Physics: Key Concepts in Mechanics, Thermodynamics, and Fluid Dynamics

Scientific hypothesis

a tentative explanation for anobservation, phenomenon, orscientific problem that can betested by further investigation

Scientific law

a hypothesis that has been well tested and not contradicted

scientific theory

a synthesis of many well-tested hypotheses

Galileo (1564-1642)

Was the first to show that experimentation was the best test of knowledge and to provide conclusive observation andexperiment regarding falling bodies

Inertia

If there is nointerference with the moving object, it would keep moving in a straight line forever, with no push or pull necessary

Net Force

external forces using (N) Newton Units and is a vector that uses both magnitude and direction

Newton's 1st Law (inertia)

Object at rest stays at rest; object in motion remains inmotion at a constant velocity unless acted upon by a net(unbalanced) external force

Newton's 2nd Law (cause and effect)

An unbalanced external force acting on an object causes the object to accelerate and can be written as F=ma or a=F/m

Newton's 3rd Law (action and reaction)

For every action (force) there is an equal and oppositereaction (force)

The Equilibrium Rule

If an object maintains its motion (constant speed,constant direction), then the net force is zero.

Static equilibrium

net force is zero while an object is at rest

Dynamic equilibrium

net force is zero while an object is moving at a constant velocity

Speed

the rate of change of position or distance with respect to time

Instantaneous speed

what you read when you look atyour speedometer

Velocity

speed and direction

Acceleration

Rate of change of velocity

Three ways to change velocity

change speed, Change direction of motion, Change speed and direction of motion

Three ways to cause acceleration

make the object move faster, make the object move slower, change the direction of motion without changing the speed

Free fall

the downward force of gravity with zero velocity at the top and has constant force/acceleration for the entire duration of "fall". This is used to calculate speed/velocity at any point in time

Forces of Gravity

always acts vertically down (towards the center of the Earth), and the normal(support) force always acts perpendicular to the plane that the object sits on.

Forces of Friction

always opposes motion and depends on two materials in contact and is always parallel to the surface. When it has static friction - the object is at rest. When it has Sliding friction - object in motion relative to another object.

Mass

the amount of "stuff" that makes up the object.The amount of "stuff" does not change with location. Thus,the mass of the object is always constant.

Weight

the measure of the force of gravity acting on a body that depends on the gravitationalacceleration (g) on that planet. The weight in space is zero

Momentum

can be interpreted as a measure of inertia and is also a vector

How do you change momentum?

we can change mass, velocity (speedor direction), or both

Conservation of Momentum

Newton's 1st Law tells us that the motion of the object does not change when there are no unbalanced external forces

When is the total momentum of the system conserved?

when there are no unbalanced external forces acting on the system

Kinetic Energy

Energy of motion; can only be zero (no motion) orpositive; can never be negative

Potential energy

Energy of position/height; we have to definereference point that we measure the height from

Law of conservation of energy

Energy cannot be created or destroyed; it may be transformedfrom one form into another, but the total amount of energy neverchanges (as long as there is no work done).

How do you change the energy of the system?

By putting work in or out of the system

Solids

Closely packed particles, rigid (often with crystal structure), definiteshape and volume, high density

Liquids

Closely packed particles, no structure, definite volume but no definiteshape, high density but often lower than for solids

Gasses

Particles are spread out (spacing between particles is roughly 10 timesthe particle size), no definite shape, no definite volume, low density

Density

the amount of "stuff" in the given volume that applies to solids, liquids, and gases. It depends mass of the atoms and spacing between the atoms. It's a property of the material—it does not matter how much material you have.

Pressure

the force applied perpendicular to a surface per unit area.

Example of Pressure

When you push on your arm using just your finger, it doesn't hurt. But ifyou use a thumbtack and push with the same force it hurts a lot more.

Liquid Pressure

Force per unit area that a liquid exerts on another object

Buoyancy

higher pressure on bottom of object exerts a net upward force. Sink when weight of submerged object is greater than the buoyant force. Float when weight of submerged object is less than the buoyant force it would have when submerged—when floating, buoyant force = weight of floating object.

Archimedes' Principle

Discovered by Greek scientist Archimedes. Relates buoyancy to displaced liquid. States that an immersed body (completely or partially) is buoyed up by a force equal to the weight of the fluid it displaces. Buoyant force = weight of displaced fluid

Bernoulli's Principle

When the speed of a fluid increases, internal pressure decreases.

Atmospheric Pressure

Recall that pressure is defined as force per unit area. Density of air decreases with increasing altitude. Because the higher you go in the atmosphere,the fewer molecules are above, so the force ofgravity is lower

Barometers

A device for measuring pressure. One of the units of pressure (used a lot inexperimental physics) is Torr. Traditional barometer uses mercury, because of itshigh and uniform density (about 13 times thedensity of water), but other liquids can be used•"Normal" (average) atmospheric pressure at sealevel is 760 mm mercury

Boyle's Law

The pressure and volume of a gas enclosed in a spaceare inversely proportional. If you increase pressure, the volumewill decrease by the same factor.

Temperature

Atoms and molecules in matter are in continuous random motion. Kinetic energy of the particles can be represented in terms of temperature.

Heat

energy transfer between different objects due to temperature gradient. By itself (without input of work), heat flows from regions of higher temperature to regions of lower temperature. When two objects are in thermal contact, heat flows from warmer to colder object. When heat is added to the substance, its temperature and internal energy increase. When heat is removed from thesubstance, its temperature andinternal energy decrease. As heat flows from one substance to another, theirtemperatures get closer and closer to each other until theyreach thermal equilibrium

calorie

amount of energy required to raise thetemperature of one gram of water by one degree Celsius

Thermal energy

the energydue to heat flow, or due tothermal motion of molecules

Internal energy

the total of all energies inside a substance

Second Law of Thermodynamics

It is possible to make heatflow from cooler regions towarmer regions by doingwork on the system or byadding energy fromanother source

Heat Capacity

Heat capacity is the amount of energy to be added or removed to change the temperature of a substance by 1°C. when heat is added, some substances increase in temperatures faster, and some slower.

Specific Heat Capacity

Different materials require different quantities of heat to raise their temperature. The quantity of heat required to change the temperature of a unit mass (1g or 1kg) of a substance by 1 degree is called the specific heat, or specific heat capacity

Specific Heat Capacity of Water

Water has the highest specific heat capacity among the common materials. Because of this property, water takes longer to cool down and heat up

Conduction

transmission of heat through anexchange of energy between atoms and molecules asa result of successive collisions

Heat conductors

How well a solid object conducts heat depends on the bonding within that material. Metals are excellent conductors of heat because they have free electrons that are able to move around

Convection

Heat transfer due to the actual motion of the "blobs" of material. Only fluids (liquids and gases)are capable of convection. As the fluid is heated, the molecules start movingfaster and "blobs" of fluid become less denseand moving upward.

Radiation - emission

Transmission of energy through empty space, usually in the form of electromagnetic waves (including radio and microwaves, visible light ,x-rays, etc.) All substances with a temperature above absolute zero emit radiant energy over a range of frequencies

Radiation - absorption and reflection

Everything that emits radiant energy also absorbs radiant energy. If more energy is absorbed than emitted, the temperatureof the object rises; if more energy is emitted thanabsorbed, the temperature of the object drops

Newton's Law of Cooling

Objects hotter than their surroundings eventulaly cool down to match the surrounding temperature

Greenhouse Effect

all objects radiate, and the frequency and wavelength of radiation depends on the temperature. High-temperature objects emit short-wavelength radiation; low-temperature objects emit longer-wavelength radiation. Infrared light has LONGER wavelength; visible light hasSHORTER wavelength; UV light has EVEN SHORTERwavelength

Temperature vs. Heat Added

Phase change and temperature change do not happen simultaneously. Graph shows energy involved in heating and phase changes of 1g of water. When energy is added to (or removed from) water, either its temperature or its phase changes

Phase change

Recall three states of matter: solid, liquid, gas. We can transition between different states of matter by adding or removing energy. Removing energy can beaccomplished by a heat pump

heat pump

"pumps" energyfrom a cooler region to awarmer region

Electric Charges

Electric charges can be positive (protons) and negative(electrons) All the electrons of all the atoms are identical; each has thesame mass and the same negative charge

Electric Force

Electric force varies inversely as the square of the distance between two charged objects. Can be both attractive (for "like" charges) and repulsive (for "unlike" charges)

Conductors and Insulators

The presence of "loose" electrons that are free to move throughout the material. Usually good conductors of electricity are also good conductors of heat, and vice versa

Electric field

Force per unit charge

Series Circuits

All bulbs (resistors) connected sequentially. Current is the same through each bulb. If you disconnect one bulb...... ALL go out Electric Circuits

Parallel circuits

All bulbs (resistors) connected to the twoends of the battery

Ohm's law

an equation relating these quantities. Current through a device (A) Voltage difference across a device. Resistance of a device

Magnetism

a fundamental force in physics that describes the attraction and repulsion of objects due to their invisible magnetic fields. The subjects of magnetism and electricity developed almost independently until Hans Christian Oersted discovered (in 1820) that electric current affects a magnetic compass.

How is magnetic forces similar to electric forces?

Like electric forces, magnetic forces act at a distance, obey inverse square law, can be attractive and repulsive,

Magnetic Poles

Give rise to magnetic forces. Instead of calling the different poles "positive" and "negative", we call them "north" and "south" for historic reasons. It's impossible for a single pole to exist separately. The strength of the magnetic force is directly proportional to the strength of the poles, and inversely proportional to the square of the distance between the poles

Magnetic Fields

If you sprinkle some iron shavings around a magnet, you will see that the shavings align themselves up in a pattern repeating the magnetic field of a magnet

Magnetic Fields

If you sprinkle some iron shavings around a magnet, you will see that the shavings align themselves up in a pattern repeating the magnetic field of a magnet. Magnetic field lines point from north pole to south pole

Vibration

a periodic wiggle in time

Wave

a periodic wiggle in time and space

Frequency

the number of cycles of a repeating event that occur in a unit of time