Energy: Basics

Definitions Energy - the ability to do work Work - the transfer of energy by applying a force through a distance But what is a “force”?

Position Position - orientation and distance an object is from some origin; measurement of position requires a coordinate system If the position does not change, the object is easily found Displacement - change in position; if position is designated with the vector r, then displacement is  r

Velocity Defn. - time rate of change of displacement; is a vector quantity; SI unit = m/s Average velocity = = Displacement  r Elapsed time  t Instantaneous velocity = limit (average velocity) t0t0 What is the average velocity of a dragster that takes 5.5 seconds to go the 400 meters down the dragstrip?

Speed Some books say that velocity is speed + direction. WRONG! Average speed = Distance traveled Elapsed time Displacement = Distance traveled Displacement on racetrack is 0, while distance travelled is not

Acceleration Defn. - time rate of change of velocity; is a vector quantity; SI unit is m/s 2 Accelerations can occur without changing the magnitude of velocity; Ex. Object going in circle at constant rate Average acceleration = vv tt

Newton’s First Law “ An object at rest, or in a state of constant motion, will continue in that state unless acted upon by an unbalanced force.” Really, Galileo’s Inverse of statement is very important: if an object is acceleration, then a net force is operating on it, even if you cannot see the reason for the force. Is there a force operating in this picture, and if so, from what direction?

Newton’s Second Law F = ma Relates kinematic variables to dynamic ones Can measure accelerations  calculate forces Note: SI unit is newtons, English is pounds Incorrect to say that X pounds = Y kilograms What force is needed to accelerate a 1000 kg car to 5 m/s 2 ? Not all forces are constant

Newton’s Third Law “ For every force, there is an equal and opposite reaction force.” Often misunderstood; actually means that one object acting on a second object will have the second object act on it Mule pulls on cart. Cart pulls back on mule with equal and opposite force. “Why pull?”, says mule, if force will be negated.

Get Back To Work Work - the transfer of energy by applying a force through a distance W = F x d if F is constant  W = F n x  d if F varies Lifting box: F = mg Distance lifted = h W = mg x h = mgh

Simple Machines Allow for the same amount of work to be done, but with smaller forces Trade-off of using a smaller force is that the force is applied through a longer distance Box lifted straight up a height h, force supplied is F = mg Force of gravity down inclined plane is F = mg sin  = mgh/L Distance pushed up plane = L

Power Power = = rate of energy usage EE tt How much power do you expend by climbing 3 flights of stairs (10 m) in 10 seconds? Can deliver the same amount of energy to a system using less power, but it takes a longer amount of time Our Western mindset usually screams for more power Ex. SUV’s require more powerful engines; larger homes require more powerful a.c.

Potential energy Energy stored within the force between two objects separated by a distance; if objects are allowed to move, force is applied through distance = work done TYPES OF POTENTIAL ENERGY: Gravitational Chemical Nuclear

Potential energy due to gravity Water behind a dam A rock at the top of a steep hill EXAMPLES: If the water or rock drops, gravity operates over a distance, thereby doing work. This work converts the potential energy to kinetic energy. Example: Gravitational potential energy

A moving object has momentum. If it hits another object, it will transfer energy to it by applying a force through a distance, i.e. work ENERGY OF MOTION Some of the bullet’s kinetic energy is transferred to the apple during the collision Kinetic energy of falling water is converted to motion of turbines when water falls on them Kinetic energy

Energy needs in the modern world

How do our current uses of energy compare with those in the “old days”?

THEN : Chemical energy in livestock (sugar, fat) NOW : Chemical energy in gasoline AGRICULTURE

THEN : Chemical energy in humans (sugar, fat) NOW : Fossil fuels, electricity from chemical energy in coal INDUSTRY

THEN : Chemical energy in biomass NOW : Electricity from chemical energy in coal LIGHT

THEN : Chemical energy in biomass (wood) NOW : Fossil fuels, electricity from chemical energy in coal HEAT & COOKING

THEN : Chemical energy in humans (sugar, fat) NOW : Chemical energy in fossil fuels LANDSCAPING

THEN : Chemical energy in humans or animals NOW : Chemical energy in fossil fuels TRANSPORTATION

THEN : Chemical energy in humans NOW : Electricity from chemical energy in coal EDUCATION

We now use energy from fossil fuels instead of energy from humans, animals or biomass MORAL:

U.S. Energy Consumption Over the last 50 years, our consumption of energy has increased (except for after energy crises) Because of more efficient devices, our consumption per person has stayed about the same over the last 30 years Source: Dept. of Energy,

One Case: Crude Oil Source: Dept. of Energy, We get energy from many different sources. One of the more important ones we will discuss is crude oil. What are the implications of this graph? What historical events occurred during this time that relate to crude oil?

Import Countries Since the mid-1970’s, we have increased our dependence of oil imports on non-OPEC countries We have increased our reliance on oil from North and South America Why?