PURPOSE:
To expose the student to new equipment and lab procedures. And to calculate the coefficient of friction of two unknowns.

INTRODUCTION:
We can define friction as the resistance to the movement of one body in relation to another body with which it is in contact. For example if we try to slide a wooden block across a table then friction acts in the opposite direction to the movement of the block. The amount of friction will depend upon the nature of the two surfaces in contact with each other.

We can measure friction in terms of a coefficient of friction, this is the ratio of the force needed to move two objects in contact with one another and the force holding the two objects together. For example, if we have a block resting on a table with the table exerting an upward force of 20N on the block and we need to apply a force of 5N to move the block then the coefficient of friction is (5/20) or 0.25. In terms of an equation for this we can write:

F_{static friction} = coefficient of static friction(µ_static) x the force exerted on the object by the surface (F_Normal)

F_{static friction} = µ_static x F_Normal #1

The F_Normal is the force of the surface, under the object, pushing up on the object at a perpendicular angle. This can be expanded to:

F_Normal = mass_object x gravity x cos(angle) #2

The angle refers to the angle of the surface on which the object is found(in respect to horizontal). If the object rests on a horizontal surface, cos(angle) = cos(0) = 1. If the object is on a 30 degree incline, cos(angle) = cos(30) = 0.87. Notice that the Normal force decreases with an increase in angle. During a lab we hook a string to a can that is looped over a pulley (see picture below) and add mass to the can until the object (wood block) starts to move to the right. At that point we have just overcome the F_{static friction} with the mass of our can and gravity.

F_{static friction} = mass_can x gravity #3

If we plug equations #2 and #3 into equation #1 we have:

mass_can x gravity = µ_static x mass_object x gravity x cos(angle)

OR

µ_static = mass_can x gravity / mass_object x gravity x cos(angle)

From experimental data we can now determine the coefficient of friction for this object.

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A platform made of wood will provide the surface to test the coefficient of friction on. A motorized lever will allow the inclination of the platform(left) and a pulley and weight(right) will provide the pulling force.

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The kinetic coefficient of friction will be tested by dragging the wood block with a Newton scale.

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Two different blocks of wood will be tested to determine their coefficients of friction.

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If you get into trouble and perform some procedure that causes the lab to fail(lab equipment will no longer operate), you can press the "Reset" button and the simulation will return to the starting position.

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PROCEDURE:

Room 1: Coefficient of Static Friction

1) You can adjust the background shading by clicking on the "Special" button to the right and selecting "Background". Click on the "Special" button and select "Print Blank Report" to obtain a web page that can be printed and used as a lab report. (the program will not be interrupted)

2) Drag the wood block (#1) on the upper left platform to the right and left. It is connected to a can hanging from a string looped over a pulley. Click on the loop of string connected to wood block #1 and note how an assistant's hand will hold the string and can in place. Drag the body of wood block #1 down to the electronic balance and record its mass. Note: you can not remove the wood block if the platform is not in its horizontal position!

3) Drag wood block #1 back up to the platform and drop it in place so that the hook catches the loop on the string. Remove the hand by clicking on it and then push wood block #1 all the way to the left.

4) On the left hand bottom of the screen you will note a set of angle control buttons. Click and hold on the "down" button and observe the angle of the platform change. As it changes a blue protractor hanging under the platform will register the current angle. Keep pressing until the platform is all the way down and registers 30 degrees.

5) The force produced by the mass of the empty can is not enough to overcome the static friction force of the platform and wood block #1. On the table is a container of metal shot. Drag one shot up and drop it in the can. Repeat the addition of shot to the can until wood block #1 slides down the platform. Note: depending on your sample, it may take 20 short or so. The combined mass of the can and shot has just overcome the force of friction between the wood block and platform.

6) The can has now disappeared below the table so adjust the angle of the platform to bring it back up into view. Drag the can from the string and place it on the electronic balance. Record the combined mass of the can and shot. The string is so thin it does not have a mass of any magnitude and can be ignored. Return the can to the string.

7) Adjust the angle of the platform to 20 degrees and slide the wood block all the way to the left. Add shot to the can, which still has the shot from before, until the wood block slides down the platform once again. Record the mass of the can & shot in the same manner as before.

8) Repeat this same procedure, recording the mass, at an angle of 10 degrees and 0 degrees. Note: because each lab has different masses of wood block and coefficient of friction, it is possible to have a situation where the same mass of can & shot will cause the wood block to slide at 10 and 0 degrees. In this case record the same mass for each.

9) Place the platform in the horizontal position. Click on the loop of string where it connects to the hook of wood block #1 causing the hand to appear. Remove wood block #1 and set it on the table. Drag wood block #2 over to the balance and record its mass. Place wood block #2 on the platform connecting the hook and loop from the string. Drag the can over to the container of shot and while holding the left lip of the can centered over and above the shot container press the "p" key. This will empty the can into the container for our next run. Return the can to the string.

10) Follow the same procedure for wood block #2 as outlined for wood block #1, recording the required data. Move to the next room.

Room 2: Coefficient of Kinetic Friction

11) The same wood blocks from the last room have been transferred to this room and have their same mass. Click on the hook of wood block #1 on the platform. An assisting hand will appear with a newton scale attached to the hook of the wood block. Here you will use the scale to determine the force needed to "keep the wood block moving". This is not to determine the force needed to start movement but to maintain movement. Drag the hand to the right in a smooth constant movement and read the value shown on the "close up" of the scale. Push the hand (which moves the block) to the left and repeat as many times as needed in order to get a good reading. Record this value under the angle of 0 degrees.

12) Adjust the platform and take readings at 10, 20 and 30 degrees, recording after each run. Return the platform to 0 degrees and click on the hook of the wood block. Once the hand has disappeared, drag wood block #1 down and place it on the table.

13) Place wood block #2 on the platform and follow the same procedure as above recording at 0, 10, 20 and 30 degrees.

14) Calculate the requested values asked for on the lab sheet and any given by your teacher. For help on these values click on the "Special" button and select "View Data & Hints". Select "File Report" to send a copy to be viewed by your teacher.