Chapter 12 Forces & Motion

Forces  “a push or a pull”  A force can start an object in motion or change the motion of an object.  A force gives energy to an object

Representing forces  We often use arrows.  Larger the arrow, the greater the force  Combined balanced forces equal zero  Normal force = the force acting perpendicular on an object in contact with another object.

Balanced Forces  Forces opposite in direction and equal in size.  Net result: no change in motion.

No change in motion!  Balanced forces will keep an object moving at a constant velocity ENGINE FRICTION

Friction:  One of the most common forces.  Always acts in a direction opposite the direction of motion.  There are four main types of friction.

Static Friction  The force that must one must overcome to get an object to move.  Examples:  Trying to move a heavy object  Tires on pavement when a car is moving  Walking

Sliding Friction  Two surfaces sliding over each other.  Caused by two factors: 1. Weight of moving object 2. Types of surfaces in contact

Rolling Friction  An object rolling over a surface  Not as strong as sliding  Can be reduced by using wheels and ball bearings.

Fluid Friction WWhen an object is moving through a fluid (either gases or liquids) eex. a fish swimming under water, air resistance

Is friction ever helpful?  YES. Examples: 1. Stopping a car 2. Driving on ice 3. Walking

Bell Work  How do we define a force?  What do we call an upward directed force?  What type of friction occurs when you slide down a slide?  What type of friction is occuring when you are driving a car?

GRAViTY  The pull of one body of mass on another.

How fast do objects fall on earth? GGalileo 16 th century DDropped two metal spheres of different sizes off the Leaning Tower of Pisa WWhich hit first? TThey hit at the same time.

Acceleration due to gravity  All objects fall at the same rate, regardless of their mass or weight  The acceleration due to gravity on earth is about 9.8 m/sec 2.  This value will change with elevation and location on earth.

Terminal Velocity  After a period of free fall, the force of gravity will be canceled by the force of air friction and objects will travel at a constant velocity

Projectile motion  Any object thrown in the air becomes a projectile.  The object will move forward due to inertia, and downward due to gravity.  Always a curved path.

Homework Questions  Name 1 way in which gravity is good. Name 1 way it makes life harder.  What is the value for gravity?  If we neglect air resistance and we drop a feather and a bowling ball from the roof, which will hit the ground first?  Why do falling objects have a terminal velocity?  Is an airplane and example of a projectile? Why or Why not?

Bell Work  Get your Notes out  Come up with an example of each type of friction  Come up with 1 example of a projectile and 1 example that is not a projectile (but that flies through the air).  Quiz Today!

Bell Work  What is a force?  What are the 4 types of friction?  What happens when air resistance and gravity balance out in free fall.

Aristotle  384 B.C. – 322 B.C.  Greek scientist and philosopher  First to work with force.  Incorrectly stated that a force is required to keep an object moving at constant speed

Sir Isaac Newton   British Physicist  Considered the “Founder of modern physics”

Galileo’s Inertia  The Tendency of things to resist changes in motion  Too little force, too little time to overcome "inertia" of tableware.

Newton’s First Law of motion AAn object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force. OOften referred to as the Law of Inertia. (the property of matter that resists any change in motion)

Newton’s First Law of motion  Motion tends to continue unchanged.  The elephant at rest tends to remain at rest.

Mass and Inertia  Inertia is also called mass  Mass – measure of the quantity of matter in an object  Mass is measured in kilograms  One kilogram is the amount of mass in a 2.2 pound weight  1 Kg = 9.8 Newton's on Earth

Weight and force  The gravitational force exerted on an object  Forces are pushes or pulls  Force is measured in newtons.  A newton is the weight of a cube of butter.

Newton’s Second Law of Motion TThe force of a moving object is directly proportional to the object’s mass and acceleration. TThe most important of Newton’s laws.

How do we label force???  F = ma  Mass is measured in kg  Acceleration is measured in m/sec 2  If we multiply these two units : 1 kg m/sec 2 = 1 newton (N)

Weight and Mass  MASS  The measure of inertia of an object.  The amount of matter in an object  Stays constant with changes in location  A scalar  WEIGHT  A measure of the force of gravity acting on an object.  Measured in newtons.  Changes with changes in location.  A vector

Bell Work  What is newton’s 1 st law?  Give an example  What is newton’s 2 nd Law?  Example?

Newton’s Third Law of Motion  For every action there is an equal and opposite reaction, or  All forces occur in pairs  examples:

Momentum AAll moving objects have momentum. IIt is the product of an object’s mass and velocity. WWhich has more momentum? A moving car or a moving train? NNot enough information. AA 1500 kg car traveling 50 m/sec? Or a 48,000 kg train traveling 2 m/sec?

Universal Forces  Electromagnetic force:  Forces that attract and repel.

Nuclear forces  STRONG  Responsible for holding the nucleus together  Strongest known force  WEAK  Very small attractive force in the nucleus

Newton’s Law of Universal Gravitation  Every object in the universe attracts every other object.  Depends on two factors: 1. Distance between the objects 2. Mass of the objects

The Value of G = 6.67x N m 2 / kg 2  Determined experimentally in 1798 by Cavendish  The magnitude G is the force between any two objects in the universe  G is a very weak force

Philipp von Jolly’s – measurement experiment  Rolled a 6 ton lead ball under a 1 kg mass.

Inverse Square Law

The Earth, Moon and Tides  Earth’s gravitational pull on the moon combined with the inertia or momentum of the moon keeps the moon in orbit around the earth.  The pull of the moon and sun causes the tides

Satellites  Satellites must be placed at a specific height above the earth’s surface in order to stay in orbit.  If it is too close, or moving too slow, it can spiral into the earth’s atmosphere and burn.

Action of the Moon and Tides  The depth of water over each point changes as the Earth rotates.  This produces two high and two low waters per day

Assignment:  Page  11-24, 26-27, 31,33