chapter 7 test

Does a rubber band have more or less elastic energy when it is stretched?

A rubber band has more elastic energy when it is stretched.

Arrange the four energy units from largest to smallest. a. joule (J) b. kilowatt-hour (kWh) c. British thermal unit (Btu) d. Calorie (kcal or C)

he four energy units from largest to smallest: b. kWh (largest), d. Calorie, c. Btu, a. joule (smallest).

Explain what it means to say that energy is conserved as a ball falls toward the ground.

As a ball falls, its potential energy is transformed into kinetic energy. The amount of kinetic energy the ball gains is equal to the amount of potential energy it loses, so the total amount of energy is constant.

A 1-kilogram ball has 8 joules of kinetic energy. What is its speed?

Ek=8J 2 8 J = 1/2(1 kg)(speed of ball) square root (16 J/1 kg) = 4 m/s

Explain in your own words why energy is considered to be "nature's money." Give an example to support your explanation.

Energy is like "nature's money" because you need energy to complete a task the way you need money to buy something. You cannot accomplish a task without energy. And sometimes you need to save or accumulate energy in order to be able to accomplish something. For example, if I wanted to run a marathon, I would need to make sure that my body has enough chemical energy to accomplish the task. Similarly, I would need to fill my car with gas in order to drive it. After I have "spent" my gas energy it is gone in the sense that it has been converted to heat or motion energy. I need to buy more gas to fill up my car!

A book on a 2-meter high shelf has 20 joules of potential energy. What is the mass of this book?

Ep=20J 2 20 J = (2 m)(9.8 m/s )(mass of the book) 1 kg = mass of the book

The table below lists normal and abnormal events. Explain why some events would never happen normally.

For example, you could use fuel energy to run a motor to push a ball up hill or you could use your own energy to pick up the apple. Also, you would have to use your own energy to stretch a rubber band, it would never stretch on its own! Systems only go from high to low energy normally. This is why a ball rolls downhill, an apple falls to the ground not up into the sky, and why a rubber band snaps back after it has been stretched.

Which of these graphs illustrates the relationship between speed and the amount of kinetic energy for a 1-kilogram object?

Graph A

If the speed of a ball increased from 1 m/s to 4 m/s, by how much would kinetic energy increase?

If the speed increased from 1 m/s to 4 m/s, the kinetic energy of the ball would increase 16 times (4 exponent 2).

List two forms of mechanical energy.

Kinetic and potential energy are forms of mechanical energy.

Name a form of energy that is part of the electromagnetic spectrum.

Light energy is part of the electromagnetic spectrum.

Imagine you are the teacher of a science class. A student brings in a newspaper article that claims the world will run out of energy by the year 2050 because all the oil will be pumped out of the planet. The student is confused because she has learned in your class that energy can never be created or destroyed. How would you explain to her what "running out of energy" means in the article?

Running out of energy just means that we are running out of certain forms of energy. Once we use fossil fuels for transportation by car or plane, for example, that fuel is "used up" because it has been converted to motion and heat. However, it has not been destroyed, just converted to forms that not longer give us the form of energy we need to keep our cars and planes moving.

1. Imagine you are holding an apple. a. Does this apple have energy? How do you know? b. How could you increase the potential energy of this apple? c. How could you increase the kinetic energy of this apple?

Sample answers: (a) Yes, the apple has energy. I know because if I take a bite of the apple, I get some food energy (or chemical energy) from eating it. Also, if I drop the apple on my foot, it might hurt! The force of the apple hitting my foot indicates that it has kinetic energy. (b) You could increase the apple's potential energy by putting it on a high shelf. (c) You could increase the apple's kinetic energy by throwing or dropping it.

Explain what it means to say that energy is conserved.

Saying energy is conserved means that the total amount of energy stays the same. Energy isn't created or destroyed, just converted from one form to another or given from one object to another.

Challenge: Some but not all of the gasoline used by a car's engine is transformed into kinetic energy. Where might some of the energy go in this system?

Some of the car's fuel energy is converted to kinetic energy or electrical energy to keep the car's engine functioning. Because a car is not 100% efficient, some of the energy is "lost" as heat energy. This is why a car's engine and other mechanical devices get hot when they are working.

A 0.5 kg ball moving at a speed of 3 m/s rolls up a hill. How high does the ball roll before it stops?

The ball has a kinetic energy of 2.25 J, so it will have a potential energy of 2.25 J when it stops. It must therefore have a height of 0.459 m.

Martha wakes up at 5:30 a.m. and eats a bowl of corn flakes. It's a nice day, so she decides to ride her bicycle to work, which is uphill from her house. It is still dark outside. Martha's bike has a small electric generator that runs from the front wheel. She flips on the generator so that her headlight comes on when she starts to pedal. She then rides her bike to work. Draw a diagram that shows the energy transformations that occur in this situation.

The chemical energy from the food is transformed into kinetic energy as Martha rides her bike. As she moves uphill, some of her kinetic energy is changed into potential energy. Some of her kinetic energy is also changed into electrical energy by the generator and then into light energy by the light bulb.

When you rub your hands together, you produce a little heat. Describe the flow of energy that causes the heat to be produced. Use the terms chemical energy, kinetic energy, friction, and heat in your answer.

When I rub my hands together I am converting chemical energy in my body to kinetic energy. This kinetic energy becomes heat due to the friction between my hands.

Do a stretched spring and a box on a high shelf both have potential energy? Why or why not? Explain your answer.

Yes, a stretched spring has one form of potential energy because it can exert a force or move something attached to the spring when it is released. A box on a high shelf has gravitational potential energy. The box can exert a force on a floor if it falls and converts its potential energy to kinetic energy.