PHY 131
Projectile Motion
Name: ____________________
Date: ______________
Lab Partners: __________________________________________
Introduction: In this lab you will investigate the motion of an object launched at various angles relative to
the horizontal. Ignoring air resistance, we would expect that the vertical motion would be governed by a
downward acceleration due to the force of gravity, while the horizontal motion would experience no
acceleration and thus the horizontal component of velocity would remain constant. Using these ideas, and
what you have already learned about constant acceleration motion in one dimension, we can derive some
equations that allow us to calculate quantities such as the object’s horizontal distance traveled, also called the
range, the maximum height attained, and the launching angle that results in a maximum range for a given
initial speed.
You will be using a projectile launcher that can be set to launch a projectile at various angles and a couple
different speeds. You will first need to figure out the launch speed of your particular launcher, then, knowing
the launch speed, you will fire the projectile at various angles (doing several trials at each angle) to
determine the dependence of the range on the angle.
Warning!!! Projectiles can hurt! Be very careful while launching your projectile so you do not hit
anyone, and be very careful while walking around so you do not get hit. To protect your eyes it is a
good idea to wear safety goggles.
Part I: The Launch Speed: You will notice that the projectile launcher consists of a spring that can be
compressed and locked in place. There are three different compression distances resulting in three different
initial speeds as the projectile leaves the launcher. Use the middle compression setting. Also, you might
use this launcher in a later lab so record its identification number in the Data section.
Set the launcher so that it launches your projectile horizontally off the edge of a table. Do a trial run to
determine where the projectile will land. Be sure to protect any walls using one of the large boards provided
by the instructor. Tape a piece of paper on the floor centered on the projectile’s landing spot. Place (do not
tape) carbon paper over this paper to mark subsequent landing spots. Now fire the projectile several times
while recording the landing spot each time. After every firing be sure that the launcher hasn’t moved. If it did
move then you’ll need to start over otherwise your distance measurements will be inconsistent.
Ideally, all spots should be at the same location, but most likely they will be spread out. For each spot,
measure the horizontal distance the projectile traveled. Calculate the average of these distances. Also,
measure the vertical distance the projectile fell. Enter this data in Table 1 in the Data section.
How would you use this data to calculate the initial speed of the projectile? Hint: The initial velocity is
purely horizontal and there is zero acceleration in the horizontal direction. A vertical velocity is gained by
the acceleration due to gravity.
Part II: Dependence of Range on Launch Angle: With the projectile launcher clamped to the table adjust
the launch angle to 20°. Fire the projectile two or three times so that you can calculate an average horizontal
distance traveled. Record this average distance in Table 2, in the Data section. You should also measure the
projectile’s initial height above the floor. Repeat the steps above for the remaining angles in Table 2.
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From your data can you determine the angle that provides maximum range? Make a plot of the relevant
quantities to show this clearly. Enter the maximum range angle in Table 3.
____________________________________________________________________________________
Data
Launcher Number ____________________
Trial #
Horizontal Distance (m)
1
2
3
4
5
6
Average =
Vertical Distance (m) =
Calculated Initial Speed (m/s) =
Table 1
Show calculations for the initial speed of the projectile.
2
Launch Angle
Average Horizontal Distance
Traveled (m)
Initial Height (m)
20°
30°
40°
45°
50°
60°
70°
Table 2
Angle for maximum range =
Table 3
Questions
1.
Is your experimental value for the maximum range angle what you expected? Explain.
2.
If the maximum angle was different from what you expected, explain what caused this difference in a
few well thought out sentences using physical, or even mathematical, reasoning.
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3.
Using one of your angles and the initial speed determined in part I, calculate the distance that the
projectile should travel. Does this result agree with your measured result? Give a mathematical
comparison.
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4.
Is the following statement always true: “The angle for maximum range is 45°.” Explain and give at
least two examples of why it is, or is not, always true.
5.
What were the most significant sources of error in this lab and what did you do to minimize their
affect?
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