Below I have marked questions that need answered with _______
and there are 6 questions at the bottom that need answers.
Record the total flight time of the cannonball. Answer:
1.01 s
Also, record the range of the cannonball. Answer:
10.1 m
Next, while leaving the height and launch angle constant, you
will be varying the launch speed and noticing the effect on the
cannonball’s flight. Before running any trials, predict the
relationships between the launch speed and the flight time and
between the launch speed and the range of the object fired
horizontally.
As the launch speed is increased, will the flight time increase,
decrease, or remain the same? Answer: same
As the launch speed is increased, will the range increase,
decrease, or remain the same? Answer: increase
Now, run the simulation at the launch speeds indicated and
complete the table below. Remember to keep the launch height at 5 m
and the launch angle at 0°. (If the projectile lands too close to
the right side of the screen to be able to use the scope, zoom out
using the magnifying glass buttons at the top left of the screen.)
Table 1. Effect of Launch Speed on a Projectile Launched
Horizontally.
Launch Speed (m/s) Total Flight Time (s) Range (m)
10 1.01 10.1
15 1 15
20 1.01 20.19
25 1.01 25.24
30 1.01 30.29
Do your results match your predictions? Answer:
yes
Step 7. To help understand your results, consider the case when
the launch speed is zero. This would correspond to one-dimensional
freefall for an object dropped from rest. Use a kinematics equation
to calculate the time it takes an object dropped from rest to fall
a distance of 5 m. Answer: _______
Now, write a statement to explain the pattern you found in the
flight times recorded in this experiment. Answer: ________
Part B: Object Launched at an Angle The purpose of this portion
of the laboratory exercise is to use the PhET simulation to
visualize a typical projectile motion problem. Step 1. Reset the
PhET simulation by clicking on the yellow eraser button to the left
of the red launch button. Now, set the cannon to a height of 0 m
and a launch angle of 35°. Don’t worry about the launch speed just
yet. Click the button showing a magnifying glass with a plus sign
to zoom in.
Step 2. Use the tape measure tool to measure the horizontal
distance between the cannonball’s launch point and Michelangelo’s
David statue. To use the tape measure tool, click and drag it to
the base of the cannon. Match the crosshairs on the tape measure to
the crosshairs on the cannon. Then, click and drag the right side
of the tool to expand the tape measure to reach from the cannon to
the bottom of David. Record the horizontal distance from the launch
point to the statue. Answer: _______
Step 3. Use the tape measure to measure the vertical height of
the statue of David. Answer: _______
Step 4. Use the kinematics equations, and what you know about
projectile motion to calculate the answer to this projectile motion
problem: At what speed must the cannonball be launched so that it
just passes over David’s head? What is the flight time of the
cannonball to this point? (Hints: The cannonball may not pass over
David’s head at the exact peak of its trajectory. You will have to
solve a system of equations to find the solution.) Answer:
_______
Step 5. Now, in the PhET simulation, adjust the launch speed to
approximately the speed you calculated. Does the cannonball pass
directly over David’s head? Answer: _______
If not, go back to your calculations and find your mistake.
Part C: Maximum Range of an Object in Projectile Motion
The purpose of this portion of the laboratory exercise is to
discern both experimentally and algebraically the launch angle that
corresponds to the maximum range of a projectile.
Step 1. Reset the simulation and zoom back out to the standard
view. Set the cannonball’s height to zero, launch speed to 15 m/s,
and launch angle to 25°. Fire the cannon and use either the tape
measure or the scope tool to determine the range of the projectile.
Then, while leaving the height and launch speed constant, adjust
the angle to each of the values indicated and complete the table
below.
Table 2. Dependence of Projectile Range on the Angle of
Projection
Angle of Projection (°) Range (m)
25 17.57
30 19.49
40 22.59
45 22.99
50 22.59
60 19.86
70 14.74
Step 2. Using Excel or Google Sheets, create a graph of range
(y-axis) versus angle of projection (x-axis) based on the data in
the table above. Include a title for your graph as well as axis
labels with units. Fit a trendline to your data.
What type of trendline best fits your data, and what is the
equation of this trendline? Answer: _______
Step 3. Using your trendline, find the launch angle that
corresponds to the maximum range. What is this launch angle?
Answer: _______
Step 4. Let us next show that this launch angle for maximum
range will be the same for all projectiles, regardless of launch
speed (as long as air resistance is ignored). First, we know that
the launch velocity, v0, can be resolved into its x and
y components.
v0x=v0cos𝜃
and v0y=v0sin𝜃
Consider a projectile that is launched from ground level and
returns back to the ground. What is the vertical displacement, 𝑦 −
𝑦0, of the projectile in this case? Answer: _______
Use the kinematics equation for the vertical component of an
object in projectile motion
y-y0=v0yt-1/2 gt2
to find an equation for the flight time of the projectile from
the instant it is launched to the instant it hits the ground. Leave
your equation in variable form. Do not plug in any numbers. Answer:
_______
Next, combine this equation with the kinematics equation for the
horizontal component of an object in projectile motion
x-x0=v0xt.
Replace x-x0 with 𝑅 (for range) and be sure to plug
in the equation above for v0x. Your final answer should
correlate the range to the launch velocity, v0, launch
angle, 𝜃, and gravitational acceleration constant, 𝑔. Answer:
_______
Use the trig identity 2𝑠𝑖𝑛𝜃𝑐𝑜𝑠𝜃 = 𝑠𝑖𝑛(2𝜃) to simplify your
equation. Answer: _______
Now, you should see that if the launch velocity and the
gravitational acceleration are held constant, your range will be
maximized when 𝑠𝑖𝑛(2𝜃) is maximized. For what value of 𝜃 is 𝑠𝑖𝑛(2𝜃)
maximized? (Recall that a sine function varies between -1 and 1;
What must 𝜃 be so that 𝑠𝑖𝑛(2𝜃) = 1?) Answer: _______
Does this value for the angle corresponding to maximum range
agree with your experimentally determined angle above? Answer:
_______
Part D: Symmetry of Launch Angles and Ranges The purpose of this
portion of the laboratory exercise is to determine why two launch
angles may give the same range.
Step 1. Re-examine Table 2 above and your corresponding graph.
Were there any pairs of launch angles that resulted in the same
range? If so, what are the pairs? Answer:_______
Based on your observations, write a rule that describes when two
launch angles will result in the same range. Answer: _______
Step 2. Next, you will want to show that this rule holds for any
launch speed. Begin with the last equation you found in Part C.
Recopy that equation here. Answer: _______
Revisit the rule you wrote in step 1. You should have found that
when the launch angle is symmetric about 45°, the ranges are the
same. Another way to state this is that when 𝜃 = 45° + 𝛥𝜃, the
range is the same as when 𝜃 = 45° − 𝛥𝜃, where 𝛥𝜃 is any angle
between 0 and 45°.
Plug 𝜃 = 45° + 𝛥𝜃 into the equation above. Simplify the equation
as much as you can, while still leaving it in variable form (i.e.
don’t plug any numbers in for v0 or 𝑔). (You may want to
use the trig identity 𝑠𝑖𝑛(90° + 𝜙) = 𝑐𝑜𝑠𝜙 which stems from the fact
that sine and cosine functions are the same, but shifted 90°.)
Answer: _______
Also, plug 𝜃 = 45° − 𝛥𝜃 into your range equation. Again,
simplify as much as you can while still leaving the equation in
variable form. (This time, you may want to use the trig identity
above, plus the additional identity that 𝑐𝑜𝑠(−𝜙) = 𝑐𝑜𝑠𝜙 which
reflects the fact that the cosine function is symmetric about the y
axis.) Answer: _______
Compare these two equations. What conclusion can you draw?
Answer: _______
Questions
1. How does the vertical portion of projectile motion compare to
one-dimensional free fall?
2. How does adding air resistance affect the motion of a
projectile? What differences in the projectile’s path, height, and
range do you observe? Experiment with the PhET simulation to draw
specific conclusions. Systematically compare cannonball flights
with the same initial conditions but with and without air
resistance.
3. With all other initial conditions held constant, how does
increasing the launch height affect range?
4. With all other initial conditions held constant, how does
increasing launch speed affect maximum height?
5. With all other initial conditions held constant, how does
increasing the launch angle affect range?
6. With all other initial conditions held constant, how does
increasing the launch angle affect flight time?