UNIT 2 MECHANICS Chapter 5 Dynamics

Chapter 5A - Forces

Fundamental Forces Classifying forces Force A push or pull on a system, may be a vector or scalar quantity The magnitude is represented by F The SI unit of force is a newton, N Three Classes Two broad categories Contact and Field forces Four fundamental

Facet of Science – Fundamental Forces Read… Facet on page 92 Four fundamental forces that are the source of all observed forces in the universe Responsible for holding together the structure of all matter in the universe Listed in decreasing strength Strong nuclear force Electromagnetic force Weak nuclear force Gravitational force

Fundamental Forces Strong nuclear force Electromagnetic force Strongest of all forces Holds protons and neutrons together Electromagnetic force Attractions and repulsions due to electrical charges Weak nuclear force Holds particles inside the proton together (quarks) Gravitational force Exerted between all forms of matter The weakest of all forces

Contact Forces Contact force Includes Acts between systems only when one system touches another Also called mechanical force Includes Compression Pushes things together Tension Pulls things apart Torsion Twists objects Shear Causes portions of a material to distort or move in opposite directions along parallel planes Friction Discussed more later

Field Forces Field forces Principal field forces A noncontact force exerted on a susceptible object by similar objects Extend outward indefinitely Diminishes rapidly with distance Principal field forces Gravitational field Surrounds every material object Everything in the universe exerts gravitational force Magnetic field Surrounds magnets and materials with electricity flowing though them Iron and other magnets Electric field Surround and affect electrical charges

Balanced and Unbalance Forces Every object in the universe has more than one force acting on it Why isn’t everything moving all over the place? Balanced forces A force that in combination with other forces acting on the same system produces a zero net force on the system External force A force exerted on a system by something in its surroundings Net force The single unbalance force acting on a system that is the sum of all forces acting on the system When balanced it equals zero Unbalanced force A force that in combination with other forces acting on the same system results in a nonzero net force and produces a change in motion of the system

Chapter 5B – Newton’s Laws of Motion

How Newton’s Laws Came to Be… Read…. Page 95, Imaging Inertia Inertia The tendency of all matter to resist change in motion Laws of motion Newton’s three laws of motion, which define the science of dynamics and apply to all areas of mechanics

The First Law The Law of Inertia Mechanical equilibrium States that objects at rest remain at rest and objects in motion continue in a straight line at a constant velocity unless acted upon by a net external force Inertia cannot be measured directly and has no units Mass is a measure of inertia Hard to see on earth because of gravity and friction Mechanical equilibrium The condition in which all forces acting on a system are balance as indicated by the system’s lack of acceleration

The Second Law The Law of Accelerated Motion States that the acceleration of a system is directly proportional to the net force acting on the system and is inversely proportional to the system’s mass Compare this to a small car and a large SUV Formula: force=mass times acceleration F=ma a=acceleration F= force m=mass Unit of force=Newton=(kg.m)/s2 Know this for future problems like Example 5-1

The Third Law The Law of Action-Reaction Normal force For every external force exerted on a system by its surroundings, the system exerts an equal but opposite force on its surroundings Also called the action-reaction principle Normal force The force exerted by a supporting surface on an object resting on it; always perpendicular to the supporting surface

Friction Friction Several kinds of friction A contact force that opposes the movement of objects past each other Vector or scalar quantity Is everywhere and affects nearly all motion Slows things down and wears things out Several kinds of friction Static-stationary objects Kinetic-sliding objects Rolling-rolling & stationary objects Fluid-liquid & a gas

Five Factors Affecting Friction Always occurs between systems in contact Depends on the kinds of materials in contact The friction vector acts on the system in the direction opposite to its motion The magnitude of friction is proportional to the normal force acting on the system of interest The friction between two stationary objects is greater than the friction between the same two objects when they are moving past each other

Chapter 5C – Gravity and Free Fall

The Gravitational Field Gravity Basic property of all matter Depends on three quantities Masses of the two objects Distance between the centers of the objects Law of universal gravitation States that the force of gravity between two objects is directly proportional to the products of their masses and inversely proportional to the square of the distance between their centers of mass

Free Fall Read… Air resistance Free fall Page 101 Friction on an object moving through the air Increases with surface area of object Free fall The condition of an object accelerated by the force of gravity alone with no other forces acting on it Can only take place in a vacuum

Gravitational Acceleration and Weight The acceleration of an object due to gravity 9.81 m/s2 regardless of mass Vector or scalar Formula: derived from Newton’s second law w=mg w=weight in newtons m=mass in kilograms g=gravitational acceleration in m/s2 Example 5-2

Free Fall and Distance How far does an object move per second?? Formula 1 – Determine velocity v=g t v= free-fall speed in m/s g=acceleration due to gravity in m/s2 t=total time of free fall in seconds Formula 2 – plug into distance formula d=v t d=distance in meters v=average speed in m/s t=total time in seconds

Free Fall and Distance Uniform acceleration A constant acceleration, such as that of an object in free fall Formula 3 (based off other two) Valid for any object starting from rest and dropping vertically in free fall if air resistance is not significant d=1/2g( t)2 g=9.81 m/s2 Example 5-3

Free Fall and Air Resistance Read… 5.13 Drag (5th Type of Friction) A form of friction exerted by a fluid on an object moving in relation to the fluid Air resistance is an example Terminal velocity The maximum constant speed of an object falling in the atmosphere Occurs when drag balances with the force of gravity Depends on the surface area compared to the mass