Dr. AH Ta
izi (Online COVID-19 version)
Engineering Processes & Tools (Engr. 10)
Laboratory Experiment
Dynamics of Gravity Powered Cars
In this lab speed and acceleration of several cars are measured and also
calculated using a mathematical model based on energy balance. The cars
start from rest on a race track at some height and coast down the track
picking up speed. Sensors measure the speed of the cars at various points
along the track and the total time lapse at the end of the run. Also, the drag
force on the cars at some speeds is measured in a wind tunnel.
To model the dynamics of the cars using energy principles we need to
identify all the possible energy quantities that are at work in this problem,
e.g. potential, kinetics, friction, drag, etc. The calculated speed should be
compared to the measured value and discrepancies are noted and justified.
The learning outcomes of this lab are:
1. Learn how to set up an experiment and collect data
2. Learn how to foresee potential problems in measurement
3. Learn how to create a mathematical model appropriate to the
experiment being conducted
4. Learn how to post-process the collected information, i.e. perform
comparison and evaluation of the experimental and mathematical data
Please refer to the information provided in the lab for the particulars of the
experiment.
Test Procedure:
1. Get familiar with your team members.
2. Weigh your racecar using the electronic balance provided. See the
given data below.
3. Measure all the dimensions of your car. See the given data below.
4. Take photos of your car from various angles as well as the measuring
tools used. Write down the accuracy of the measurements. See the
given picture below.
5. Measure the length of the track, release height of the car, and the
slope of the track. See the given data below.
6. Place your car on the track, release it, and write down the total time
lapse. IGNORE this item.
Dr. AH Ta
izi (Online COVID-19 version)
7. Place your car on the track and place the photo-gate system at the
pre-marked locations. Release your car and record the velocity of your
car at the photo-gate location. Make sure you measure the distance
traveled to the photo-gate. Refer to the lab discussion.
8. Repeat part 7 for various distances along the track as marked.
9. Place your car on the section of the track that is horizontal and using
the force sensor push on the car gently until the car moves. Record
this force, which is the rolling friction force. Refer to the lab discussion
and use the data given below.
10. Place your car in the wind tunnel and run the tunnel at the
airspeeds of about 30 and 20 mph. Record the drag force as well as
the front and back lift force on the car. The lift forces will not be used
in computation but they could allow for some discussions. Using these
drag forces you will calculate the velocity of your car at various
locations along the track. Note that you have already measured
velocities at these locations so hopefully your calculations and the
measurement will compare favorably. Refer to the lab discussion and
use the data given below.
11. Write a report to document your work. You need to make
calculations of your car’s velocity using the energy equations and data
obtained. Compare your calculated velocities with the measured values
and discuss discrepancies.
Note on the Written Report:
You should use the report template posted online. You may include pictures,
graphs, tables, etc. as appropriate. Make sure all your dimensions are
accurate. Please remember that you are communicating with your customer
(or your company’s official) so make sure your report is properly done!
Table 1. Data for the car experiment
Car
mass
(kg)
Friction
force
(N)
Drag
force
(N)
Distance
etween
stations
(m)
Number
of
stations
Car
Material
Track
Material
XXXXXXXXXXSteel Aluminum
XXXXXXXXXXSteel Aluminum
XXXXXXXXXXSteel Aluminum
XXXXXXXXXXSteel Aluminum
Dr. AH Ta
izi (Online COVID-19 version)
Table 2: Station heights from
the floor for the car experiment
Station 1
Height
170 mm
Station 2
Height
150 mm
Station 3
Height
110 mm
Station 4
Height
80 mm
Station 5
Height
60 mm
Station 6
Height
40 mm
Station 7
Height
20 mm
Station 8
Height
10 mm
Station 9
Height
0 mm
Station 10
Height
0 mm
Slide 1
*
Potential Energy
Kinetic Energy
Frictional Losses
Aerodynamic Losses
Images from: highlandtoday.com, ag-de
y-cars.com, de
ytalk.com
*
*
Weight (use electronic scale)
Dimensions (use caliper and ruler)
Start Height (use measuring tape)
Time of Travel (use stop watch etc.)
Speed (measure at 4 places using photo-gates)
*
Friction
Body Shape (why?)
Release Height
Weight
Any other?
*
E
or in Measurement
Human Factors
Inaccuracies in Mathematical Model
E
or in Computation
*
Let’s do some calculations to see if we could predict the car speed from the measured data
*
List the computed values and discuss
*
2
(J)
m = mass of object (kg)
g = gravitational acceleration (9.81 m/s
)
h = height of object (m)
PEmgh
=
2
1
(J)
2
v = velocity of object (m/s)
KEmv
=
. d (J)
f = rolling friction of wheel (m XXXXXXXXXX
027 for plastic on steel
w = force on the surface (wheel weight +
external force) (N)
R = radius of wheel (m)
d = distance traveled
w
FEf
R
=
d (J)
F = Friction force (measured using force
gauge)
d = distance traveled
f
f
FEF
=
2
2
3
1
(N)
2
C = Drag Coefficient
A = Projected Frontal area (m)
v = velocity (m/s)
= Air density (kg/m)
dd
d
FCAv
=
: Wind tunnel measurement (gram)
= F * g/ XXXXXXXXXXN)
d
F
F
2
2
1
(N)
2
C =
1
2
Measured in wind tunnel
v = Wind tunnel air speed
dd
d
d
d
FCAv
F
Av
F
=
=
2
**
1
**
2
distance traveled (meter)
mass (kg)
car height (meter)
car velocity (m/s)
& friction force (Newton)
df
df
df
PEKEFdFd
mghmvFdFd
d
m
h
v
FF
=++
=++
=
=
=
=
=