Section 1: The Nature of Energy

• Energy is the ability to cause change.

• There are many different forms of energy:– chemical– thermal– kinetic– potential– electrical

Potential and Kinetic Energy

Kinetic Energy

• energy in the form of motion

• KE = ½ mass x velocity2

KE = ½ m x v2

• SI unit—Joule (J)

Potential Energy

• stored energy due to position

• types of potential energy– elastic: energy stored by something that can

stretch or compress (ex. rubber band, spring)– chemical: energy stored in chemical bonds

(ex. gasoline, food)– gravitational: energy stored by objects that

are above Earth’s surface(ex. apple or leaf in tree)

Gravitational Potential Energy (GPE)

• GPE = mass x 9.8 m/s2 x height

• SI unit—Joule (J)

• Bjorn is holding a tennis ball outside a 2nd story window (3.5 m from the ground) and Billie Jean is holding one outside a 3rd story window (6.25 m from the ground). How much more GPE does Billie Jean’s tennis ball have? (Each tennis ball has a mass of 0.06 kg).

Section 2: Conservation of Energy

• Energy is most noticeable as it transforms from one form to another.

• mechanical energy = total energy (kinetic + potential)

Bonus: How do your kinetic and potential energy relative to an elevator change as you go up?

Energy Conversion

Law of Conservation of Energy

• Energy cannot be created or destroyed.

• The total amount of energy in the universe remains constant.

Work (Chapter 5-1)

• Work is the transfer of energy that occurs when a force makes an object move.

• Two conditions must be met for work to be done on an object:– the object has to move– the movement must be in the direction of the

force

Work and Energy

• When work is done, a transfer of energy always occurs.

Calculating Work

• Work = force x distance W = F x d

• SI unit: Joules (J)

• One joule is about the amount of work required to lift a baseball a vertical distance of 0.7m.

Example Problems

• You move a 75 kg refrigerator 35 m. This requires a force of 90 N. How much work was done while moving the refrigerator?– 3150 J

• When you and a friend move a 45 kg couch to another room, you exert a force of 75 N over 5 m. How much work did you do?– 375 J

Bonus

• Suppose you used a force of 50 N to shoot an arrow, and the arrow flew 25 meters. As you shot the arrow, the bow string moved the arrow 1 m. Did you do 1250 J of work or 50 J of work? Explain.– You did 50 J of work, because

after the arrow left the bow, itwas flying loose in the air and

was not experiencing any force from you.

Power

• Power is the amount of work done in a certain amount of time (the rate at which work is done).

• Power = work / time P = W / t

• SI unit: watts• Example: How much power is required to push

a car for 10 s if the amount of work done during that time is 5,500 J?– 550 W

Temperature and Heat(Chapter 6)

• All matter is made up of tiny particles in constant motion, meaning that they have kinetic energy.

• temperature: average kinetic energy of the particles

• thermal energy: total energy of all the molecules in an object

• A larger mass has more thermal energy than a smaller mass at the same temperature, because there are more particles in the larger mass.

• When the temperature of an object increases, the average kinetic energy of the molecules increase, so the thermal energy increases.

Heat

• heat: energy that is transferred from an object at a high temperature to one at a lower temperature

• Heat and work are similar. Both are energy being transferred. Both are measured in joules.

• calorimeter: instrument used to measure changes in thermal energy

Transferring Thermal Energy

• conduction: transfer of energy through matter by the direct contact of particles

• Conduction occurs because of the collisions between particles.

• insulators: material that doesn’t allow heat to flow through it easily

Conduction

Convection

• fluid: anything that flows (liquids, gases)

• convection: transfer of energy in a fluid by the movement of the heated particles

• Convection currents are rising-and-sinking actions that transfer heat from warmer to cooler parts of a fluid.

Radiation

• radiation: transfer of energy by electromagnetic waves

• Electromagnetic waves can travel through space even when no matter is present.

• When radiation strikes a material, some of the energy is absorbed, some is reflected and some may be transmitted through the material.