Chapter 7 Review

31. How much gravitational potential energy is contained by a 1,000-kg car traveling at 20 m/s?

50

Jake, who weighs 680 newtons, climbs a 6-meter ladder in 8 seconds. What was Jake's power?

510 watts

Ball #1 is rolled down the steeper side of a hill while ball #2 rolls down the opposite side which is less steep. Both balls roll out onto a flat surface at the same level. Assuming there is no friction, how do the speeds of the balls compare when they reach the flat surface?

The speed will be the same for both balls.

Exercise may transform the chemical energy in the food you eat into

both kinetic and potential energy.

If one horsepower is equal to 746 watts, how much horsepower does a highly trained athlete generate by doing 350 joules of work per second for an hour?

0.469 horsepower power = work time = 350 joules second = 350 watts 1 horsepower = 746 watts power = 350 watts × 1 horsepower 746 watts = 0.469 horsepower

Kevin jumps straight up in the air to a height of 1 meter. At the top of his jump, he has potential energy of 1,000 joules. Answer the following questions based on these numbers. What is Kevin's weight?

1,000 newtons

A marble rolls along the track below with no friction. The sequence of positions on the track listed in order from the lowest kinetic energy to the highest kinetic energy is:

1,2,3,4,5,6

Jordan lifts a 100.0-kilogram barbell from the floor to a height of 2.00 meters in 1.50 seconds. The amount of power he generates is about ____ watts.

1,310

List the changes in energy that take place in each of the following situations: 1. Liana lights a match. 2. Nate switches on the TV. 3. Stacy turns the crank on an electrical generator connected to a light bulb.

1. The match has potential energy in the form of chemical energy. Liana does work, using kinetic energy to strike the match. The ignited match releases thermal energy in the form of heat and light. 2. Nate does work to switch on the TV. The TV is powered by electrical energy, which results from the conversion of chemical energy in fossil fuels. The TV releases energy in the form of heat and sound and light. 3. Stacy does work, converting her potential energy (from eating food) to kinetic energy (moving the crank). The generator produces electrical energy, which is converted to light and heat energy produced by the bulb..

Jasmine, who weighs 400 newtons, moves up a 5.0-meter climbing wall in 15 seconds. The amount of power generated by Jasmine as she climbs the wall is about ____ watts.

130

A 60-newton block at rest at the bottom of a frictionless ramp is pushed up the incline: How much potential energy does the block gain?

180

The rubber ball in Figure 7-2 is replaced with a steel ball with a mass of 2 kg. If the steel ball is still thrown straight up with a velocity of 20 m/s, how high will it travel?

20 m The steel ball will travel the same height as the rubber ball. The initial velocity of the ball is what determines the height. The mass cancels out of the equation.

Natalie lifts a 15-kg rock from the ground onto a 1.5 meter high wall. What is the amount of potential energy she has given the rock?

220 joules

32. Kendra has a mass of 50 kg. What is her kinetic energy while she is walking at 2 m/s?

25

If Kevin's potential energy at the highest point of the jump is 1,000 joules, what is his speed when he leaves the ground

4.4 m/

29. How much kinetic energy does a 1,000-kg car have when it is traveling at 30 m/s?

450,000

30. How much potential energy is held by a 2.0-kg can of paint on top of a 2.0-meter ladder?

4= mgh = 2.0 kg× 9.8 m/s 2× 2.0 m PE = 39 J

A bowler lifts her bowling ball a distance of 0.5 meters using 35 joules of energy. What is the mass of her bowling ball?

7 kg

An automobile uses 75,000 joules of chemical energy from gasoline to produce 10,000 joules of useful output energy. How much energy is "lost" to the system? Where does the extra energy go?

750,000

If a machine's maximum power rating is 2,000 watts, how much work could it do in 40 seconds?

80,000 joules

Which of the following stores energy?

A battery, Food, A stretched rubber band

A gallon of gasoline contains chemical energy. If you pour a gallon of gasoline into your car, you could drive for about 20 miles on the highway (about 20 minutes). You could choose instead to run a lawn mower for an hour with one gallon of gasoline. If the gasoline were to come in contact with a fire, the entire gallon would burn in less than a minute. Which reaction has the most power?

Burning the gasoline in a fire

How are work and energy related?

Doing work means transferring energy by applying a force over a distance. The energy may be transferred to the object you apply the force to, or it may go somewhere else. Energy can be thought of as stored work

What is transferred when work is done?

Energy

Which of the following use the same units of measurement?

Energy and work

Isaiah and Fred have a race to carry identical objects from a point at the bottom of a hill to the same point at the top. Isaiah runs straight up a very steep part of the hill and finishes first. Fred chooses a more gradual ascent and, walking, arrives after Isaiah. Who has done more work? Explain your answer.

Isaiah and Fred do the same amount of work. The work is done against gravity: work = mass × g × height. Since the objects are identical, they have the same mass and are lifted the same distance against gravity. The value for g, the acceleration due to gravity, is a constant for the objects. The work done is independent of the path. (Because Isaiah finished the work in less time, he did produce more power.)

Jennifer, who weighs 450 newtons, and Judy, who weighs 600 newtons, are standing at the bottom of a flight of stairs that is 4.00 meters high. Jennifer runs up the stairs in 3.0 seconds. Judy runs up the same stairs in 4.0 seconds. Compare the work done and the power generated by each girl's run up the stairs. Support your answer by showing calculations for each quantity. a. Who does more work? b. Who is more powerful?

Judy does more work. She does 2,400 joules of work. work = force × distance work = 600 N × 4.00 m work = 2,400 J Jennifer does only 1,800 joules of work work = force × distance work = 450 N × 4.00 m work = 1800 J

A 4.0-kg steel ball is dropped from a platform 15 meters above the ground. a. Find the kinetic energy of the ball when it hits the ground. b. What is the speed of the ball when it hits the ground?

The KE of the ball when it hits the ground equals the potential energy of the ball at the top of the platform. KE = PE = mgh = (4 kg)(9.8 m/s2)(15 m) = 588 J = 590n J (2 sig figs) b. KE = 1 2 mv 2 v = 2KE m = 2(590 J) 4 kg = 17 m/s

A father warns his son against speeding in the family automobile by saying "Having an accident at 80 miles per hour is four times as dangerous as an accident at 20 miles per hour." Do you agree with the father or not? Give support for your answer based upon principles of energy conversion and conservation.

The father is wrong! It is sixteen times as dangerous. Increasing speed from 20 to 80 mph (quadrupling the speed) increases the kinetic energy by 16 times. Ek = 1/2mv2

A unit used to measure power is the:

Watt

When a ball is thrown into the air, the potential energy gained must come from its kinetic energy. As the ball gains potential energy, its speed will

decrease

Kinetic energy and potential energy are often considered to be forms of mechanical energy. List three other forms of energy and state a source for each.

heat from the Sun b. electrical from a generator c. chemical from a battery d. nuclear from splitting atoms at a power plant e. light from a fluorescent bulb

Potential energy is energy due to the:

height of an object

What happens to the kinetic energy of a moving car if you double the mass of the car?

it increases by two times

Energy is measured in units of

joules

The brakes on an automobile become hot when they are used continuously because

kinetic energy is being converted to thermal energy

Kinetic energy increases with:

mass and speed

A stretched spring stores

potential energy