Push Cart Lab
12
When an object is in motion, the force between itself and the surface it is on is called the force of kinetic
friction. We have learned that this crucial force, dependant on the coefficient of kinetic friction and the weight
of the object, is the cutoff between whether or not the object in motion will maintain its speed (C.V.) or change
its speed (acc/dec).
N
N
N
Ff
FA
Fg
Ff
FA
Ff
Fg
Fg
FA > Ff
FA = Ff
Fnet = FA − Ff
Fnet = 0
Acc.
C.V.
Fnet = Ff
Dec.
In this experiment, we will utilize the effects of friction on an object in motion with no applied force. That is to
say, the net force is the frictional force. Therefore, we can seek to determine the coefficient of kinetic friction
between the cart and the floor by simply calculating the rate of deceleration of the cart.
Learning Targets:


Students will utilize Galileo’s Equations of Uniform Acceleration and their measurements to
determine the deceleration rate of the cart.
Students will use the deceleration rate to determine the coefficient of kinetic friction between the
cart and the floor.
Procedure
A. Computing the Final Velocity of the Push
1. In groups of 4, record the names of each member of the group in Table 20-1. Have each person
weight themselves on the scales in the room; divide their weight in pounds by 2.2 lbs/kg to get their
mass in kg. Record their mass in Table 20-1.
2. With each person taking a turn as the “rider,” have another partner push them from rest to the 4m
line. Make sure the pusher lets go at this line and does not get an “extra” push. Record the time of
the push in Table 20-1. As the partner on the cart comes to rest, measure and record the stopping
distance in Table 20-2.
Table 20-1
Rider
Mass
(kg)
Acceleration
Distance
(m)
4
Time of Push
(sec)
Acc.
(m/s2)
Final Velocity
(m/s)
4
4
4
3. If we assume the force of each push was constant, we can then assume that the acceleration of each
1
push was also constant. Using Galileo’s 2nd Equation,
x = vo t + 2 at 2 , solve for the
acceleration rate of each rider. Record your results in Table 33-1.
4. Use Galileo’s 1st Equation, vf = vo + at and the acceleration calculated in Step 3, find the Final
Velocity of each rider. Record your results in Table 20-1.
B. Computing the Deceleration Rate and the Coefficient of Kinetic Friction
1. At the point of release, the rider has reached a maximum velocity, which we calculated as v f
above. This final velocity then is the velocity at which the rider begins to slow down from
once the pusher releases the cart. Therefore, the old vf is the initial velocity for our
deceleration portion of this lab. Record the old vf as the new vo in Table 20-2.
2. Since the final velocity of the rider was 0 m/s and the force of friction was uniform (making
the acceleration uniform), we can utilize Galilio’s 3rd Equation, vf2 = vo2 + 2ax to solve for
the deceleration rate of each rider. Record this rate in Table 20-2.
3. Since the Force of Friction is the Net Force for this part of the lab (refer to the 3 rd free body
diagram in the introduction), we can solve for the coefficient of kinetic friction by setting
Fnet equal to Ff.
Fnet = Ff
4. Find your group’s average coefficient of kinetic friction. Record this value in both Table 20-2
and the board.
Table 20-2
Initial Velocity Stopping Distance
(m/s)
(m)
Deceleration Rate
(m/s2)
Average Coefficient
Coefficient of
Kinetic Friction
Interpretation Questions
1. Does the coefficient of kinetic friction depend on the weight of the rider? Explain.
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2. Are your calculated coefficients of friction relatively equal? Why or why not? Should they be?
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3. What are some sources of error during this experiment?
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