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.

© J

IM C

UM

MIN

S/F

PG IN

TER

NAT

ION

AL/P

NI

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.

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

Figure 6.1 Wooden

block connected to

a spring scale

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

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.

STC/MS™ EN E R G Y, MA C H I N E S , A N D MO T I O N 51

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.

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

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

STC/MS™ EN E R G Y, MA C H I N E S , A N D MO T I O N 53

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.

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