KINESIOL 2A03 Chapter Notes - Chapter 4: Force Platform, Ground Reaction Force, Normal Force

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15 Sep 2016

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The reason an object doesn"t move is the applied force is less than the maximum force produced by the interactions of the surfaces of the object and floor. Friction has an element of variability due to the unevenness of surfaces and unknown surface forces that create friction. When any two surfaces are in contact there are interactive forces that create frictional force. Fluid friction: occurs between layers of fluids in motion. Dry friction: created by contact forces that develop between two unlubricated or dry surfaces. Frictional characteristics of a single surface cannot be defined without knowing the other contacting surface. Tangential axis: line that is at the level of the surfaces and parallel with the surfaces. Normal axis: line that is perpendicular to the surfaces. Normal-tangential system may be oriented at some angle to the horizontal. Friction occurs whenever two surfaces in contact attempt to move across one another.

Related Questions

I am so lost onwere to even begin. I just need help trying to find the Ñ² (angle)for part part 1 of the experiment if anyone can just explain how todo that and maybe give me a step-by-step example so I can finishthe rest. Thank you! Experiment 6 ~ Friction

Purpose:

In this lab, you will make some basic measurements of friction.You will measure the coefficients of staticfriction between several combinations of surfaces using aninclined plane, and you will measure the coefficient ofkinetic friction between two of the combinationsof surfaces you used in the static friction part of thisexperiment.

Part 1: StaticFriction

Theory:

The coefficient of static frictionm_s can be measured experimentally foran object placed on an inclined plane (a.k.a. ramp, a.k.a. hill).The coefficient of static friction is related to the criticalangle q_c for the ramp, at whichthe object just begins to slide. Using what we havecovered in class, you can derive this relationshipyourself! At this critical angle, static friction preventing theobject from sliding down the hill is just exactly equal to thecomponent of the objectâs weight along the hill. If the componentof the weight along the hill were just a little greater, it wouldovercome friction, and the object would start to slide down. Thefree body diagram for this situation will look like the figuresketched below. Use this diagram to find an equation that relatesm_s toq_c.

F_N = NormalForce = mg cos(Î¸)

F_F =Î¼_s x F_N = Î¼_s mg cos(Î¸)

Î£F_x = mA_x = F_Gx - F_F = ?

F_Gx = mgsin(Î¸)

Hint #1: the angle q of the incline is the same as theangle between the normal force N and the weightmg. Why?

Hint #2: The weight mg can be broken into twocomponents. One component is along the incline(W_H). The other component is âpressinginto the hillâ, and is equal in magnitude to the normal forceN. (This is an example of a situation where the normalforce is not equal to the weight!) Remember that you need to usethe normal force to calculate friction, sincef=mN. Now calculate what the angle ofthe incline should be, so that the component of the weight down thehill is just exactly balanced by friction.

Equipment:

For this lab you will need to share the following ramps andblocks with other lab groups:

Ramps Blocks

Aluminum Aluminum

ParticleBoard Brass

StainlessSteel Plexiglass

Plastic Copper

Rubber

Steel

Wood

Experiment:

Test at least 5 different combinations of materials. Record thematerial of the block and the ramp for each combination.

1. Measure and record the length of theinclined plane you will be using. (This is thehypotenuse of the right triangle!)

2. Place the block on the plane and begin tiltingthe plane just until the point when the block begins to slide.

3. Measure the height of the raisedend of the inclined plane at this point. (This is theopposite side of the right triangle!) You cancalculate the angle from the measurements of the two sides.

4. Repeat the height measurement at least 3 timesfor each combination of materials. Record all the measurements inthe table below. Then,

(a) calculate the angle from the measurements of the hypotenuseand opposite side you just obtained

(b) use the equation you derived above in order to calculate thecoefficient of kinetic friction from the angle.

Block: woodenblock Block: cooper

Ramp:particleboard Ramp: particle board

Length of ramp:0.61(m) Length of ramp: 0.61(m)

Height(m)	Ñ²	Ms	Height(m)	Ñ²	Ms
0.24			0.26
0.25			0.31
0.245			0.345

avg m_s: ______ s : _______ avg m_s: ______ s : _______

Block:aluminum Block:particle board block

Ramp: solidewood Ramp: solide wood

Length of ramp:0.605(m) Length of ramp: 0.605(m)

Height(m)	Ñ²	s	Height(m)	Ñ²	s
0.15			0.23
0.16			0.165
0.16			0.195

avg m_s: ______ s : _______ avg m_s: ______ s : _______

5. Remember that in class we said that friction does not dependon surface area. You can easily test whether or not this iscorrect. Choose a block/ramp combination not already tested, andmake sure the block is not a cube or cylinder. Place the block sothat the side with the larger surface area is in contact with theboard. Now measure the angle at which the block just begins toslide (by measuring the hypotenuse and opposite sides of thecorresponding right triangle). As before, calculate the coefficientof friction from the angle you measured, using your equation.Repeat this process with the side of the block with the smallersurface area in contact with the board. Enter this data in thesmall tables below.

Block (larger end) :_________ Block (smaller end):__________

Ramp:_______________ Ramp: ________________

Length of ramp:______(m) Length of ramp: ______(m)

Height(m)	s	Height(m)	s

avg m_s: ______ s : _______ avg m_s: ______ s : _______

Part 2: KineticFriction

Theory:

You can calculate the coefficient of kinetic friction,m_k, using a variation of the methodyou used for the coefficient of static friction. For thecoefficient of kinetic friction, you can use the same free bodydiagram as the one drawn on the first page. But now, thecombination of W_H and the force offriction will need to add up such that the block will slide at aconstant speed. Think of Newtonâs first and second laws when youset up this equation. Derive a relationship between the criticalangle and the coefficient of kinetic friction.

Experiment:

You can measure m_k using aprocedure similar to the one you used to measurem_s. This time, just pick twocombinations of materials. The combinations you pick for this partof the experiment have to be combinations you alreadyused for the static friction part of the experiment, because thepoint is to compare m_sandm_k!

1. Measure and record the length ofthe inclined plane you will be using.

2. Raise the plane high enough for the blockto begin moving. Begin lowering the plane just until the pointwhen the block begins to slow down. (Alternatively, raise theboard while tapping the block until block moves at a constantrate.)

3. Measure the height of the raisedend of the inclined plane at this point.

4. Repeat the height measurement at least 3times for each combination of materials. Record all themeasurements in the table below. Then,

(a) calculate the angle from the measurements of the hypotenuseand opposite side you just obtained

(b) use the equation you derived above in order to calculate thecoefficient of kinetic friction from the angle.

Block (from part 1) :_________ Block (from part 1):__________

Ramp:_______________ Ramp: ________________

Length of ramp:______(m) Length of ramp: ______(m)

Height(m)	k	Height(m)	k

avg m_k: ______ s : _______ avg m_k: ______ s : _______

Analysis:

Calculate the mean value and standard deviation of thecoefficient of static friction that you measured, for each set ofmaterials.

ramp/board pair		Standard deviation




(larger end)
(smaller end)

Calculate the mean value and standard deviation of thecoefficient of kinetic friction that you measured, for each set ofmaterials.

ramp/board

pair

Standard

deviation

Questions:

1. How do the values of m_s compareto the values of m_k? (Of course, youcan only compare them for the same pairs of materials.)

2. Is the relationship betweenm_sand m_kwhat you expected?

3. Of the two parts of the experiment, measurement ofm_sand measurement ofm_k, which had more sources of error?What were some of the sources of error?

4. Could m_k orm_s ever be greater than 1? Explain whyor why not.

5. Is the coefficient of friction the same as when the block wasstanding on its larger (or smaller) end?

6. Think about your results. Do they make sense when youconsider your everyday experiences?

1.

While helping a friend move, you push a 11.5 kg dolly with a forceof 11.0 N. Assume there are no other forces acting on the dolly. Ifthe dolly starts at rest, how far does it move in 3.50 s?
1 m

2.

At a lumberjack competition an axe with a mass of 1.25 kg is thrownat a target with a speed of 41 m/s. When it hits the target, itpenetrates to a depth of 0.055 m.
(a) What was the average force exerted by the target on theaxe?
1 N

(b) If the mass of the axe is halved, and the force exerted by thetarget on the axe remains the same, by what multiplicative factordoes the penetration depth change?
2
It doubles. It increases by a factor of the square root of two. Itdoes not change. It decreases by a factor of the square root oftwo. It halves.
Your work in question(s) will also be submitted or saved.
3

Your 1500 kg car pulls a 540 kg trailer away from a stop sign withan acceleration of 2.00 m/s2. (Note the answers should be given inkN.)
(a) What is the net force exerted by the car on the trailer?
1 kN

(b) What force does the trailer exert on the car?
2 kN

(c) What is the net force acting on the car?
3 kN

4.

Two boxes sit next to, and touching, each other on a frictionlesshorizontal surface. The lighter box has a mass of 5.31 kg and theheavier box has a mass of 7.29 kg.
(a) Find the contact force between these boxes when a horizontalforce of 6.1 N is applied to the light one.
1 N

(b) If the 6.1 N force is applied to the heavy box instead, is thecontact force between the boxes the same as, greater than, or lessthan the contact force in part (a)?
2

(c) Verify your answer to part (b) by calculating the contact forcein this case.
3 N

5.

A child sits in a wagon and together they have a mass of 25 kg. Thewagon is pulled with a constant speed by a handle that is at anangle of 22Â° above the horizontal. If the normal force exerted onthe wagon is 210 N, what is the force F applied to thehandle?
1 N

6.

An object acted on by three forces moves with constant velocity.One force acting on the object is in the positive x direction andhas a magnitude of 6.7 N; a second force has a magnitude of 4.5 Nand points in the negative y direction. Find the direction andmagnitude of the third force acting on the object.
1 N
2Â° (counterclockwise from the +x axis)

7.

A chain is used to lift a 1200 kg car vertically upward with anacceleration of 0.90 m/s2. What is the tension in the chain?
1 N

8.

A 22.5 kg box (m1) rests on a table.

(a) What is the weight of the box?
1 N
What is the normal force acting on it?
2 N (upward)
(b) A 12.5 kg box (m2) is placed directly on top of the 22.5 kgbox.
Determine the normal force that the table exerts on the 22.5 kgbox.
3 N (upward)
Determine the normal force that the 22.5 kg box exerts on the 12.5kg box.
4 N (upward)

9.

Your friend has a weight of 127 lb when standing on a scale. Youtake the scale to an elevator where you notice your friend's weighton the scale reads 108 lb. What are the direction and magnitude ofthe elevator's acceleration?
Direction 1
Magnitude 2 m/s2

10.

When you lift a package with a force of 82 N, the packageaccelerates upward at a rate of a. If, instead, you lift with aforce of 93 N, the package now has an acceleration of 8 multipliedby a.
(a) Find the weight of the package.
1 N

(b) Find the acceleration a.
2 m/s2

11.

One 3.1 kg paint bucket is hanging by a massless cord from another3.1 kg paint bucket, also hanging by a massless cord, as shownbelow.

(a) If the buckets are at rest, what is the tension in eachcord?
(lower cord) 1 N
(upper cord) 2 N

(b) If the two buckets are pulled upward with an acceleration of1.30 m/s2 by the upper cord, calculate the tension in eachcord.
(lower cord) 3 N
(upper cord)5

12)
Your friend has a pair of fuzzy dice with a mass of 1.32 g hangingfrom his rear-view mirror.
(a) When he accelerates from a stoplight, the dice deflect backwardtoward the rear of the car hanging at an angle of 6.18Â° relative tothe vertical. Find the tension in the string holding thedice.
N

(b) At what angle to the vertical will the tension in the string betwice the weight of the dice?
Â°

QUESTION 1

Rosina Gonzales quit her job in Pennsylvania and moved to California to be close to her family. She is currently looking for work, so she would be considered

		frictionally unemployed.
		cyclically unemployed.
		a discouraged worker.
		seasonally unemployed.

1 points

QUESTION 2

Discouraged workers

		are counted as unemployed workers when the unemployment rate is calculated.
		would decrease the unemployment rate if they were added to the number of unemployed workers.
		are counted as one-half of a worker in the unemployment statistics.
		would increase unemployment rate if they were added to the number of unemployed workers.

1 points

QUESTION 3

In measuring the unemployment rate, part-time workers are ________ and discouraged workers are ________.

		included as employed; included as unemployed
		excluded; excluded
		excluded; included as unemployed
		included as employed; excluded

1 points

QUESTION 4

Mary has decided that the she does not like Iowa and has decided to quit her job as a medical technician and move to Arizona. Mary's unemployment as she searches for a new job is best classified as

		frictional.
		cyclical.
		structural.
		seasonal.

1 points

QUESTION 5

When a golf caddy in Minnesota (where golf courses close in the winter) is unemployed in December, it is most likely the case that the golf caddy is

		cyclically unemployed during long droughts.
		frictionally unemployed during spring and summer.
		structurally unemployed during spring and summer.
		seasonally unemployed.

1 points

QUESTION 6

After graduating from college, Yunis, age 22, started working for his parent's real estate business as an unpaid assistant. He works 25 hours a week helping manage rental units . In the Current Population Survey, Yunis is considered

		part of the labor force and employed.
		part of the labor force and unemployed.
		a discouraged worker.
		not part of the labor force.

1 points

QUESTION 7

The working-age population includes

		youngsters between the ages of 14 and 16 if they are working at least part time.
		only employed people over the age of 16.
		those in jails and hospitals.
		employed and unemployed people over the age of 16.

1 points

QUESTION 8

Involuntary part-time workers are workers who

		work less than 35 hours but would like to work full time.
		work more than 35 hours but would like to work less than 35 hours.
		work less than 35 hours by choice.
		work have lost their jobs within the last four weeks and are seeking another job.

1 points

QUESTION 9

Frictional unemployment is the result of

		the normal process of jobs being created and destroyed.
		an economic recession.
		people not getting along (having friction) with their employers.
		the economic decline of major industries

1 points

QUESTION 10

Bill just graduated with his degree in economics. Through Career Services he submitted his resume to several companies and he will visit them during the next two weeks. Bill is considered

		not in the labor force.
		cyclically unemployed.
		frictionally unemployed.
		structurally unemployed.

1 points

QUESTION 11

Harry works at the video rental store for 20 hours per week. He's asked his boss to allow him to work 40 hours per week, but has been told that business is too slow. Harry is considered

		an involuntary part-time worker.
		a marginally attached worker.
		a discouraged worker.
		not in the labor force.

1 points

QUESTION 12

Discouraged workers who are over 16 years old are

i.not counted as unemployed.

ii.part of the working-age population.

iii.part of the labor force.

		i and ii
		ii and iii
		ii only
		i only

1 points

QUESTION 13

Higher unemployment benefits create

		higher seasonal unemployment as workers switch jobs.
		a lower number of job leavers.
		incentives for longer job searches and higher frictional unemployment.
		incentives for shorter job searches and higher structural unemployment.

1 points

QUESTION 14

Seasonal unemployment includes people who become unemployed from

		changes in the business cycle.
		technological changes.
		the seasonal weather patterns.
		normal changes in the labor force.

1 points

QUESTION 15

Cyclical unemployment is

		higher when the economy is expanding.
		always greater than the total of structural and frictional unemployment.
		the total of structural and frictional unemployment.
		created by a recession.

1 points

QUESTION 16

Which of the following people would be classified as employed in the Current Population Survey?

		April, who just graduated from college and is looking for work
		Jason, who was laid off from work less than 6 months ago but who has stopped looking for work
		Rich, who is working 20 hours a week but wants a full-time job
		Misty, who just quit her job to return full time to school

1 points

QUESTION 17

People who are willing and able to work but are not looking for work because they have been discouraged by their previous futile efforts are called

		discouraged workers.
		involuntarily unemployed.
		part-time lookers.
		unemployed workers.

1 points

QUESTION 18

Discouraged workers are included in the calculation of the

i.unemployment rate.

ii.labor force participation rate.

iii.working-age population.

		i and ii
		i only
		iii only
		ii only

1 points

QUESTION 19

As firms search for the best employee to fill an opening and the unemployed search for the job that best fits their skills, the economy experiences

		changes in the business cycle.
		cyclical unemployment.
		frictional unemployment.
		structural unemployment.

1 points

QUESTION 20

Suppose the U.S. population is 275 million. If 210 million people are of working age, 135 million are employed, and 6 million are unemployed, what is the labor force participation rate?

		64 percent
		67 percent
		76 percent
		49 percent

1 points

QUESTION 21

During a recession the unemployment rate generally ________ and during an expansion the unemployment rate generally ________.

		rises; rises
		rises; does not change
		falls; rises
		rises; falls

1 points

QUESTION 22

In January of 2001, the population of the United States was 276.8 million, the working-age population was 210.2 million, the total number of people employed was 140, and the total number of people unemployed was 5.0 million. What was the labor force participation rate?

		56 percent
		69 percent
		78 percent
		67 percent

1 points

QUESTION 23

If Brian, age 24, had no job but was available for work and had looked for a job the week before the survey, Brian is classified in the Current Population Survey as

		unemployed.
		employed.
		a discouraged worker.
		not in the labor force.

1 points

QUESTION 24

Julie works part-time for economic reasons. She would be considered

		an involuntary part-time worker.
		not in the labor force.
		a job seeker.
		a discouraged worker.

1 points

QUESTION 25

Suppose the population is 300 million people, the labor force is 200 million people, the number of people employed is 185 million, and the working-age population is 170 million people. What is the unemployment rate?

		5 percent
		7.5 percent
		8.8 percent
		92.5 percent

1 points

KINESIOL 2A03 Chapter Notes - Chapter 4: Force Platform, Ground Reaction Force, Normal Force

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