Chapter 12: Gravity, Friction, and Pressure 389

VOCABULARY

friction p. 389fluid p. 392air resistance p. 393

BEFORE, you learned

• Gravity is the attractive forcemasses exert on each other

• Gravity increases with greatermass and decreases withgreater distance

• Gravity is the centripetal forcekeeping objects in orbit

NOW, you will learn

• How friction affects motion• About factors that affect

friction• About air resistance

KEY CONCEPT

Friction is a force thatopposes motion.

THINK ABOUT

What forces help you to walk?

As a person walks, she exerts a backward force on the ground.A reaction force moves her forward.But some surfaces are harder to walkon than others. Ice, for example, isharder to walk on than a dry surfacebecause ice is slippery. How can differ-ent surfaces affect your ability to walk?

Friction occurs whensurfaces slide against each other.

Have you ever pushed a heavy box across the floor? You probablynoticed that it is easier to push the box over some surfaces than over others. You must apply a certain amount of force to the boxto keep it moving. The force that acts against your pushing force is called friction. is a force that resists the motion between twosurfaces in contact.

When you try to slide two surfaces across each other, the force offriction resists the sliding motion. If there were no friction, the boxwould move as soon as you applied any force to it. Although frictioncan make some tasks more difficult, most activities, including walking,would be impossible without it. Friction between your feet and theground is what provides the action and reaction forces that enable you to walk.

Friction

SUPPORTING MAIN IDEASTake notes about friction,including details and examples.

Forces and Surfaces If you look down from a great height, such as from the window of anairplane, a flat field appears to be smooth. If you were to walk in thefield, however, you would see that the ground has many bumps andholes. In the same way, a flat surface such as a piece of plastic maylook and feel smooth. However, if you look at the plastic through astrong microscope, you see that it has tiny bumps and ridges. Frictiondepends on how these bumps and ridges on one surface interact withand stick to the bumps and ridges on other surfaces. There are severalfactors that determine the friction between two surfaces.

Types of Surfaces Friction between two surfaces depends on thematerials that make up the surfaces. Different combinations ofsurfaces produce different frictional forces. A rubber hockey puck sliding across ice has a smaller frictional force on it than the samepuck sliding across a wooden floor. The friction between rubber andice is less than the friction between rubber and wood.

Motion of the Surfaces You need a larger force to start somethingmoving than you do to keep something moving. If you have ever tried to push a heavy chair, you may have noticed that you had topush harder and harder until the chair suddenly accelerated forward.

As you apply a force to push a chair or any other object that is notmoving, the frictional force keeping it from sliding increases so theforces stay balanced. However, the frictional force has a limit to how

390 Unit 3: Motion and Forces

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Learn more about friction, forces, and surfaces.

reminder

Remember that balancedforces on an object do notchange the object’s motion.

Friction and Motion

Before Object Moves While Object Moves

When an object is standing still, there is a maximumforce needed to overcome friction and start it moving.Any force less than this will be exactly balanced by theforce of friction, and the object will not move.

Once the object is moving, the frictional force remainsconstant. This constant force is less than the maximumforce needed to start the object moving.

friction

acceleration

friction

appliedforce

appliedforce

large it can be. When your force is greater than this limit, the forces onthe chair are no longer balanced, and the chair moves. The frictionalforce remains at a new lower level once the chair is moving.

Force Pressing the Surfaces Together The harder two surfaces arepushed together, the more difficult it is for the surfaces to slide overeach other. When an object is placed on a surface, the weight of theobject presses on that surface. The surface exerts an equal and oppo-site reaction force on the object. This reaction force is one of thefactors that determines how much friction there is.

If you push a chair across the floor, there will be a certain amountof friction between the chair and the floor. Increasing the weight ofthe chair increases the force pushing the surfaces together. The forceof friction between the chair and the floor is greater when a person issitting in it than when the chair was empty.

Friction depends on the total force pressing the surfaces together,not on how much area this force acts over. Consider a rectangularcardboard box. It can rest with its smaller or larger side on the floor.The box will have the same force from friction regardless of whichside sits on the floor. The larger side has more area in contact with the floor than the smaller side, but the weight of the box is morespread out on the larger side.

check your reading What factors influence frictional force? Give two examples.

Chapter 12: Gravity, Friction, and Pressure 391

Friction and Weight

Less Weight More Weight

The force of friction depends on the total force push-ing the surfaces together. Here the weight of thechair is the force pressing the surfaces together.

The weight of the chair increases when someone sitsin it. The force of friction is now greater than whenthe chair was empty.

weightweightweight

friction friction

appliedforce

appliedforce

How does the shape of an object affect how it falls?Write a hypothesis that explains how shape affects the speed of falling objects. Design an experiment that tests your hypothesis.

PROCEDURE

Figure out how you can use the three sheets of paper to test your hypothesis.Remember to control all other variables, including the mass of the paper.

Write up your procedure.

Conduct your experiment.

WHAT DO YOU THINK?• What were the results of your experiment?

• Did the results support your hypothesis? Explain your answer.

• Write a statement that summarizes your findings.

CHALLENGE What other variable might affect falling time? How couldyou test it?

3

2

1

Friction in AirFriction in AirSKILL FOCUSDesigning experiments

MATERIALS3 identical sheets

of paper

TIME30 minutes

392 Unit 3: Motion and Forces

Friction and Heat

Friction between surfaces produces heat. You feel heat produced byfriction when you rub your hands together. As you rub, friction causesthe individual molecules on the surface of your hands to move faster.As the individual molecules in an object move faster, the temperatureof the object increases. The increased speed of the molecules on thesurface of your hands produces the warmth that you feel.

The heat produced by friction can be intense. The friction thatresults from striking a match against a rough surface produces enoughheat to ignite the flammable substance on the head of the match. Insome machines, such as a car engine, too much heat from friction cancause serious damage. Substances such as oil are often used to reducefriction between moving parts in machines. Without motor oil, a car’sengine parts would overheat and stop working.

Motion through fluids produces friction.As you have seen, two objects falling in a vacuum fall with the sameacceleration. Objects falling through air, however, have different accel-erations. This difference occurs because air is a fluid. A is asubstance that can flow easily. Gases and liquids are fluids.

fluid

Friction produces sparksbetween a match head anda rough surface. The heatfrom friction eventuallylights the match.

KEY CONCEPTS 1. How does friction affect

forward motion? Give anexample.

2. Describe two ways to changethe frictional force betweentwo solid surfaces.

3. How does air resistance affectthe velocity of a falling object?

CRITICAL THINKING4. Infer What two sources of

friction do you have to over-come when you are walking?

5. Synthesize If you push achair across the floor at a constant velocity, how doesthe force of friction comparewith the force you exert?Explain.

CHALLENGE6. Synthesize If you push a

book against a wall hardenough, it will not slide downeven though gravity is pullingit. Use what you know aboutfriction and Newton’s laws ofmotion to explain why thebook does not fall.

When an object moves through a fluid, it pushes themolecules of the fluid out of the way. At the same time, themolecules of the fluid exert an equal and opposite force on theobject that slows it down. This force resisting motion through afluid is a type of friction that is often called drag. Friction influids depends on the shape of the moving object. Objects canbe designed either to increase or reduce the friction caused by afluid. Airplane designs, for example, improve as engineers findways to reduce drag.

The friction due to air is often called Air resistance differs from the friction between solid surfaces.Air resistance depends on surface area and the speed of anobject in the following ways:

• An object with a larger surface area comes into contact withmore molecules as it moves than an object with a smaller surfacearea. This increases the air resistance.

• The faster an object moves through air, the more molecules itcomes into contact with in a given amount of time. As the speedof the object increases, air resistance increases.

When a skydiver jumps out of a plane, gravity causes the skydiverto accelerate toward the ground. As the skydiver falls, his body pushesagainst the air. The air pushes back—with the force of air resistance.As the skydiver’s speed increases, his air resistance increases. Eventually,air resistance balances gravity, and the skydiver reaches terminal veloc-ity, which is the final, maximum velocity of a falling object. When theskydiver opens his parachute, air resistance increases still further, andhe reaches a new, slower terminal velocity that enables him to land safely.

check your reading How do speed and surface area affect air resistance?

air resistance.

When the force of airresistance equals theforce from gravity, a skydiver falls at a constant speed.

Chapter 12: Gravity, Friction, and Pressure 393

gravityair resistance