PHYSICS LAB 1

AH-05B: Coefficient of Friction (Wood and Felt on Plastic Track)

Rev: 1-10-2021

OBJECTIVE

The purpose of this experiment is to calculate the coefficients of kinetic and static friction between the track and two different surfaces (wood and felt), for two different surface areas as they slide relative to each other. We use the track in a horizontal position and then as an inclined plane.

VIDEO: You can see the experimental process in this video in which this experiment is done on a Formica table instead of the track. This is only to give you an idea on how to do the experiment. Do NOT take the data from the video. https://youtu.be/BAl3o6-9RF0

MATERIALS

1. ME-6960 PASTrack 4. ME-9807 Friction Block

2. ME-9448B Pulley 5. ME-8979 Weight hanger and weights

3. ME-9495A Angle Indicator 6. Black String

1- ME-6960: PASTrack Unassembled PASTrack Assembled

2- ME-9448 Super Pullley 3- ME-9495 Angle Indicator

4- ME-9807 Friction Block 5- ME-8979 Weight hanger and weights

INTRODUCTION

In general, friction is the force that slows down the motion of an object. The force of friction is directed along the surface of contact between the object and surface and directed opposite to the direction of motion of object (or the direction of impending motion). We deal with:

a) Static friction ( fs)

This exists when the object is at rest relative to the surface. This force must be overcome in order to make the object start moving. Its value increases to match the external force trying to cause the object to move. It is given by: fs ≤ µs n, where n is the normal force, and µs is the coefficient of static friction. The maximum value of static friction is fs = µs n.

b) Kinetic friction ( fk)

This exists when the object is in motion and is given by fk = µkn, where µk is the coefficient of kinetic friction and n is the normal force which presses the two surfaces together. In general, µs > µk because it takes a larger force to start an object sliding (static friction) than to keep it sliding (kinetic friction). Both coefficients depend only on the materials in contact and are independent of the area of contact, or the normal force.

In this experiment, using the track in a horizontal position, we measure the frictional force fk and fs for different values of the normal force n. By using these, we calculate the two coefficients of friction between block and board.

Another way to find µs is to set up the board as an inclined plane. The coefficient of friction µs is related to the maximum angle θm to which the board can be elevated before the block starts to slip by the equation:

θm

θm

µs = tan θm ( 1 )

EXPERIMENTAL PROCEDURE

Kinetic Friction

1) Assemble the Track. To do this, read the manual that came with the track, and see the video: Assembling the Track. At the end, you may dis-assemble it, for which you can see the video: Dis-assemble the track. Both videos are in the module: LAB VIDEOS.

2) Take the mass of the Friction block as 110 grams.

3) Attach a pulley at the end of the track. Tie a string to the Friction Block and pass it over the pulley. Attach weight hanger to the string by wrapping the thread in the notch in the plastic hanger.

4) First, keep the large wooden side of the friction block on the track.

5) Slowly increase the weights on the weight hanger, until the block starts slowly moving with constant speed after given a small push. Don’t forget to include the mass of the hanger. Use your judgement to see if the speed is constant.

6) Repeat the above procedure by adding 100, 200 and 300 grams on the friction block.

7) Repeat steps 5 and 6 with felt side of the friction block at the bottom.

8) Repeat steps 5 and 7 with the small sides of the friction Block. This time, use only 200 grams on the Friction Block. Do a total of 3 independent trials and record your data.

Use the data to plot a graph between friction force on the Y-axis (equal to the hanging mass in kilograms times g) and normal force on the X-axis (equal to the mass of friction block plus any masses on it times g), and hence find the coefficient of kinetic friction.

Static Friction

9) Set up the friction block again with the wide wood side on the track, and place a mass of 200 g on it. Place weights gently on the hanger and increase them slowly until the block just starts its motion without any push. Do a total of 3 independent trials and record your data.

10) Repeat step 9 with the felt side ion the track.

Use the data to calculate the coefficient of static friction.

11) Remove the pulley and use the track as an inclined plane. Attach the angle indicator to the track. Place 200 g on the friction block, and use tape to hold them there. With the Large wood side on the track, gently and smoothly tip the track until block starts to move (once it starts to move, static friction changes to kinetic friction, and the block accelerates down the track). Measure the angle θm at which the block just starts to slip, and record it in the table. Do a total of 3 independent trials. Use angle θm to calculate the coefficient of static friction.

12) Repeat step 11 with the large felt side at the bottom.

CALCULATIONS

1) For kinetic friction, plot the points and draw the best fit graph of the friction force ( fk ) on the Y-axis versus the normal force ( n ) on the X-axis, and find the coefficient of kinetic friction between block and board from the slope of the curve. (For how to make Best Fit Graph, see Lab-1: Error and Data Analysis)

2) For static friction, use fs = µs n to calculate µs .

3) The percent difference is given by:

Percent difference = [difference of the two values / average value] x 100 %

RESULTS:

Make a table of results showing the measured values of friction coefficients, their errors, and values that you find on the internet for these surfaces.

ADDITIONAL INFORMATION

Video of this experiment done on a table instead of the track: https://youtu.be/BAl3o6-9RF0

Coefficient of Static Friction:

Manual device: https://www.youtube.com/watch?v=xOXI7UBkZNM

Automatic device: https://www.youtube.com/watch?v=yVdiWN6rHjs

Notice how the force changes as soon as the object starts to move (static friction changes to kinetic friction): https://www.youtube.com/watch?v=F8n3mVdMtDI

AH-05B - Coefficient of Friction REPORT FORM

Name: _______________________________

LARGE WOOD SIDE ON TRACK

Mass of Friction Block (Mb): _________ Mass of hanger (Mh): ________________

Normal and Friction force for kinetic friction

Mass on Friction Block ( M )	Normal Force ‘n’ (newtons) (Mb + M)*g	Suspended mass + mass of hanger (grams) Ms + Mh	Friction Force ‘fk’ (newtons) (Ms + Mh)*g
0 kg
0.100 kg
0.200 kg
0.300 kg

µk from graph of friction force Vs normal force (i.e. fk Vs n): ________

Normal and Friction force for static friction

Mass on friction block (grams) (M) grams	Normal Force ‘n’ (newtons) (Mb + M)*g	Suspended mass + mass of hanger (grams) Ms + Mh	Friction Force ‘fk’ (newtons) (Ms + Mh)*g	µs
200
200
200
Average

Inclined plane for static friction

Trial No.	Angle θm	µs
1
2
3
Average

Percent difference between the two values for µs : _____________

Average of the two values of µs : _____________

LARGE FELT SIDE ON TRACK

Mass of Friction Block (Mb): _________ Mass of hanger (Mh): ________________

Normal and Friction force for kinetic friction

Mass on Friction Block ( M )	Normal Force ‘n’ (newtons) (Mb + M)*g	Suspended mass + mass of hanger (grams) Ms + Mh	Friction Force ‘fk’ (newtons) (Ms + Mh)*g
0 kg
0.100 kg
0.200 kg
0.300 kg

µk from graph (of fk Vs n) ________

Normal and Friction force for static friction

Mass on friction block (grams) (M) grams	Normal Force ‘n’ (newtons) (Mb + M)*g	Suspended mass + mass of hanger (grams) Ms + Mh	Friction Force ‘fk’ (newtons) (Ms + Mh)*g	µs
200
200
200
Average

Inclined plane for static friction

Trial No.	Angle θm	µs
1
2
3
Average

Percent difference between the two values for µs: _____________

Average of the two values of µs: _____________

SMALL WOOD SIDE ON TRACK

Mass of Friction Block (Mb): _________ Mass of hanger (Mh): ________________

Normal and Friction force for kinetic friction

Mass on friction block ( M ) kg	Normal Force ‘n’ (newtons) (Mb + M)*g	Suspended mass + mass of hanger (grams) Ms + Mh	Friction Force ‘fk’ (newtons) (Ms + Mh)*g	µk
0.200
0.200
0.200
Average µk

SMALL FELT SIDE ON TRACK

Mass of Friction Tray (Mb): _________ Mass of hanger (Mh): ________________

Normal and Friction force for kinetic friction

Mass on friction block ( M ) kg	Normal Force ‘n’ (newtons) (Mb + M)*g	Suspended mass + mass of hanger (grams) Ms + Mh	Friction Force ‘fk’ (newtons) (Ms + Mh)*g	µk
0.200
0.200
0.200
Average µk

RESULTS

	Coefficient of Kinetic Friction	Coefficient of Static Friction
	Large Wood Surface	Large Felt Surface	Small Wood Surface	Small Felt Surface	Large Wood Surface	Large Felt Surface
Experimental values
Values found from internet
Percent error