IZMIR UNIVERSITY OF ECONOMICS
FACULTY OF ENGINEERING AND COMPUTER
SCIENCE
Electrical and Electronics Engineering
PHYS100: General Physics Lab. # 9
Elastic & Inelastic Collisions
Assoc.Prof.Dr. Diaa Gadelmavla
PHYS100: Lab # 9
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Contents:
1. Objectives
2. Theory
3. Lab instructions
a. Equipment setup
b. Lab procedure
c. Presenting results
4. Questions
PHYS100: Lab # 9
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1. Objectives:
a. Determine the impulse exerted on an object by knowing its mass
and measuring its change in velocity.
b. Verify the law of conservation of momentum for both elastic and
inelastic collisions
c. Determine the conditions necessary in order for momentum to be
conserved.
d. Analyze the obtained data and check for any sign of friciton forces
PHYS100: Lab # 9
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2. Theory
The mass and velocity of objects affect their motion, and thus a combination of
both defines the linear momentum of an object (the product of its mass and
velocity):
p = mv
The momentum is a verctor quantity and has the same direction as the velocity
vector v. The concept of momentum conservation in physics is a basic law. It is
not analytically derived but empirically proved (from experiments) and to be
valid for all isolated systems (if the net external forces acting on the system is
zero – all external effects are ignored).
For a system of two objects the total momentum is given by:
ptot = p1 + p2
Which stays constant for any isolated system.
Usually the concept of momentum conservation is used for collision problems,
for any isolated system the total momentum before collision is equal to the total
momentum after collsion:
pi = pf
or for a system of two objects:
p1i + p2i = p1f + p2f
The left side represents the total momentum of the two objects before collision
and the right side represents the total momentum after the collision. In reality,
the momentum of each object changes but the total value is always constant. The
concept of momentum conservation in an isolated system is interpreted as any
loss of momentum by some objects will be gained by the others.
For a system of any number of objects, the above equation can be generalized
as:
PHYS100: Lab # 9
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If the system is not completely isolated from external forces, there may be a
change in momentum and this defines a quantity called impulse I:
3. Lab Instructions
The volocities of two gliders, moving without friction on an air-cushion
track, are measured before and after the collision, for both elastic and
inelastic collision.
a. Equipment setup
Figure. 1: Experimental set up for investigating the laws of collision.
b. Lab procedure
Lab activity # 1: Velocity determination
1. Adjust the levelling of the air track:
a. Connect the air supply to the air fitting on the end of the air track.
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b. Place the 300 gm glider on the air track and turn on the air source.
The glider will float and likely drift down to one end of the track or
the other.
c. Loosen the locking wing nut on the leveling screw and twist the
screw until the glider wanders on the middle of the track.
d. Tighten the locking wing nut, the track is now ready to be used.
2. Set up the photogates as a method of timing for this experiment. And then
choose the glider to be used.
3. Mount a flag along the spine of the glider. Measure the length of the flag.
If no flags are available, lower the photogate so that the beam is broken
by the glider.
4. Set up two photogates 0.5 m apart and connect them to seperate timers.
Ensure that they are mounted at the same height, so that the same cross
section of the glider blocks the beam at each gate.
5. Turn on the air source and place the glider on the air track. Be sure that
the gilders move smoothly on the air track.
6. Start the glider towards the zero end of the track and launch it towards the
end stop. This can be done by simply pushing the glider by hand, as the
initial velocity is irrelevent to the final analysis.
7. Determine the velocity of the object, it is calculated by dividing the
distance the object travels by the time it takes to cover that distance:
v=L/t
L: is the length of the glider or the flag
t : the amount of time that the sensor is blocked
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Lab activity # 2: Elastic Collision
Figure 2. The experimental setup used for the elastic collision.
In this part of the experiment, we will observe the elastic collision between two
gliders.
1. Keep the photogates in the same positions as in the first part of the
experiment and attach rubber bumpers to Gliders 1 and 2, and then
position Glider 2 at rest between Photo gate 1 and Photo gate 2. Both
Gliders 1 and 2 will be equipped with vertically positionedmeasurement flags.
2. Make sure that Glider 2 (the one that is going to be hit) is placed
between the two photo gates. Glider 1 should be outside the photo
gates (see Figure 2).
3. The first time measurement (“time before”) will be made by giving
Glider 1 a push, push it gently (see explaination below). As Glider 1
passes through Photo gate 1, a time interval will be measured. The
initial velocity, momentum and kinetic energy of Glider 1 can be
computed from the velocity measured, and from the mass of Glider 1.
The reason, why you want to push Glider 1 gently. You want it to
hit Glider 2 so that Glider 2 will start moving, but Glider 1 will stop
moving. Think of a situation where one pool ball hits another and
then stops – but the second ball, the one that was hit, starts moving.
That’s what you want to do with the gliders. Basically you are
transferring all the kinetic energy of Glider 1 to Glider 2.
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4. The second velocity you will measure is the velocity of Glider 2 as it
passes through the second photo gate. This is our “time after”. The
momentum and kinetic energy of Glider 2 can be computed from the
velocity measured, and from the mass of Glider 2.
5. Record your data into the following table:
m1 =
PHYS100: Lab # 9
kg
m2 =
kg
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Lab activity # 3: Inelastic Collision
Figure 3. Experimental setup used for the inelastic collision.
1. Give Glider 1 a push. As it passes through Photo gate 1, a time interval
(the “before” time) will be measured. The velocity and momentum of
Glider 1 can be computed from time data measured and the mass of the
glider.
2. Once Glider 1 strikes Glider 2, the two should stick together. The
resulting momentum of the coupled Gliders 1 and 2 can be computed
from their total masses and the velocity measured at Photo gate 2 (the
“after” time). Once the gliders have stuck together, you can treat them
as a single object. Since the recorder is measuring time, the velocity
recorded is automatically computed using the 2.5 cm flag width (∆x).
3. Repeat this experiment few times, and record your data in table 2.
m1 =
PHYS100: Lab # 9
kg
m2 =
kg
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c. Presenting results
1. For each run of your elastic & inelastic collisions, calculate the
percent difference between the initial momentum and the final
momentum. Does your data indicate conservation of momentum?
2. For run of your elastic & inelastic collisions, calculate the percent
difference between the initial energy and the final energy. Does your
data indicate conservation of energy?
3. List some possible source of error in this part of the experiment. Are
these sources of error random or systematic?
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4. Questions
Find an expression for the final velocities (V1f , V2f ) of two balls (masses
m1 and m2) undergo an elastic 1D collision in term of the initial velocities
(V1i , V2i ) and masses of the two balls.
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