The Force of Friction - Kyrene School District · For you and your lab partner 2 ... answers to the following questions: A. ... Figure 6.2 Pulling a wooden block across a surface

6The Force of Friction

LESSON

OBJECTIVES FOR THIS LESSON

Observe the properties of sliding friction.

Measure the force of friction on awooden block pulled across differentsurfaces.

Measure the force of friction on loads ofdifferent weights.

Measure the force of friction on awooden block with different base areasin contact with a surface.

48 STC/MS™ EN E R G Y, MA C H I N E S , A N D MO T I O N

INTRODUCTIONIn Lesson 5, you investigated the properties ofgravity and elastic forces in a rubber band. In thislesson, you will explore another force—friction.Friction is a force you experience every day.People spend time, money, and effort to reduceit. You might be surprised to learn that friction isnot always bad. It can be very helpful.

You will investigate the force of friction as youpull a wooden block at a constant speed across asurface. As you do so, you will investigate threevariables—surface type, weight (load), and surfacearea (base area)—and determine how each affectsthe frictional force between the block and the sur-face. In each trial, you will change one variableand keep the other two the same. This method isan example of good experimental design. As in theprevious inquiries, you will record observations,collect data, and draw conclusions based on whatyou discover about friction. Understanding fric-tion will help you understand motion, which youwill study later in this module.

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What force stops the baseball player as he slides into

third base?

MATERIALS FOR LESSON 6

For you1 copy of Student

Sheet 6.1: What a Drag!

1 copy of StudentSheet 6.2: Chang-ing the Load

For you and your labpartner2 wooden blocks

with attachedscrew hook

1 0- to 2.5-N springscale

1 0- to 10-N springscale

1 piece of waxedpaper

1 piece of papertowel

1 piece of finesandpaper

1 piece of coarsesandpaper

1 meterstick1 rubber band1 piece of masking

tape

Getting Started

1. Discuss what you know about frictionwith your lab partner and then with therest of the class.

2. Identify a situation in which frictionworks against you. Then identify one inwhich friction works for you. Think of situations where friction is very low oralmost zero. What would happen if therewere no friction at all?

3. Your teacher will share information withyou about how to use the spring scales tomeasure the force of friction. Listen care-fully and ask questions if you are not surehow to make the measurements.

STC/MS™ EN E R G Y, MA C H I N E S , A N D MO T I O N 49

LESSON 6 TH E FO R C E O F FR I C T I O N

Inquiry 6.1Pulling a Block AcrossDifferent Surfaces

PROCEDURE

1. Lay the spring scales on the tabletop.Does the pointer register zero? If not,adjust it to read zero.

2. In this inquiry, you will investigate theforce of friction on a block as you pull itacross each of the following surfaces: theplain tabletop, waxed paper, a paper towel,fine sandpaper, and coarse sandpaper. Tapethe four surface strips (waxed paper, papertowel, fine sandpaper, coarse sandpaper) tothe tabletop. Measure and mark off another27-cm length on the bare tabletop.

3. You will need to decide which spring scaleto use for your measurements. If you arenot sure which to use, try making somemeasurements of the forces needed to pullthe block using the scale. The best scaleto use is one that will register the force topull the block, yet not go off scale whenyou exert the maximum force needed topull the block.

4. Attach the hook on the spring scale to theround screw hook on the wooden block,as shown in Figure 6.1.

5. How do you think the frictional force onthe block will compare as you pull theblock across the different surfaces? Writeyour prediction in your science notebook.

6. Before you start to collect data, practiceby pulling the wooden block with thespring scale across a surface at a steadyrate, as shown in Figure 6.2. Make sureyou hold the spring scale parallel to thetabletop and pull horizontally on theblock. As you move the block at a steadyspeed, observe the force reading. You willprobably find that the force is not per-fectly steady. When the spring scale read-ing is not steady, it is best to do severaltrials and average your results. How manytrials should you do for each surface?Discuss this with your partner and decideon the number of trials needed in order toobtain accurate force data.


Figure 6.1 Wooden

block connected to

a spring scale


Inquiry 6.2Changing the Load

PROCEDURE

1. Think about and discuss with your labpartner the following question: What willhappen to the effort force needed to pullthe block if you change the weight of theblock? Write your prediction in your science notebook.

2. This inquiry requires that you and yourlab partner work as a team and collabo-rate with the other teams in your class.Each team should collect data. But thereis not enough time for each individualteam to check all surfaces with differentloads. Therefore, you will need to worktogether as a class and assign differentsurfaces to different teams. Each surfaceneeds to be assigned to at least two teams.

3. Design a data table on Student Sheet 6.2:Changing the Load to record your measure-ments. Discuss with the class how you willmeasure the weight of the load in each trial.

7. Pull the block across each of the fivesurfaces and collect force data for eachsurface. Be sure to pull the block acrossthe entire length of the surface. Recordyour data in Table 1 on Student Sheet6.1: What a Drag! Calculate the averageforce for each surface.

8. How can you graphically represent theaverage force data for each surface? Whatkind of graph should you use? Constructa graph on Student Sheet 6.1.

9. In your science notebook, record youranswers to the following questions:

A. Which surface required the greatestforce to pull the wooden block across it?

B. Which surface required the least force?

C. Did the weight of your wooden blockchange as the surfaces changed?

D. Review the variables for this lesson.Which variables did not change as youtested each surface?

10. Be prepared to discuss your answers tothe questions in Step 9 with the class.


Figure 6.2 Pulling a wooden block across a surface to measure the force of friction

4. Change the weight of the load by stackingmore blocks, one at a time, on the originalblock. Each time you add a block, measurethe force it takes to pull this load at asteady speed across the surface. Shareblocks with another team so both teamscan gather data for a load of up to fourblocks.

5. Graph your data.

6. Compare your data and graph with thatof a team that used the same surface.Discuss with your partner and the otherteam the relationship between the load(total weight of the block or blocks) andthe frictional force. Record the descrip-tion in your science notebook.

7. Compare your data with the data ofgroups that used different surfaces. Whatdo you find? Record your findings in yourscience notebook.

8. Think about the variables in this investiga-tion. What did you keep constant as youchanged the weight of the blocks? Recordyour answer in your science notebook.

9. Share your results with the class.

Inquiry 6.3Changing the Surface Area

PROCEDURE

1. Look at your block. You can turn theblock on its wide side and pull on it, orstand it on one of its narrow sides andpull on it. When the block is on its wideside, the area in contact with the surfaceis greater than when it is on its narrowside. You pulled the block across the sur-faces on its wide side in previous trials.Predict what will happen to the force offriction if you pull the block on a narrowside across the surface. Write your predic-tion in your science notebook.

2. Construct a data table in your notebookto record the description of each surfacearea of the block (wide or narrow) andthe measurement of the force needed topull the block at a steady rate across thesurface (waxed paper, fine sandpaper, andso on). Different teams should use differ-ent surfaces. Make sure you record thesurface you use. Be prepared to share thedata you collect with the class. Your classshould design a class data table on theboard or on a transparency.

3. Put a rubber band around the block sothat it is below the center (about one-fourth of the way to the top of the blockas measured from the table). Attach thespring scale hook to the rubber band andpull the block so that it moves smoothlyacross the table, as shown in Figure 6.3.Measure the frictional force as the blockslides at a constant speed along the sur-face. Do this for each side of the block.




REFLECTING ON WHAT YOU’VE DONEWrite answers to the following questions in yourscience notebook:

A. What have you learned about friction in thislesson? In your science notebook, summarizein several paragraphs what you have learnedabout the force of friction. In your summary,include factors that affect frictional force andexplain how you measure it.

B. In this lesson, you measured sliding friction.Why does the force on the spring scale measurethe force of friction while the block moves at asteady speed?

C. Suppose you used ice as a surface for theblock to slide on. What results would you getin this lab? Consider results for all three variables—surface type, weight of the block,and surface area.

4. Record your data, and then discuss theresults with your team and share yourresults with the class. Add your results tothe class data table.

5. Answer the following question in yournotebook: How did your prediction com-pare with the results?

6. Think about the variables. Which variablesdid you keep constant this time? Recordyour answer in your science notebook.

Figure 6.3 Attach a rubber band and a

spring scale to the block as shown here to

pull the block on one of its narrow sides.




By controlling the amount of the body that is exposed to

the drag of the air, a skydiver can change the terminal

velocity. Here, a skydiver falls in a spread-eagle position.

Since the maximum amount of body is exposed to the

air, the air friction is greater and the terminal velocity is

smaller than when the body is pointed straight down.

NATURE PUTS ON THE BRAKESSkydiving is a sport that many people enjoy.But it takes courage! When skydivers leap fromplanes, you might think that the constant pullof gravity would make them fall faster andfaster until their parachutes opened andallowed them to glide safely back to the earth.But that’s not quite what happens.

At the beginning of a skydive, a diver’s falldoes speed up or accelerate rapidly. But as thediver falls, the acceleration continually decreasesuntil the diver stops speeding up. The diverthen falls at a constant velocity. (“Velocity” isthe speed at which an object is traveling in asingle direction—in the case of a skydiver,down!) The constant velocity that the skydiverreaches is called terminal velocity. It usuallytakes a skydiver about 10 seconds to reach ter-minal velocity.

Forces on SkydiversIt is the forces on skydivers that make themeventually reach terminal velocity. To the



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Terminal velocity is a result of the interaction of two

forces: gravity and air friction (drag).

Air Friction Gravity

observer, gravity is the more obvious force. Ifgravity were the only force acting on skydivers,their velocity would continue to increase at arate of 9.8 meters per second each second thatthey fall. Ten seconds into the fall, they wouldbe moving at 98 meters per second if gravitywere the only force on them. As they continuedto fall, they would continue to go faster andfaster.

But gravity isn’t the only force on skydivers.Air friction, or drag, pushes up on their bodiesas they fall through the air. (“Drag” is anotherword for the force of friction between the sky-diver and the air.) The force of gravity is con-stant, but drag increases with the skydiver’sspeed. As skydivers fall, they eventually reachthe point where the size of the drag forceequals the size of the force of gravity. The dragcontinues to push up while gravity continues topull down. But now, the two forces counterbal-ance each other. As a result, skydivers fall at aconstant velocity—terminal velocity.

Controlling Terminal VelocitySkydivers can make their terminal velocityfaster or slower by changing body position as



Because the

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Left: Skydivers leaping from their plane

Below: Parachutes are designed to take advantage of air

friction, allowing skydivers to land safely.

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they fall. For skydivers falling with their backsor stomachs parallel to the earth, terminalvelocity is about 50 meters per second after 10seconds. If skydivers were going head first, ter-minal velocity would reach twice that rate, or100 meters per second.

Why the difference? Because terminal velocityalso depends on the surface area of the diveragainst which the air pushes during the fall.The greater the surface area, the greater thedrag. The smaller the surface area, the less thedrag and the faster the fall. To understand thecomparison, think of swimmers—they feelmuch more resistance entering the water with abelly flop than with a nosedive.

Falling at terminal velocity without a para-chute is still too fast to land safely on theground. So skydivers do need that parachute.Parachutes help slow skydivers even more bygreatly increasing their surface area. Large para-chutes are more powerful than small ones. Thelarger the parachute, the greater the air resis-tance acting on the diver, and the slower theterminal velocity. The probability of a smooth,safe landing increases proportionately! �

QUESTIONS

1. What forces act on a skydiver to make the diver reach terminal velocity?

2. How is air friction different from the slidingfriction studied in this lesson?



Rock climbing is notusually thought of as ateam sport. For mostclimbers, however, itis. The climber has apartner, called abelayer.

As the climberascends the slope, thebelayer often remainsbelow. The job of thebelayer is to keep theclimber from falling.The climber orbelayer places ananchor at the top ofthe climb or at someother convenient loca-tion. The belayerholds one end of arope. The other end ofthe rope is threadedthrough the anchorand then attached tothe climber’s waist. Asthe climber ascendsthe slope, the belayerholds the rope. If theclimber should slipand fall, the belayerwould pull on therope and prevent theclimber from falling tothe ground below.

Rock Climbing:Two People, One Powerful Force

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At a time like this, friction comes in handy.




Climbers also increase the force of friction bywearing shoes with specially designed soles.These soles cover more of the foot than thesoles of everyday shoes do. This makes it easierfor the climber to get some part of the sole ofthe shoe on the rock. This is important becausethe soles are designed to create a lot of frictionwith the rock.

Going Up! Both partners make full use of a powerful forcecalled friction. Climbers use friction to maketheir way up to the top. While ascending asteep rock face, the climbers constantly look for“holds”—cracks and crevices into which theycan wedge their feet, or small rocks or ledgesthat they can grasp or step on. If the rock faceis smooth, climbers have to rely only on thefriction between their hands and feet and therock face to keep from falling.

Climbers’ shoes are designed to grip the rock surface and to prevent their feet from slipping.

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hands. More often belayers thread the ropearound special metal devices that increase thefriction on the rope and decrease the force thebelayer must exert if the climber falls.

Climbers and belayers aren’t usually thinkingabout friction when they’re moving up a moun-tainside. They’re focusing on their goal: reachingthe top. Friction helps keep them safer andmakes the climb easier. And, unlike those spe-cial shoes and mountaineering equipment, it’sabsolutely free! �

Holding On!While climbers make their way up the moun-

tain or cliff, belayers keep an eye on theirprogress. Belayers use one of several methods toincrease the friction on the rope and ensure thatthe climbers will be safe. For example, belayerscan wrap the rope around their hips. The frictionbetween their clothing and the rope wrappedaround them is far greater than the frictionbetween their hands and the ropes. Should theclimbers slip, belayers can control the fall betterthan they could if the rope were just in their

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Climbers prepare their gear.

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Belaying devices for climbing


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The Force of Friction - Kyrene School District · For you and your lab partner 2 ... answers to the following questions: A. ... Figure 6.2 Pulling a wooden block across a surface