Unit 7 - Energy

What is energy? Energy: the ability to do work or cause change Has different forms and can change between them (like currency or things of value: Dollars, Euros, gold, IPhones, buildings) Measured in Joules (J) 1 J = 1 N·m = 1 kg·(m/s2)·m = kg·m2/s2

Forms of energy (2 categories) POTENTIAL ENERGY: (stored energy due to interactions between objects or stresses within a system). Includes… Gravitational potential energy: stored energy due to gravitational interaction between two objects

Forms of energy (2 categories) POTENTIAL ENERGY includes… Elastic potential energy: stored energy of compressed or stretched object (springs and rubber bands)

Forms of energy (2 categories) POTENTIAL ENERGY includes… Chemical potential energy: stored energy of chemical bonds (gasoline, firewood)

Forms of energy (2 categories) Speaking of chemical energy… Often measured in calories or Calories 1 calorie (cal) = 4.18 J 1 Calorie = 1 kcal = 1000 calories Calories: what is found on nutrition labels

Forms of energy (2 categories) 2. OTHER FORMS (not exhaustive list) Kinetic energy: energy of motion Thermal energy: energy of heat (also often measured in calories) Electrical energy: energy of electric charges Radiant energy: energy of light Sonic energy: energy of sound

Energy can’t be lost, only “lost” Law of Conservation of Energy: Energy cannot be created or destroyed, but may change forms. Energy may be “lost” as heat, but it does not disappear

KE = ½·m·v2 Mechanical Energy 1. Kinetic Energy (KE) Mechanical energy: energy related to an object’s position and movement. Includes: 1. Kinetic Energy (KE) KE = ½·m·v2 KE of a moving object is directly proportional to mass If mass increases 3x, KE increases 3x KE is exponentially proportional to velocity of object If velocity increases 4x, KE increases 16x

Mechanical Energy GPE = m·g·h 2. Gravitational potential energy (GPE) m = mass, g = acceleration due to gravity (9.81 m/s2), h = height relative to a reference point reference must be used/mentioned! The bigger the h or the bigger the m, the more GPE

Mechanical Energy EPE = ½ kx2 3. Elastic potential energy (EPE) k = spring constant, unique constant for each spring/material, x = distance stretched or compressed EPE is exponentially proportional to x If spring is stretched 3x as much, EPE increases 9x

Mechanical Energy If no energy is lost to friction or air resistance or as heat, mechanical energy is conserved (in reality this is never true) KEi + GPEi + EPEi = KEf + GPEf + EPEf We will usually only deal with two kinds (not all three) for any given problem (ignore one term on each side)

Mechanical Energy lost… In real life, energy is lost as heat. Examples: swinging on a swing without pumping eventually results in person sitting at rest because of air resistance and friction of bearings Roller coaster hills have to get smaller and smaller because of loss of energy to friction and air resistance

Practice Side 2 of energy transformation worksheet

Work and Energy Work: a specific kind of energy transfer involving movement of object Because work (W) describes an energy transfer (Δ energy), it has the same units as energy (Joules, J) (just like the price of something and the amount paid have the same units)

Positive Work Definition E transfer when a force is applied over a distance on an object, acting in the same direction as the movement. If the object doesn’t move, work has not occurred, even if energy is used in attempting to move it W = F x d

Positive Work Example A boy exerts a northward force of 10 N on a box 2 meters in the northward direction. 2 ft

Zero Work Example A boy exerts a northward force of 10 N on a box but doesn’t move it. He still uses energy, but has not performed work.

Negative Work Occurs when the force is applied in the opposite direction of the motion Example: the force of friction does work to slow down an object. Example: the force of gravity does negative work on an object launched vertically

Practice A waiter is holding up a tray that weighs 15 N. The waiter carries the tray across the room for 5.0 m. How much work on the tray has been performed?

Work-Energy Theorem: W = ΔKE = KEf – KEi = ½ mfvf2 - ½ mivi2 The work performed on an object is equal to the change in kinetic energy of the object W = ΔKE = KEf – KEi = ½ mfvf2 - ½ mivi2

Practice Practice: A 55.3 kg object is dropped and strikes the ground at a velocity of 28.0 m/s. How much work did gravity do on the object (ignore air resistance)? From what height was the object dropped?

Power The amount of energy transferred or work performed in a given amount of time Rate of energy transfer Power = Work (energy)/time 1 Watt (W) = 1 Joule/sec

Practice What is the power of a man lifting a 23 N box 0.005 m in 2.0 s? Does he have more or less power than a woman who lifts a 11.5 N box 0.005 m in 0.90 s?

Machines Contraptions that make work easier, but do not decrease the amount of work done. For a 100% efficient machine, Win = Wout Findin=Foutdout A machine can function by decreasing the necessary applied force (Fin), but at the expense at making the distance it is applied greater (and therefore taking longer)

Mechanical Advantage Measure of how much “easier” a machine makes a task M.A. = Fout/Fin M.A. greater than 1 means the machine makes work easier, but at the expense of distance, and therefore speed. M.A. less than one makes the work harder, but it can perform the task faster.

Example of M.A. < 1 A bicycle has a M.A. < 1. You have to put a greater force on the pedals per turn, but the wheels move the bike more than you feet move.

Machine Examples: Inclined Plane Push a mass for a longer distance with less force (effort) to reach a certain height.

Machine Examples: Pulleys Weight of mass is distributed over several ropes, decreasing tension and force required to move Rope must be pulled more than the distance the weight is lifted If the weight is pulled instead, work is harder, but faster (theatre curtains, M.A. < 1)

Machine Examples: Lever A plane is placed on a fulcrum If the fulcrum is closer to the load, the effort is less than the load. (M.A. > 1)

% Efficiency In the real world, a machine’s output work is less than the input work due to energy loss as heat. Friction is responsible for much of the inefficiency. % efficiency = Wout/Win x 100%