Free-Fall Laboratory
Answer Key
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
[Note: The purpose of these questions is to activate prior knowledge and get you thinking.]
1. Suppose you dropped a feather and a hammer at the same time. Which object would hit
the ground first? [1 pt.]
Answers will vary. [The hammer would hit the ground first.]
2. Imagine repeating the experiment in an airless tube, also known as a vacuum. Would
this change the result? If so, how? [1 pt.]
Answers will vary. [In a vacuum, both objects would hit the ground at the same time.]
Activity A:
1. Click Play (
Get the Gizmo ready:
The Free-Fall Laboratory Gizmo™ allows you to
measure the motion of an object in free fall. On
the CONTROLS pane check that the Shuttlecock
is selected, the Initial height is 3 meters, and
the Atmosphere is Air.
) to release the shuttlecock. How long does it take
to fall to the bottom? [1 pt.]
0.90 seconds
2. Select the GRAPH tab. The box labeled h (m) should be checked,
displaying a graph of height vs. time. What does this graph show in
terms of height as time progresses? [1 pt.]
The height decreases over time.
3. Turn on the v (m/s) box to see a graph of velocity vs. time.
Velocity is the speed and direction of the object. Velocity is also
referred to as instantaneous velocity. Because the shuttlecock is
falling downward, its velocity is negative.
Does the velocity stay constant as the object drops? [1 pt.]
No
Activity B:
Get the Gizmo ready:
 Click Reset (
).
 Select the CONTROLS tab.
1. Drop each item through Air from a height of 3 meters. Record how long it takes to fall
below. For the tennis ball, try to click Pause (
) when it hits the ground. [5 pts.]
Shuttlecock
Cotton ball
Tennis ball
Rock
Pebble
0.90 s
1.36 s
~0.79 s
0.79 s
0.80 s
2. Why do some objects fall faster than others? [1 pt.]
Hypotheses will vary.
2
3. A vacuum has no air. How do you think the results will change if the objects fall through
a vacuum? [1 pt.]
Predictions will vary.
4. On the Atmosphere menu, select None. Drop each item again, and record the results
below. [5 pts.]
Shuttlecock
Cotton ball
Tennis ball
Rock
Pebble
0.78 s
0.78 s
0.78 s
0.78 s
0.78 s
5. What happened when objects fell through a vacuum? [1 pt.]
Every object took the same amount of time to fall to the ground.
6. Objects falling through air are slowed by the force of air resistance. Which objects
were slowed the most by air resistance? Why do you think this is so? [2 pts.]
The cotton ball and shuttlecock were slowed the most by air resistance. They were slowed
the most because they were the lightest and largest objects.
3
7. Select the Shuttlecock. Check that the Initial height is 3 meters and the Atmosphere
is None. Click Play and wait for the Shuttlecock to fall. Select the BAR CHART tab and
turn on Show numerical values.
A. How long did it take the shuttlecock to fall to the bottom? [1 pt.]
0.78 seconds
B. What was the acceleration of the shuttlecock during its fall? [1 pt.]
-9.81 m/s2
C. What was the velocity of the shuttlecock when it hit the bottom? [1 pt.]
-7.68 m/s
8. If the acceleration is constant and the starting velocity is zero, what is the relationship
between the acceleration of a falling body (a), the time it takes to fall (t), and its
instantaneous velocity when it hits the ground (v)?
The instantaneous velocity is equal to acceleration multiplied by time.
v = at
9. Click Reset. On the CONTROLS tab, set the Initial height to 12 meters. Click Play.
A. How long did it take for the shuttlecock to fall 12 meters? [1 pt.]
1.56 seconds
4
B. Assuming the acceleration is still -9.81 m/s2, what is the instantaneous velocity
of the shuttlecock when it hits the ground? Show your work below. [1 pt.]
v = at
v = (-9.81 m/s2) (1.56 s)
v = -15.3 m/s
C. Select the BAR CHART tab. What is the final velocity of the shuttlecock? [1 pt.]
-15.35 m/s
D. Is your calculated value close to your actual value? [1 pt.]
Yes. [Any differences are due to rounding.]
5
Activity C:
Get the Gizmo ready:
 Click Reset.
 Set the Initial height to 12 meters.
 Set the Atmosphere to Air.
1. Select the Shuttlecock. Choose the BAR CHART tab, and click Play. What do you notice
about the velocity and acceleration of the shuttlecock at about half way down? [2 pts.]
About halfway through the fall, the acceleration is close to zero and the velocity stops
changing.
When objects fall through air for a long time, they will eventually stop accelerating.
Their velocity at this point is called terminal velocity.
2. Form hypothesis: How will an object’s size and mass affect its terminal velocity? [1 pt.]
Hypotheses will vary.
3. Click Reset. On the CONTROLS tab, select Manual settings. Set the height to 100
meters and the air density (ρ) to 1.3 kg/m3, close to actual air density at sea level.
For each combination of mass and radius in the charts below, find the terminal velocity
(vterminal) of the object. Use the BAR CHART tab to find the terminal velocity. (Hint: Turn
on Show numerical values.) [6 pts.]
Mass
Radius
vterminal
Mass
Radius
vterminal
1.0 g
3.0 cm
-1.61 m/s
10.0 g
2.0 cm
-8.24 m/s
10.0 g
3.0 cm
-5.47 m/s
10.0 g
5.0 cm
-3.19 m/s
50.0 g
3.0 cm
-12.30 m/s
10.0 g
10.0 cm
-1.11 m/s
6
4. Your data shows how mass and radius affect terminal velocity.
A. What was the effect of increasing mass? [1 pt.]
As mass increased, terminal velocity increased.
B. What was the effect of increasing radius? [1 pt.]
As radius increased, terminal velocity decreased.
5. Apply: If you wanted to use a device to slow your fall, what properties should it have?
[2 pts.]
You should use a device that is light in weight and has a large radius. A parachute is a
good example of such a device.
7