Physical ScienceCh. 4: Energy

• Energy is the ability of an object to do work. The SI unit for measuring energy is the Joule (j).

• 1 J is the amount of energy required to move a 1 N weight, 1 meter.

• Kinetic energy is energy in motion.

• The amount of kinetic energy which an object has depends on the objects mass and velocity.

i.e. the bigger it is or the faster it is moving, the more kinetic energy it has.

• What other property that we’ve talked about is kinetic energy closely related to?

• Potential energy is stored energy. It is the result of an objects position or condition.

Examples: -a ball sitting on the top shelf

-anything that is flammable

-a skydiver ready to jump

• Gravitational potential energy is due to an objects position above the ground.Energy was required to elevate the object to that height, and since that energy can’t be destroyed (Law of Conservation of Energy), then it is stored in the object until released.

• Why does is burn whenyou slide down the ropein gym class?

• P.E. due to the elasticity of a material is called elastic potential energy.

• Examples of E.P.E.:

– A drawn bow– A rubber band– A compressed spring

• Chemical potential energy is energy released through a chemical reaction.

• Mechanical energy is the combined total of the kinetic and potential energy in an object or a system.

K.E. + P.E. = M.E.

• The Law of Conservation of Energy states that energy can not be created or destroyed, but may change from one form to another.

Therefore, the total amount of energy in the universe is constant.

• Example:

A bouncing ball will continually alternate between kinetic and potential energy as it goes up and down. (Changing from one form of energy to another.)

• Explain what happens to the kinetic, potential, and mechanical energies of a pendulum as it swings back and forth.

• Sometimes energy might seem to be lost, like the bouncing ball gradually coming to a stop, or the pendulum gradually stopping. This energy is not actually lost, it is just transferred into a different form, such as thermal energy (heat).

• An automobile engine takes gasoline (chemical potential energy) and burns it (thermal energy) and uses the gases from the burning to turn drive pistons which turn the axles (kinetic energy).

• A calorimeter is a device used to measure changes in thermal energy (find the calories in food), just like a thermometer is used to measure changes in kinetic energy.

• Changes in thermal energy are measured in joules, but the English unit is the calorie.

1 J = .2399 calories

1 calorie = 4.19 J

• So, calorimeters can tell how many calories of energy are released by certain foods. This will be the same as the amount of energy the foods will release in your body.

• This also tells you how much energy you need to produce to burn up these foods.

• So if a Big Mac has 850 calories, then that’s how much energy it will produce in your body. That’s also how much energy you’ll need to produce to burn it off.

• The average person takes in 2,000-2,500 calories per day.

• Here’s the problem, certain foods contain large percentages of proteins and carbohydrates, which your body burns with relative ease. However, your body has a harder time breaking down fatty foods. So if you eat a diet high in fat, then you’ll either have to work harder to get rid of the calories, or you’ll gain weight.

Calculating Changes in Thermal Energy

Q = M x t x Cp

Q = change in thermal energy

M = mass

t = change in temperature

Cp = specific heat of the material

• A perpetual motion device will continue running indefinitely on an initial source of energy, without ever having to draw more energy from an external source.

These are also called free energy machines.

• Perpetual motion machines will eventually solve all of our energy problems, and provide an everlasting source of cheap energy. Right (?)………….

• Wrong! It sounds great in theory, but creating a perpetual motion device is against the law.

• Which law would that be?

• Nevertheless, this has not stopped some “scientists” from continually trying to create a free energy machine.

• How do you think the following devices are designed to work?

Work• Work is the amount of

force which is applied over a given distance.

W = F x D

• Work is a form of energy and therefore is measured in joules (j).

• And if you remember from Newton’s 2nd Law of Motion,

Force = Mass x Acceleration

So you could also say that Work = M x A x D

• In a nuclear fission reaction (like an atomic blast), atoms are split apart and produce a tremendous amount of energy. Do you think that this is a violation of the Law of Conservation of Energy? Why or why not.

• In order to push her car 300 m, Kim had to do 27,600 J of work. How much force was she producing?

• Ralph’s diet consists primarily of proteins and carbohydrates, while Homer eats a lot of fatty foods. What effect, if any, will this have on the boys if their daily caloric intake is the same (2,000 cal./day)? Explain your answer.

• What happens to the amount of gravitational potential energy of a projectile as it flies through the air?

• The kinetic energy?

• In an automobile engine the amount of kinetic energy produced from chemical potential energy (gasoline) is relatively low, due to undesirable energy transfers.

What are some ways that you could decrease the amount of undesirable energy transfers occurring in your engine?

• As a pendulum swings back and forth, the mechanical energy is 120 J. At 1 point during the swing, if the pendulum is experiencing 10 J of kinetic energy, how much potential energy will there be?

• What type of potential energy are we talking about?• About where in the swing would this scenario take

place?

• A parachutist jumps out of a plane and accelerates for 5 sec., after which time he reaches terminal velocity. After falling for 10 seconds at that speed, he deploys the parachute and drifts to the ground. Tell what happens to each of the following during these 3 phases:

• Kinetic energy• Potential energy• Mechanical energy

• A car with a mass of 1,200 kg accelerates at 10 m/s/s over a distance of 500m. How much work was done by the car’s engine?

A car’s engine takes in 15,000 J of chemical potential energy and produces 4,000 J of kinetic energy.

•How much thermal energy was produced by the engine?

•What would an oil change do for the system?

• A red Pacer accelerates at 3 m/s/s over a distance of 500 m. If the car has a mass of 900 kg, how much work was done by the car’s engine?

• If a 7 kg bowling ball is dropped from a window which is 45 m above the ground, how much work is being done by gravity?

• Heili is running the marathon at the 2008 world championships. He carb loads for 2 days prior to the event to insure a good energy supply during the race. As his body burns these calories, what 2 types of energy are produced as a result?

• The formula used to calculate the gravitational potential energy of an object is GPE = M x 9.8 x H, where M is mass, G is acceleration due to gravity, and H is height above the ground.

• A baseball player pops up the first pitch. If the ball (.25 kg) reaches a height of 15 m, how much GPE did it have at it’s highest point?

• How much kinetic energy did it have when it was caught?

• How much mechanical energy did it have?