EXPERIMENT 7.1.1

MEASURING HEIGHT WITH A STOPWATCH

Student Information:

Name:

Email:

Teacher's Email:

PURPOSE:

To use free fall physics and the acceleration due to gravity to measure the height of a room when a direct measurement is not possible.

MATERIALS:

A stopwatch that reads to hundredths of a second (many smartphones have this feature)
A chair or stepladder
A rock or other heavy object to reduce air resistance
A tape measure

SAFETY:

Be careful when standing on chairs or stepladders.
Choose an object that won't damage the floor when dropped.
Make sure the area below is clear of obstacles and people.
Have a friend spot you if possible.

BACKGROUND:

Sometimes it is hard to measure the height of something because you do not have a ruler long enough. In this activity, you will use a stopwatch and a rock to measure the height of the ceiling in a room.

This experiment applies the physics of free fall. When an object falls, it accelerates due to gravity at a constant rate of approximately 9.8 m/s² (or 32 ft/s² if using imperial units). Using the time it takes for an object to fall, we can calculate the distance it traveled using the equation:

d = ½gt²

Where:

d = distance (height)

g = acceleration due to gravity (9.8 m/s² or 32 ft/s²)

t = time of fall in seconds

This equation is derived from the kinematic equation for an object starting from rest and accelerating at a constant rate.

Stopwatch showing time to hundredths of a second

FIGURE 7.13 - Stopwatch
Make sure your stopwatch shows the time to the hundredths of a second.

PROCEDURE:

Stand on the chair or stepladder and hold your rock so that it touches the ceiling of the room. With the stopwatch in your other hand, simultaneously drop the rock and start the stopwatch.
When the rock hits the ground, stop the stopwatch. Record the time from the stopwatch in the data table below.
Repeat timing the drop of the rock 10 times. It is important to repeat this 10 times to reduce experimental error in timing because the time is so small.
Once you have timed the rock drop 10 times, calculate the average time by adding all your measurements and dividing by 10.
Use the time you just calculated and equation 7.1 (d = ½gt²) to determine the distance over which the rock fell.
If the tape measure you have is marked off in feet, use 32 ft/s² for the acceleration. If it is marked off in meters, use 9.8 m/s² as the acceleration.
Once you calculate the height of the room, measure it with the tape measure. How close are your numbers? They should be within 15% of each other.

DATA COLLECTION:

Trial	Time (seconds)
1
2
3
4
5
6
7
8
9
10

Average Time (seconds):

Units for Calculation:

Calculated Height:

Measured Height (with tape measure):

Percent Difference:

ANALYSIS QUESTIONS:

1. Why is it important to repeat the time measurement 10 times?

2. What factors might contribute to experimental error in this experiment?

3. How would air resistance affect your measurements? Would it make your calculated height higher or lower than the actual height?

4. If you performed this experiment on the Moon (where g = 1.6 m/s²), how would your results differ?

5. Why did we use a heavy object like a rock instead of a lighter object like a feather?

CONCLUSION:

Write a paragraph discussing your results. How close was your calculated height to the measured height? What might have caused any differences? How could you improve the accuracy of this experiment?

SUBMIT YOUR RESULTS:

Click the button below to submit your experiment results to your teacher.