FRICTION
In this experiment, you will apply a horizontal force to a block to pull the block at
constant velocity across a horizontal plane (the table). You will add books to the block to
find if the force changes with the mass being pulled. (What do you think might happen
to the required force?)
Problem: How is friction affected by mass? (kinetic friction? You are only testing one
aspect of friction.)
Hypothesis:
Materials: Wooden block, textbooks (4), 5 or 10 N spring scale, balance
Procedure:
Determine the mass of your block using a balance. Multiply this number by 9.80 to find
the weight. (do units matter here?) What units should force be in? Shouldn’t they know
how to do this by now?
Pull the block five times using the spring scale and record each measurement on the
scale. Calculate the average of these values and put it into the pull force column of the
data table (at the Physics 30 level do they need this level of directive detail?).
Add one textbook to the block. Measure the total new mass and total weight.
Pull the block five times during each trial and find the average (of what?).
Repeat up until four textbooks are on the block.
Using the following ideas and your wide edge data for the next part of the procedure; 1)
the magnitude of the force of the pull will equal the magnitude of the force of kinetic
friction, and 2) the magnitude of the weight will equal the magnitude of the normal force.
Plot and fit a graph of FRICTION FORCE vs NORMAL FORCE (Wednesday in the
computer lab), place the line of best fit on your graph, and on the printed sheet, show
your work in determining the equation of the line as practiced in your notes (y = mx + b).
Observations:
Force to pull blocks – wide edge
Total
Mass of
textbooks
+ block
(kg)
Pull 1
(N)
Pull 2
(N)
Pull 3
(N)
Total
Weight (N)
Pull 4
(N)
Average
Pull Force
(N)
Pull 5
(N)
Average
Pull Force
(N)
Analysis:
1) Why can we say that weight is equal to the normal force? Because it’s a
definition?
2) a) As you were carrying out the measurements above, what did you notice about
the reading on the scale just as the box began to move?
b) What does this tell you about static friction when compared to kinetic friction?
3) If __________ kg of mass were placed on the block, what would you expect the
force of friction to be? Use your equation of fit to calculate the answer. Show all
work for predictions.
4) What does your graph tell you about the relationship between friction and the
normal force?
5) Seeing as the narrow edge of the block is also wood hypothesize as to whether
friction would be different for that side. So will you tell them the answer or let
them find out through activity? I vote for activity – they know the equipment now
– why not let them find out.
Application *Remember to site your sources:
1) Determine and explain three differences between all-season tires and winter
tires.
2) How does the coefficient of friction play a role in winter driving?
3) Based on your knowledge of friction, would you use winter tires or all-season
tires on your vehicle? Depends on finances, and what their parents think about
snow tires. We have long history of driving in the winter with all season radials. A
bad history mind you, but a history nonetheless.
4) Make a recommendation as to which winter tires are best (research it!), and
support your answer using factors that affect the coefficient of friction. (Of course
there are winter tires, and all weather tires, and all season tires. All weather are
not rated the same as all season tires. So much to learn – check the Canadian
Tire website.
Conclusion: Why must you have a conclusion? How is friction affected by mass?
Is that what you want a conclusion for.