Laboratory Activity 6: Gravity
Group member names:
-Bo White
-Aaron Hawkins
-Jeremy Cossel
Objectives

Explore how gravity affects different objects.

Develop a quantitative understanding of the gravitational force.

Learn about proportional and inversely proportional graphs.
Equipment

Computer

Basketball

Ultrasonic motion sensor

Racquetball

USB Link

Tow ball
Activity One: Motion of a falling object
Velocity
Acceleration
Prediction 1: Using the line tool on the drawing toolbar of Word, draw a line on
both the velocity and acceleration graphs below predicting the shape of a curve
for a ball being dropped.
Time
Time
Set up the equipment: Obtain a USB Link and attach it to the computer. If there
is not already a Motion Sensor mounted on the ceiling that you can use, get one
and hook it on the frame of the drop ceiling. Put the detector on Person setting.
Open the experiment file called FallingBall.ds to display acceleration and
velocity graphs. When you use the activity, you should notice two things. First,
the coordinate system in the file has been reversed so that up (towards the
detector) is the positive direction. Second, the rate of data collection has been
increased, since the motion of the falling ball is faster than that of the carts that
we have worked with.
Experiment 1: One partner should hold the ball near the detector, careful to
keep it at least 30 cm away, while the other takes data. When you are ready, the
first person should let the ball drop straight down as the second starts the data
collection. When you have a nice graph, highlight the section of the graph that
the ball was falling so that it is marked yellow, copy the graph and paste it in the
section below.
Question 1: What type of motion is the falling ball (constant velocity, constant
acceleration, increasing or decreasing acceleration)? Is the sign of the
acceleration and velocity positive or negative? Have you seen any other objects
in a lab move with this type of motion?
Constant negative acceleration. We have seen it, but they’ve always been
positive when moving away from the origin, and this time it’s negative.
Experiment 2: Measure the average acceleration of the ball during the time it is
falling freely. Do this by click-dragging a rectangle around the data points with
the mouse, and then selecting “mean” from the “Statistics” button
at the top of
the window. (Don’t forget units.)
Average acceleration
-9.8 m/s^2
Now measure the average acceleration by finding the slope of the velocity graph.
Select the region of the velocity graph in which the ball is falling freely, and then
use the fit button
to do a linear fit. As you know, the slope of the line is the
acceleration.
Average acceleration
-9.5431 m/s^2
Question 2: How do the two values compare? Should they be the same or not?
The acceleration is correct, but there is a little bit of human error in the second
value. They should be the same but, they aren’t.
Experiment 3: Now take the ball and hold it below the detector around waist
level. While your partner takes data, carefully through it straight up so it goes up
and back down in the view of the detector. It may take several practice tries until
you can throw it straight up without it coming too close to the detector. Once you
have a good graph, copy it and paste it below.
Being Thrown
At top
On way
up
Going
down
Hits
ground
Question 3: Look at your graph and identify with labels or arrows the time in
which the ball was being thrown, when it is on the way up, when it is at the top of
the flight, when it is coming down, and when it hits the ground.
Prediction 4: Suppose you were given a light object, such as a racquetball, and
a heavy object about the same size, such as the ball for towing. If you dropped
them at the same time, which object would hit the ground first?
Both would hit the ground approximately at the same time.
Get a racquetball and tow ball from your teacher and weigh them, recording the
masses below
mass racquetball
41g
mass tow ball
771g
Experiment 4: Take the two objects, hold them with the bottoms at the same
level, and drop them at the same time onto a piece of cardboard.
Question 4: Did one of the objects hit the ground significantly before the other?
Does the acceleration due to gravity depend on the mass of the object? Does
the force due to gravity depend on the mass of the object?
Both hit at exactly the same time. Nothing matters, everything is going to fall at
the same rate no matter what the mass is.
Summary
The following questions will help you get the main ideas out of this lab. You
should find these straightforward questions, but take the time to talk it over with
your team and write complete answers to these questions. You may find your
answers here to be the most useful part of this lab down the road.
Summary 1: For an object falling near the surface of the earth, how does the
force on it change depending on its speed and direction of motion?
The force changes according to what the mass is. Acceleration always stays the
same.
Summary 2: For an object falling near the surface of the earth, how does the
force on it change depending on its mass? Using what you know from Newton’s
laws, can you write down a mathematical expression that gives the force of
gravity for an arbitrary object?
No matter what the mass, the force will always be the same. F=ma, because
acceleration will always be the same, but the change in mass will cause the net
force to be different.
Summary 3: How do you know your above statements are true? What
experimental observations and/or logical reasoning can you give to justify what
you said in summary questions 1&2?
We believe that our answers are correct from the results of our graphs.