  • 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?
straw grass through wood
Straw (grass) Through Wood

Small mass, great acceleration

  • One Ounce Projectile

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

  • Newton’s Jar


  • Car on a ramp


  • Eeyore on a car on a ramp

Momentum; inertia in motion

conservation of momentum
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…)
  • Newton’s Cradle

Conservation of Momentum

  • Swinging Rock

Potential and Kinetic energies; Conservation of Momentum

mass in a circular motion
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
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
Rotational Inertia Demos:
  • Bicycle tire
  • Gyroscope
  • Top
  • Frisbee
  • Football
  • Moment of Inertia
  • X-Zylo
angular momentum
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
  • 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
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…

  • Spinning on a chair with weights
  • Arms Out?
  • Pulling arms in?
centripetal force
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
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
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
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
What is the acceleration of these?Acceleration is Inversely Proportional to mass…

5 N

Acceleration = 6 kph

Acceleration = ?

5 N

5 N

Acceleration = ?

solutions acceleration is inversely proportional to mass
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
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
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
  • Syringe with water
  • Rocket Balloon
  • Beaker of Water on Balance
common forces
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 forces1
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
Other Physics concepts of interest:

The quality of an object that enables it to do work

  • Power


potential energy
Potential Energy
  • The stored energy that a body possesses because of its position with respect to other bodies
kinetic energy
Kinetic Energy
  • The energy of motion
  • Kinetic Energy = 1/2 mv2