Download Friction Lab

Friction Lab: Setting Up An Experiment
1. Define friction. ___________________________________________________________________________________________
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2. You will perform a series of experiments to see what can be learned about friction. You will be
using a force measurer (to measure force), a surface board, and different masses. Your teacher
will demonstrate the proper use of the equipment.
3. Practice using the equipment by measuring the amount of force it takes to slide the wooden
block and one unit across the rubber surface on your surface board. Take three readings.
Remember:
a. Slide the surface board away from the force measurer slowly and evenly.
b. Be sure that the string, block, and force measurer are always lined up correctly.
c. Keep the surface of the block and surface board free of dust and grit.
d. Always zero your force measurer before each use.
Trial
Weight
1
Block + 1 unit
2
Block + 1 unit
3
Block + 1 unit
Force Reading
4. Your force reading may have been different from each other. This may surprise you since they
were all supposed to measure the same thing. Keep in mind, though, that it is difficult to measure
things without some error. Scientists repeat measurements and find an average for those
measurements to cut down errors.
How will you find your average force reading for your three trials in question #3?
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You need your teacher’s initials to continue. _________________________
5. What is your average force reading for question #3? ______________________________________
6. Now let’s change one variable in this experiment. Do you think that changing the weight on the
block will affect the force readings? ___________________________________________________
7. Write a hypothesis for what you think will happen to the force reading as we add weight on the
block. As the weight on the block increases, the force reading will _____________________________________
8. You will now test your hypothesis. Look at the set up of the data chart below before you begin.
Notice that you will be changing the mass you place on your block. Be sure to increase the mass as
you work down the chart. Also be sure to record the mass you use. YOU WILL USE THE RUBBER
SURFACE FOR ALL TRIALS on the chart.
Total Weight
Trial 1
Trial 2
Trial 3
Average
Block + I unit
Block + 2 units
Block + 3 units
Block + 4 units
9. Why did you always use the same surface (rubber) in question #8 above?
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10. Graph your results on graph paper. On which axis does mass go on? _____________________________
On which axis does Average Force reading belong on? ______________________________
11. What happened to the force readings as you added more weight?
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12. What does this tell you about what happens to the force of friction as the weight/mass on a
surface increases?
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13. Check your hypothesis in question #7. Does your data suggest that the prediction you made is
correct? ______________________________________________________
14. Ask your teacher to help you draw the best-fit line for your graph.
15. Use your best-fit line to predict how great friction would be if you used a block + 6 units
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16. Suppose you added 8 units to the block. How great would friction be? ____________________
17. Data collected in a good experiment should let you predict what will happen in situations not
actually tested. Why did the graph you made help you to predict what would happen in
experiments you have not actually done? (How could you answer #15 and #16 without
performing the experiments)?
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18. Look carefully at your surface board. Notice that some surfaces are rougher (bumpier) than
others. Predict what you think the relationship between surface roughness and friction will be.
As the surface becomes rougher (bumpier), friction will ___________________________________________
19. Describe an experiment you could do to test this prediction.
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20. Should the weight/mass on the block be changed or kept the same in your experiment? Why?
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21. Any experiment to test the prediction in #18 should involve varying the surface roughness
while keeping the weight/mass on the block the same. Check your answers to #19 and #20
22. Carefully study the data chart below. Choose the amount of weight/mass you will have on your
blocks through the entire experiment. Our weight/mass will always be Block + _______ units
23. You need your teacher’s initials to continue. _______________________________
24. Fill in the data chart as you complete the experiment.
Surface
Force Reading with block + ___________ units
Trial 1
Trial 2
Trial 3
Average
Paper
Cork
Sandpaper
Rubber
25. Did the amount of friction change as the surface on which the block moved was changed?
26. As the surface became rougher (bumpier), what happened to the amount of friction?
27. Do the results of your experiment agree with the hypothesis you made in question #18?
28. In this experiment, between which two objects is there friction?
29. So far, you have been doing experiments in which only one variable was changed and the result
was measured.
Which variable was changed in question #8?
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Which variable was changed in questions #24?
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30. Define variable. Use a resource book.
31. Let’s see what happens when two variables are changed at the same time in an experiment.
Study the data chart below. Notice that you will change both the weight/mass on the block and
the surface texture at the same time.
Variable I
Variable II
Block + 1 unit
Sandpaper
Block + 2 units
Rubber
Block + 3 units
Cork
Block + 4 units
Paper
Results (force reading in Newtons)
32. Using only the information you gathered in question#31, what is the effect of weight on the
amount of friction in this experiment?
33. What is the effect of the surface texture on the amount of friction in question #31?
34. You will need your teacher’s initials to continue. ________________________________
35. Why can’t we draw any conclusions from your data in #31?
36. You ought to be able to identify the variables in a situation. You should also be able to set up an
experiment in which you purposely change only one variable. Read the following story
carefully to see if you can identify flaws in an experiment.
Daisy Dingdong’s father owns the town ice rink. She noticed how easily things, such as
hockey pucks, would slide across the ice. She reasoned that cold surfaces must produce less
friction than hot ones.
To test her guess, Daisy decided to use the freezer compartment of her refrigerator. First,
she emptied it and measured how much force it took to pull a wooden block across the bottom
of the freezer. She did this to find out how much friction there was on a cold surface.
She then measured how much force it took to pull the same wooden block across the floor.
This gave her a measure of the amount of friction on a surface at room temperature.
Finally, Daisy measured the force it took to move the block across the bottom of a heated
oven.
After completing the experiment, Daisy found that she couldn’t decide what her data meant.
37. What variable did Daisy keep constant (unchanged) in this experiment?
38. Which variable did Daisy change on purpose?