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4-Nov-18 PHYS101 - 08
© KFUPM – PHYSICS Department of Physics revised 04/11/2018 Dhahran 31261
1
Elastic Collision
Objective
To study the laws of conservation of momentum and energy in an elastic collision.
Introduction
If no net external force acts on a system of particles, the total linear momentum of the system
cannot change. This is known as the law of conservation of linear momentum. In other words,
if the net external force on the system is zero, the total linear momentum of a system of
particles before and after a collision is equal. If the total kinetic energy of the system is also
conserved during the collision then it is called an elastic collision.
In this lab, you will study the properties of elastic collision in one dimension. Figure 1 shows
the experimental set up. A glider of mass m1, moving with speed v1i, collides with another
glider of mass m2, moving with speed v2i. Immediately after the collision, the first glider
moves with speed v1f and the second glider with speed v2f. You will determine the speeds
using motion sensors.
Figure 1
Flag
Elevation Screw USB Interface
Motion Sensor 2
v1i v2i
i
Motion Sensor 1
Glider 1 , mass m1
Glider 2 , mass m2
Air Track Air Blower
Rubber Band
Blade
Flag
4-Nov-18 PHYS101 - 08
© KFUPM – PHYSICS Department of Physics revised 04/11/2018 Dhahran 31261
2
If friction may be neglected, then in the collision of two gliders the total linear momentum of
the system is conserved. That is,
m1 1i + m2 2i = m1 1f + m2 2f
Note that the velocities are vector quantities. In an elastic collision, the total kinetic energy of
the system before and after the collision is also conserved. That is,
2
22
2
11
2
22
2
11 v2
1v
2
1v
2
1v
2
1ffii mmmm
Exercise 1: m1 ≈ m2 and v2i = 0
In this exercise, you are provided with two gliders of almost equal masses on an air track.
You will keep one of the gliders at rest before collision, and verify the conservation laws of
momentum and energy.
1. Measure the masses of the gliders m1 and m2 using a triple-beam balance. Record the
values in your report.
2. Download the file collision1.ds from the link in phys101 homepage and save it in the
desktop. This file has been preconfigured for optimum experimental parameters such
as position and time scales.
3. From the Start button, go to All Program DataStudio English. This will open
DataStudio. Click on Open Activity, navigate to the folder desktop and to the file
collision1.ds, and open it. This will open the position-time graph.
4. Level the air track as follows: turn on the air blower, and then adjust the elevation
screws at the ends of the track until glider 2 either remains stationary at the mid-point
of the track or moves slowly back and forth about this point. Leave this setting fixed
for the duration of the experiment.
5. Make sure the range switch of the motion sensor is set to work with the (short
range) option.
6. As shown in Figure 1, bring the glider 1 closer to the motion sensor 1 while the glider
2 stays at the center of the air track. Gently push the glider 1 away from the sensor 1
and click the Start button at the top left corner in DataStudio. Click the Stop button
when the glider 2 reaches the motion sensor 2, and switch off the air blower. It is
important to push the glider gently to obtain good data.
7. Your data will look like Figure 2.
4-Nov-18 PHYS101 - 08
© KFUPM – PHYSICS Department of Physics revised 04/11/2018 Dhahran 31261
3
Figure 2
8. Choose the appropriate straight line portions of the data by pressing the left button of
the mouse and drawing a rectangle as shown. Right click in the graph area and choose
Fit and then select Linear Fit. The magnitude of the slope of the fit gives the
corresponding speed. Draw the rectangle as close as possible to the location of the
collision and select just enough data points to obtain the speeds immediately before
Glider 1 comes to rest
Glider 1 is at rest
Glider 1 is moving away from Sensor 1 Positive slope
Collision occurs here
Glider 2 is at rest
Glider 2 is moving toward Sensor 2 Negative slope
Sensor starts sensing glider’s motion here
4-Nov-18 PHYS101 - 08
© KFUPM – PHYSICS Department of Physics revised 04/11/2018 Dhahran 31261
4
and immediately after the collision. Why do you think this step is important? See
Figure 3 for a close-up version of one of the graphs after fitting. You may avoid the
point right at the instant of collision.
9. Record the values in your report, with the correct sign for velocities and momentum.
Choose the direction to the left (moving away from the sensor 1) as positive.
10. Calculate the total momentum of the system before and after the collision and their
percent difference. Record the results in your report.
11. Do the same for the total kinetic energy of the system. Record the results in your
report.
Figure 3
Exercise 2: m1 ≈ m2 and v2i ≠ 0
In this exercise, you will study the collision between two gliders of almost equal mass,
moving in the same direction.
1. Delete all the data runs from the DataStudio file and get it ready for Exercise 2.
2. Switch on the air blower and bring the glider 1 closer to the motion sensor 1 and
glider 2 to the middle of the air track. Gently push the glider 1 towards glider 2 and
glider 2 towards motion sensor 2 such that they can collide well before reaching
sensor 2 (v1i and v2i both positive with v2i < v1i). Click the Start button at the top left
4-Nov-18 PHYS101 - 08
© KFUPM – PHYSICS Department of Physics revised 04/11/2018 Dhahran 31261
5
corner in DataStudio and click the Stop button when the glider 2 reaches sensor 2,
and switch off the air blower.
3. Fit the appropriate straight line portions of the data, as you did in Exercise 1, to
determine the speeds of the gliders just before and just after the collision.
4. Record the values in your report, with the correct signs for velocity and momentum.
Choose the direction to the left (moving away from the sensor 1) as positive.
5. Calculate the total momentum of the system before and after the collision and their
percent difference. Record the results in your report.
6. Do the same for the total kinetic energy of the system. Record the results in your
report.
Exercise 3: Impulse
Consider the collision between two gliders. They experience equal and opposite force (t)
that varies with time during the collision. The impulse of the force is defined as
dtt)(F J
Where the integration is over the duration of the collision. Therefore, impulse is a measure of
both the magnitude and the duration of the collision force, and is given by the area under the
Force versus time curve. Note also that the impulse is a vector quantity. That is, it has a
direction as well as magnitude.
Momentum of a body changes if a net force acts on it. Therefore, the momentum of each
glider changes due to the collision. It can be shown that the change in momentum of a glider
is the impulse of the force on that glider. That is
p J
Since the total momentum of the two-glider system is conserved, the change in momentum of
glider 1 should be equal and opposite to the change in momentum of glider 2. That is, the
impulse on glider 1 is equal in magnitude and opposite in direction of the impulse on glider 2.
In this exercise, you will determine the impulse during the collision between two gliders of
different mass from force versus time graph. You will also verify the force exerted between
them is equal in magnitude but opposite in direction at each moment, and thus the impulse on
the gliders are also equal and opposite to each other.
1. Download the file collision3.ds from the link in phys101 homepage and save it in the
desktop. This file has been preconfigured for optimum experimental parameters such
as force and time scales.
2. Replace the two gliders used in Exercise 1 and 2 by two different gliders of unequal
masses and with force sensors mounted on it (provided to you separately, ask the
4-Nov-18 PHYS101 - 08
© KFUPM – PHYSICS Department of Physics revised 04/11/2018 Dhahran 31261
6
instructor if you can’t find them). Make sure only the bumpers of the force sensors
will be in contact when the gliders collide (see Figure 4).
Figure 4
3. Plug the cable of the force sensors 1 and 2 to the USB interface module, after
unplugging the motion sensors. You will be prompted to include force sensor in the
graph. Click No button. Let the cable of the force sensor rest on the table. This step is
important, in each experimental run, to minimize the unwanted force exerted by the
cable on the glider.
4. From the Start button, go to All Program DataStudio English. This will open
DataStudio. Click on Open Activity. Click No to saving the current activity. Navigate
to the folder collision3.ds on the desktop. This will open the Force Graph windows.
5. Press the zero button on each force sensor to zero the force sensor. This step is very
important prior to each experimental run.
6. Switch on the air blower and bring the gliders closer to each other and the bumpers
facing each other. Gently push the gliders toward each other while clicking the Start
button at the top left corner in DataStudio. Click the Stop button when the gliders get
separated after collision, and switch off the air blower.
7. Your data will look like Figure 5, after adjusting the x and y scales to zoom the area
where collision occurred.
Bumpers
s
Big Glider
- Small Glider
Force
Sensors
4-Nov-18 PHYS101 - 08
© KFUPM – PHYSICS Department of Physics revised 04/11/2018 Dhahran 31261
7
Figure 5
8. Expand the region that corresponds to the collision. Notice the large value of force
acting for a very short duration of the collision. The impulse is given by the area
under the force versus time curve. Use the Statistics tool to find the area under the
curve.
9. Record the values of the impulse from each force sensor in your report.
10. Write a conclusion based on the result of step 9.
Statistics Tool