Finding the Frictional graph when pulling a wagon activity, Physics/Adv Physics
In lecture we studied the friction-time graph. We pulled a weight horizontally and looked at the following pics
and graph:
(a)
starting to pull
(b)
string is tight—
mass and block not
yet moving (peak
static friction not
yet reached)
(c)
motion—we have
kinetic friction now
Friction-time graph when pulling a weight
Static friction grows as
you pull harder and
harder but weight not
moving
string now
tight but
weight not
moving
Peak static friction. At this
point, my pull breaks
through the static friction
bonding sites and weight
begins to move
Kinetic friction. This is the frictional
force needed to keep the object moving
at constant speed. Notice this is less than
the force to start the object moving.
Finding the Frictional graph when pulling a wagon activity, Physics/Advanced Physics, p. 1
Can we make a similar graph when pulling somebody in the wagon? We are going to try.
Keep the handle at an angle of 0 degrees (pull directly horizontally). Attach the spring scale to the wagon's handle
and have somebody get totally in (not dragging your feet). IN THIS CASE, YOU ARE GOING TO PULL WITH A SMALL FORCE
INITIALLY AND BUILD THIS FORCE UP SLOWLY (OVER A FEW SECONDS). Keep pulling until you start to move the
wagon/person combination and then keep pulling for a few more seconds at a constant speed.
VIDEO the spring scale. MAKE SURE YOU CAN SEE THE NUMBERS ON THE SPRING SCALE! Walk with the puller.
(a) Weight of wagon/person (person steps on bathroom scale with wagon) in lbs: ______________ lb
(b) Determine the normal force: In this case the weight (line b above) and the normal force are EQUAL.
Normal force = Weight (line b above)
or
Normal force = ___________ lbs
------------------------------------------------------------------------------------------------------------------
Data: Watch the video on logger pro and pause the video from regular intervals (0.1 or 0.2 seconds or so)
starting from 0 seconds and read the spring scale from the video. Make sure to note the largest value
(peak static friction). See next page for data table. Write in your values
----------------------------------------------------------------------------------------Graph your data here. Hope we get something like our graph on p. 1! When you are done with the graph,
use it to help you on page 5.
Finding the Frictional graph when pulling a wagon activity, Physics/Advanced Physics, p. 2
Data Table: SEE MY DATA TABLE ON THE NEXT PAGE.
You may not need all of these spaces on the data table.
Use the video to read the spring scale and intervals around
(0.1 or 0.2 seconds). Watch the scale carefully and read to
the nearest tenth of a lb. Use this data to make your graph
on p. 2.
This is the
time
Read the spring scale in lb.
Read it to the tenth. This would
be around 0.6 lbs
Finding the Frictional graph when pulling a wagon activity, Physics/Advanced Physics, p. 3
My sample graph
Finding the Frictional graph when pulling a wagon activity, Physics/Advanced Physics, p. 4
Let's see if we can find the frictional texture (, mu). We said mu is the quantity of bumps, ridges, and
contact points on the object and surface. The higher this number the higher the friction. We said that
there are two of these mus, µstatic and µkinetic.
µstatic occurs at the peak of our graph right when the object starts to move. This is generally larger than
µkinetic because there are more time to build up the contact points and interlock the bumps that cause
friction.
Finding µstatic
In our notes, we said that the following equation is true:
Ffs (peak static friction) = s * (Fn (normal force)) or
s =
Ffs (peak) peak force that just starts object moving,see graph (lbs)
Fn (normal force),this is line b on page 2 (lbs)
s = __________________
---------------------------------------------------------------------------------------------------------------
Finding µkinetic
In our notes, we said that the following equation is true:
Ffk (kinetic friction) = k * (FN (normal force))
k =
or
Ffk (kinetic) (approx force that just keeps object moving,see graph) (lbs)
Fn (normal force),this is line b on page 2 (lbs)
k = __________________
----------------------------------------------------------------------------------------------------------------Open this website for a nice coefficient of friction table (scroll down): http://blog.mechguru.com/machinedesign/typical-coefficient-of-friction-values-for-common-materials/
or check the table in your notes and see if you can find something close to your values.
Something close: ____________________________________________________________________________
Finding the Frictional graph when pulling a wagon activity, Physics/Advanced Physics, p. 5