Gravity and Motion

Reading PreviewKey Concepts. What determines the strength of

the force of gravity between twoobjects?

. What two factors combine tokeep the moon and Earth inorbit?

Key Terms. force. gravrtyr law of universal gravitation. mass. weight. inertiao NeMon's f irst lawof motion

Asking Questions Before youread, preview the red headings. Ina graphic organizer l ike the onebelow, ask a question for eachheading. As you read, writeanswers to your questions.

Earth revolves around the sun in a nearly circular orbilmoon orbits Earth in the same way. But what keeps Earththe moon in orbit? Why don't they just fly off into space?

The first person to answer these questions was thescientist Isaac Newton. Late in his life, Newton told ahow watching an apple fall from a tree in 1666 had madethink about the moon's orbit. Newton realized that therebe a force acting between Earth and the moon that kepmoon in orbit. A force is a push or a pull. Most everydayrequire objects to be in contact. Newton realized that thethat holds the moon in orbit is diflerent in that it acts ovtrdistances between objects that are not in contact.

GravityNewton hypothesized that the force that pulls an apple bground also pulls the moon toward Earth, keeping it inThis force, called gravity, attracts all objects towardother. In Newtont day, most scientists thought thatEarth were different from those elsewhere in theAlthough Newton did not discover gravity, he was theperson to realize that gravity occurs everl'where. Nlaw of universal gravitation states that every object inuniverse attracts every other object.

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Can You Remove the Bottom Penny?'1. Place 25 or so pennies in a stack on a table.2. Write down your prediction of what will happen if you

attempt to knock the bottom penny out of the stack.3. Quickly s l ide a ruler along the surface of the table and

strike the bottom penny. Observe what happens to theof oennies.

4, Repeat Step 3 several times, knocking more pennies frombottom of the stack.

Think lt OverDeveloping Hypotheses Explain what happened to the stadpennies as the bottom penny was knocked out of the stack

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Gravity

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The force of gravity is measured in units callednewtons, named after Isaac Newton. The strength ofthe force of gravity between two objects depen-tls ontwo factors: the masses of the obiects and thedistance between them.

Gravity, Mass, and Weight According to the lawof universal gravitation, all of the objects aiound you,ncluding Earth and even this book, are pulling onrou, just as you are pulling on them. Why don'iyourotice a pull between you and the book? iecause'the'trength of gravity depends in part on the masses of:ach of the objects. Mass is the amount of matter inen obiect.

Because Earth is so massive, it exerts a muchreater force on you than this book does. Similarlv.:arth exerts a gravitatio,lrq.l fp_r_ce _o- 1h-e,mppg.^laqre=nough to keep the moon in orbit. The moon ulio:rerts a gravitational force on Earth, as you will learn..rter in this chapter when you study the t ides.

The force of gravity on an object is known as itsweight. Unlike mass, which doesn't change, an-.bject's weight can change depending on its loiation.- or example, on the moon you would weigh about.ne sixth of your weight on Earth. This is beiause theroon is much less massive than Earth, so the pull ofre moon's gravity on you wouJd be far less than that-'f Earth's gravity.

Gravity and Distance The strength of gravity is.lTected by the distance between two objects is weil as--ieir masses. The force of gravity decreases rapidly asistance increases. For example, if the distance--€tween two objects were doubled, the force of qrav--r between them would decrease to one fourth if its,riginal value.

d8;q!ijt* r what is an object's weisht?

FrcuRE 7Gravity, Mass, and DistanceThe strength of the force of gravitybetween two objects depends on theirmasses and the distance between them.Inferring How would the force of gravitychange if the distance between the obi;cBdecreased?

Gravity attracts all

lf mass increases,

Hforce decreases.

FTGURE 8Earth Over the MoonThe force of gravity holds Earthand the moon together.

Gravity Versus DistanceAs a rocket leaves a planet's surface, the forceof gravity between the rocket and the planetchanges. Use the graph at the right to answerthe ouestions below.

1. Reading Graphs What two variables arebeing graphed? In what units is eachvariable measured?

2. Reading Graphs What is the force of gravityon the rocket at the planet's surface?

3. Reading Graphs What is the force of gravityon the rocket at a distance of two units(twice the planet's radius f rom its center)?

4. Making Generalizations In general, howdoes the force of gravity pulling on therocket change as the distance between itand the planet increases?

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Inertia and Orbital MotionIf the sun and Earth are constantly pulling on onebecause of gravity, why doesn't Earth fall into the sun?larly, why doesnt the moon crash into Earth? The faasuch collisions have not occurred shows that there mustanother factor at work. That factor is called inertia.

lnertia The tendency of an object to resist a change inis inertia. You feel the effects of inertia every day. When youriding in a car and it stops suddenly, you keep movingIfyou didn't have a seat belt on, your inertia could cause youbump into the cart windshield or the seat in front of you.more mass an object has, the greater its inertia. Al object wigreater inertia is more difficult to start or stop.

Isaac Newton stated his ideas about inertia as a scilaw. Newtonis first law of motion says that an object atwill stay at rest and an object in motion will stay in

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lrbital Motion Why do Earth and the moon remain in:eir orbits? Newton concluded that two factors-inertia and

.ravity-combine to keep Earth in orbit around the sun and:re moon in orbit around Earth.

As shown in Figure 9, Earth's gravity keeps pulling the-roon toward it, preventing the moon from moving rn a:aight line. At the same time, the moon keeps moving ahead

'-cause of its inertia. If not for Earth's gravity, inertia would.-lse the moon to move off through space in a straight line. In. same way, Earth revolves around the sun because the sun's

- :vity pulls on it while Earth's inertia keeps it moving ahead.

Target Reading Ski l l asking euest ions'se your graphic organizer about the headings

' help answer the que:t ions below.

Reviewing Key Conceptsi. a. Summarizing \Arhat is the law of universal

gravitation?b. Reviewing What two factors determine the

force of gravity between two objects?c. Predicting Suppose the moon were closer

to Earth. How would the force of sravitvbetween Earth and the moon be dlfferent?

2. a. ldentifying What two factors act togetherto keep Earth in orbi t around the sun?

b. Applying Concepts Why doesn't Earthsimply fall into the sun?

c. Predicting How would Earth move if the sun(including its gravity) suddenly disappeared?Explain your answer.

FTGURE 9Gravity and InertiaA combinat ion of gravi ty andinert ia keeps the moon in orbi taround Earth. l f there were nogravity, inertia would cause themoon to travel in a straight l ine.f nterpreting Diagrams Whatwould happen to the moon if itwere not moving in orbit?

Chapter 19 a 669