Experimental physics

Static Friction, μs  and Kinetic Friction μk

Introduction

In this experiment you will be measuring the angle at which an object starts to move down a slope. You will be using this angle to calculate the coefficient of static friction between the surface of your object and the  slope. You will repeat your experiments by finding the angle at which the object

The investigation is designed to give you an understanding of:

•     Separating forces into components

•     Frictional forces

•     Newton’s second law

Theory

The following information may be helpful.

What you will need

Assemble this equipment before you start the exercise:

•     A ramp (this may be a plank of wood, a flattened cardboard box or even a very large textbook)

•     A protractor

•     A box that fits on your ramp (e.g., an empty tissue box)

•     Objects that can be used to evenly distribute mass over the box (could be weights, stones, books)

•     A set of kitchen scales

Maintaining the integrity of your work

Turnitin Submission

Your submission must be typed, not handwritten, so that it can be read by Turnitin. Hand drawn diagrams are accepted but data, tables, graphs and explanations must be typed.

Collaboration

You may either do this investigation alone or with up to one other person currently enrolled in the course (i.e., you may not work as a group of three or more).

▲  If you work with someone else, you need to acknowledge your collaborator (include their name and student number in the report and include them in your photo). In this case you must conduct the analysis and answer the questions individually, even if your data is the same. If you have identical answers that will be considered plagiarism, so while it is OK to collect the data together you need to answer the questions and plot the graphs etc. as individuals.

Photographic evidence

You must include two (2) photographs in your report:

•    A close-up image of your UNSW photo ID card with your name and photo clearly visible.

•    A photograph of yourself with your student card, the equipment and a piece of paper clearly

showing the date and your name and student number. If you worked with a partner this photo must include both of you.

▲  You MUST provide these photos in your assessment. Assessments without the photos will not be marked and you will receive a zero.

<  Sample photo of equipment, faces of people who completed the experiment, your student cards and paper with names, student numbers and date. You also need to include a close-up of your student card.

If the experiment asks you to include additional photos, such as to record and report observations, then you must include a label in the image showing your name and zID (just like in the photo above except you do not need to be in it)

Risk Assessment

Read over the experimental method and then complete a risk assessment in a table similar to the one below and include it in your report.

You must minimize all risks before you start. Add as many rows to the table as you need. If any of the risks rank above medium, you need to adapt the experiment to make it safer before proceeding.

By proceeding with the experiment, you are agreeing to follow these risk control methods and conduct the experiment safely. If you have any doubts about your ability to complete the experiment safely then you should not proceed.

▲  In your report you need to include your risk assessment and state that you have agreed to follow the risk controls. Without these, your report will not be marked.

Procedure

Phase 1: Static Friction

1.    Set up your ramp. You will need to be able to change the angle of the ramp so that you can accurately measure that angle at which the object just starts to move down the slope. You will need to work out  how to do this, you could use a car jack, chair or a stack of books to support one end of the ramp.

2.    Record the masses of the object and the box that you place on the slope.

3.    Place the object in the box on the slope. Take a photograph of it to submit in your report (include your name, zID and student card in this image, too). Increase the angle of your slope from horizontal until the box just starts to move. Measure this angle using the protractor and record it in the results table. In your report, describe the method you used to make your reading of the protractor as accurate as possible.

4.    Repeat the measurement twice (so that you have three measurements in total).

5.    Place more mass on the object and repeat steps 3 and 4.

6.    Repeat steps 3-5 until you have recorded results for 5 different masses (keep the surfaces in contact the same).

Results

Record your results in a table like the one below.

Phase 2: Kinetic Friction

7.    Set up your ramp. You will need to be able to change the angle of the ramp so that you can accurately measure that angle at which the object just starts to move down the slope. You will need to work out  how to do this, you could use a car jack, chair or a stack of books to support one end of the ramp.

8.    Place an amount of mass in the box; you should use the same mass(es) as you did in Phase 1.

9.    Place your box on the slope, at a specific spot; lightly tap the object so that it moves and then comes to a stop.

10.  Repeatedly tap the box (returning it to the original position on the ramp each time) while increasing the angle of the ramp. There will be one angle for which the box keeps sliding; record this angle!

11.  Repeat the measurement twice (so that you have three measurements in total).

12.  Place more mass on the object and repeat steps 9 - 12.

13.  Repeat steps 9-12 until you have recorded results for 5 different masses (keep the surfaces in contact the same).

Results

Record your results in a table like the one below.

You may find the following equations useful:

Analysis

1.    Show that μs  = tan θs, using a diagram.

▲  If you find it difficult to type your working or draw diagrams in your document, then you can do these with pen/pencil and paper and insert an image ofit into your report.

2.    For each mass calculate the coefficient of static friction between the object and the ramp. Calculate the uncertainty in this coefficient for each mass.

3.    Calculate the overall coefficient of static friction between the two surfaces that you have chosen.

Include an uncertainty.

For the uncertainty in the friction coefficient, use 

4.    For each mass calculate the coefficient of kinetic friction between the object and the ramp. Calculate the uncertainty in this coefficient for each mass.

5.    Calculate the overall coefficient of kinetic friction between the two surfaces that you have chosen.

Include an uncertainty.

For the uncertainty in the friction coefficient, use 

Questions

▲l.  If you find it difficult to type your working or draw diagrams in your document, then you can do these with pen/pencil and paper and insert an image ofit into your report.

1.    For static friction:  Draw a labelled diagram showing all the forces acting on the object when the slope is at an angle halfway between 0° and the angle at which the object started to move (for the highest mass case). Label your forces as weight force, normal reaction force and frictional force.

2.   What is the net force acting on the object in the case you have just drawn?

3.    For static friction: Calculate the component of the gravitational force acting parallel to the slope just before the object starts to move (for the case with the highest mass).

4.    For static friction: Calculate the normal reaction force experienced by the object on the slope just before it starts to move (for the case with the highest mass).

5.   What is the relationship between μs  and μk? Is that what you expect?.

6.    Plot a graph of μs versus mass and μk  verses mass.. Does this graph agree with your expectation? Why or why not?

▲  Include a screenshot of this graph in your report.

What to submit

▲  You should type up your report to submit it to Turnitin. The report needs to be in a single file. It is better to submit a .pdf rather than a .docx Word document as Word documents can display different on different computers. Your tutor will mark the report as it is displayed on their machine.

The report should include:

•    A statement of the aim of the investigation

•    The risk assessment and a statement saying that you will follow the risk controls recommended in the risk assessment

•    A description of how you measured the angle accurately

•    Everything in the results and analysis section of the investigation

•    Details of your analysis of the results and predictions (i.e., show how you did the calculations)

•    A photograph of yourself and partner (if you had one), your student card, a piece of paper with the date, name and student number and the equipment as well as a close up of your student card.

•    Answers to the questions

•    A conclusion stating what you have found out

You report should not include:

•     The procedure (as this has been given to you, only comment if you make a change to this procedure)

Rubric

(This rubric has been programmed into TurnItIn):