Teacher Portal:
Earth’s Forces
Investigation 2 – PreLab
MINDSET
This Investigation is designed to:
- compute average speed from measures of distance and time.
- explain that acceleration is speeding up, slowing down, or changing direction.
- demonstrate that an unbalanced force causes an object to accelerate.
- demonstrate that objects accelerate at 9.8 m/s2 due to the force of gravity on Earth.
- demonstrate that the mass of an object is independent of its acceleration at 9.8 m/s2 due to the force of gravity on Earth.
- allow students to conclude that because the force of gravity increases in proportion to an object’s mass, the acceleration due to gravity is the same for all objects, regardless of their mass.
SCIENTIST’S GLOSSARY
- Force: a push or a pull on an object.
- Gravity: the force that exists between all objects and tends to pull them together.
- Motion: how an object is moving.
- Mass: the quantity of matter in an object or substance.
- Weight: the force of gravity on an object or substance.
- Balanced forces: when forces are equal and act in opposite directions.
- Unbalanced force: when a force does not have an equal force acting in the opposite direction on the same object.
- Distance: length of the path traveled.
- Speed: how quickly an object is moving.
- Acceleration: speeding up, slowing down, or changing direction.
BE PREPARED
Watch the Investigation 2 Teacher Video (below) and Student Video (at end of PreLab SHARE IT) to prepare for the PreLab.
SET FOR SUCCESS
- Tell students that they will continue the Earth’s Forces CELL.
- Ask students to share the kinds of things they think they might learn in this Investigation.
Begin the PreLab Concept Slides to start students on their learning journey. Then watch the Pre-Lab Student Video as a class.
NAVIGATE IT
Once the slide presentation is launched
- use your left and right arrows to advance or go back in the slide presentation, and
- hover your mouse over the left edge of the presentation to get a view of the thumbnails for all the slides so that you can quickly move anywhere in the presentation.
- Click HERE to launch the slide presentation for the CELL.
SHARE IT
SLIDE VEFORCE2-pre-1
This is the first slide for CELL Earth’s Forces, Investigation Two. The PreLab for this Investigation is somewhat different than most other PreLabs in that most of the period is devoted to a simple classroom demonstration and analysis of distance and speed.
______________________________________________
SLIDE VEFORCE2-pre-2
Students should follow along with the teacher-guided classroom demonstration. Students should participate in the demonstration and record all of their results in their Student Data Records.
The demonstration begins with placing a piece of masking tape on the floor in the classroom or hallway. This is the Start position. A second piece of masking tape is then placed on the floor at a distance of 5 meters from the Start position… this is the Stop position.
Next, students should read the definitions of both Speed and acceleration in their Student Data Records:
“Speed refers to how quickly an object is moving.”
“Acceleration refers to a change in motion. An object is accelerating if it speeds up, slows down, or changes direction.”
______________________________________________
SLIDE VEFORCE2-pre-3
A student volunteer is asked to walk from the Start to the Stop position without speeding up, slowing down, changing direction, or stopping, until they reach the Stop position.
Ask the other students to describe the motion of the student volunteer when he or she is standing at the start line. Students should indicate that he or she is stationary, or not moving.
Ask students to describe the motion of the volunteer at the end of the walk. Student answers may vary. He/she is stationary or not moving.
Ask students: If he/she was not moving to begin with and was not moving at the end, describe his/her motion in between. Student answers may vary. He/she speeded up and then slowed down.
Ask students: What is the term used to describe speeding up or slowing down? Students should indicate they are forms of acceleration.
______________________________________________
SLIDE VEFORCE2-pre-4
Ask students: What is another way to accelerate? Students should indicate that changing direction is also a form of acceleration.
The student volunteer is asked to walk from the Start to a point about half the way to the Stop position and then to make a left or right turn from the straight path. In other words, the student changes direction during the walk.
Ask students: What is the result of balanced forces? Students should indicate that the balanced forces mean that an object will either remain stationary or will keep moving at the same speed.
Ask students: What is the result of unbalanced forces? Students should indicate that the unbalanced forces result in acceleration. That is, unbalanced forces speed up, slow down or cause an object to change direction.
Ask students: If a person speeds ups, slows down, or changes direction, what does this tell us about the forces on the person? Students should indicate that the forces on the person must be unbalanced.
Ask students: If a person walks at a constant speed, what does that tell us about the forces on them? Students should indicate that the forces on the person are balanced.
______________________________________________
SLIDE VEFORCE2-pre-5
Ask a second student volunteer to take the place of the first volunteer and to stand behind the start line. Explain to the class that you want to record the time it takes to walk 5 m at a constant speed.
- In order to avoid including speeding up and slowing down in the measurements, the student must begin walking before the start line and continue past the end line.
- Ask the student to walk at a slow, but constant speed.
- Start the stopwatch as the student crosses the start line, and stop the stopwatch when the student crosses the end line.
- Ask students to record the result in Problem 2 of their Student Data Records.
______________________________________________
SLIDE VEFORCE2-pre-6
Ask the same student to walk at a faster, but constant speed. Start the stopwatch as the student crosses the start line, and stop the stopwatch when the student crosses the end line.
Record the data for this second timed Trial.
______________________________________________
SLIDE VEFORCE2-pre-7
Ask a second student volunteer to take the place of the first volunteer and to stand behind the start line. Explain to the class that you want to record the time it takes to walk 5 m at a constant speed. In order to avoid including speeding up and slowing down in the measurements, the student must begin walking before the start line and continue past the end line. The stopwatch is started as the student passes the Start positing and stopped when the student passes the Stop position.
Ask students to record the result in Problem 2 of their Student Data Records.
______________________________________________
SLIDE VEFORCE2-pre-8
Use the data collected in the three timed Trials to calculate the speed in each case.
Ask students: If a person took 2.5 s to walk 5 m, what is their speed? Speed = 5 m / 2.5 s = 2 m/s. Ensure the correct units are included.
Ask students to finish the PreLab by completing the conclusions sections (questions 3 and 4) in their Student Data Records.
Ask students to consider the following questions as they perform the Investigation 2 Lab:
- How does gravity cause acceleration?
- Can acceleration due to gravity be measured?
WATCH IT
Play the following Student Video in preparation for the lab. Discuss as necessary to answer student questions.
PRELAB EXERCISE
Supplies and Equipment:
Class materials:
- 1 Masking Tape
- 1 Meter stick
- 1 Stopwatch
- 1 Calculator
Individual materials:
- 1 Scientist’s Glossary
- 1 Scientist Data Record
- 1 Calculator
Preparation:
- Place masking tape at the start and end of a 5 m straight walkway.
Instruction:
A. Begin by explaining to students that in this Investigation they will continue to explore and measure motion. Specifically, this Investigation will focus on the concepts of speed and acceleration, how the two can be measured, and how forces impact both of them.
1. Direct students to their Scientist’s Glossary. Ask a volunteer to read the definition of speed.
“Speed refers to how quickly an object is moving.”
2. Direct students to their Scientist’s Glossary. Ask a volunteer to read the definition of acceleration.
“Acceleration refers to a change in motion. An object is accelerating if it speeds up, slows down, or changes direction.”
3. To facilitate students’ understanding of motion and in particular, the concepts of speed and acceleration, conduct the following in-class activity:
a. Gather the class along the sides of the 5-meter walkway that has been created.
b. Ask a student volunteer to stand at the start line. Ask the other students to describe the motion of the student volunteer when he or she is standing at the start line. Students should indicate that he or she is stationary, or not moving.
c. Ask the student volunteer to start walking and then stop walking at the end line. When he or she reaches the end line and has stopped, ask students to describe the motion of the volunteer at the end of the walk. Student answers may vary. He/she is stationary or not moving.
d. Ask students: If he/she was not moving to begin with and was not moving at the end, describe his/her motion in between. Student answers may vary. He/she speeded up and then slowed down.
e. Ask students: What is the term used to describe speeding up or slowing down? Students should indicate they are forms of acceleration.
f. Ask students: What is another way to accelerate? Students should indicate that changing direction is also a form of acceleration.
g. Ask the student to again walk from the start to the end line. At some point along the walkway, ask the student to change direction by turning and going toward the left or right.
h. Ask students: What is the result of balanced forces? Students should indicate that the balanced forces mean that an object will either remain stationary or will keep moving at the same speed.
i. Ask students: What is the result of unbalanced forces? Students should indicate that the unbalanced forces result in acceleration. That is, unbalanced forces speed up, slow down or cause an object to change direction.
j. Ask students: If a person speeds ups, slows down, or changes direction, what does this tell us about the forces on the person? Students should indicate that the forces on the person must be unbalanced.
k. Ask students: If a person walks at a constant speed, what does that tell us about the forces on them? Students should indicate that the forces on the person are balanced.
Note: Some students may realize that during walking at a “constant speed,” a person speeds up as they push against the ground and slow down as they strike the ground. Therefore during these times, the forces are not balanced, however, these forces will balance out if the person maintains a constant speed. In other words, if the forces that slow a person down when he or she strikes the ground are equal to the forces that speed the person up as they push off, then his or her speed will be, on average, constant.
__________________________________________________
4. Continue the in-class activity by explaining to students that the class will now think about the forces that are present when a person walks at a constant speed.
a. Ask a second student volunteer to take the place of the first volunteer and to stand behind the start line. Explain to the class that you want to record the time it takes to walk 5 m at a constant speed. In order to avoid including speeding up and slowing down in the measurements, the student must begin walking before the start line and continue past the end line.
b. Ask the student to walk at a slow, but constant speed. Start the stopwatch as the student crosses the start line, and stop the stopwatch when the student crosses the end line.
c. Record this time on the chalkboard.
d. Repeat steps 4a through 4c but ask the student to walk at a slightly faster, but still constant, speed.
e. Repeat steps 4a through 4c, but ask the student to walk slightly faster, but still constant, speed.
5. Students should quietly return to their seats and locate Problem 1 in their Student Data Record.
_____________________________________________________
B. Compile the data collected during the in-class activity in the Table in Problem 1 in the students’ Student Data Record. Use the data to assist students in calculating speed and in discussing the relationship between speed, distance, and the time taken to travel a distance.
1. Students should copy down the three recorded times in the table of Problem 1 in the Student Data Record.
2. Explain and demonstrate on the chalkboard how to calculate speed:
a. The equation for speed is: Speed = Distance / Time.
b. For example, if someone took 10 seconds to walk 5 meters, the equation would be: Speed = 5 m / 10 s = 0.5 m/s.
c. Explain that another way to understand this answer is to say that during every second, the person will walk 0.5 meters. This means that in 2 seconds they will walk 1 meter. In 10 seconds, they will walk 5 meters.
d. Emphasize that the units in which speed is measured come directly from the equation. That is, the equation divides meters by seconds, to give m/s.
e. Ask students: If a person took 2.5 s to walk 5 m, what is their speed? Speed = 5 m / 2.5 s = 2 m/s. Ensure the correct units are included.
f. Ask students to calculate the speed for the three times and to record their answers in the table.
g. Explain to students that we can predict the distance an object will travel in a certain time if we know its speed and its speed remains constant.
h. To do this, rearrange the equation above to get: Distance = Speed x Time. For example, if a sprinter runs at a speed of 10 m/s for a time of 10 s, they will travel 100 m (10 m/s x 10 s = 100 m). Ask students: If a car is moving at 10 m/s, how far will the car travel in 5 s? Distance = 10 m/s x 5 s = 50 m (Ensure the correct units are included).
i. Ask the students to calculate the distance covered in 10 s, for the three speeds in table 1. Discuss answers as a class.
C. Guide students to drawing conclusions from their data as they review the table and answer the questions that follow it.
a. Ask students to use the data and the calculations in the table, to answer problem 3 in the Scientist Data Record. Discuss the students’ answers and correct any wrong answers.
b. Ensure that the equation Speed = Distance / Time is written on the board. Leave room to re-write the equation in the process of solving for different variables.
c. Explain to students that we can predict the time an object will take to travel a certain distance, if we know its speed and its speed remains constant. We rearrange the equation above to get: Time = Distance / Speed.
d. For example, if a sprinter runs at a speed of 10 m/s, they will take 10 s to travel 100 m (100 m / 10 m/s = 10 s).
e. Ask students: If a car is moving at 30 m/s, how long will the car take to travel 60 m?
Time = 60 m / 30 m/s = 2 s (Ensure the correct units are included).
f.Ask students to calculate the time taken to walk 100 m, for the three speeds in table 1.
g. Direct students to answer problem 4 in the Scientist Data Record. Discuss students’ answers as a class.
______________________________________________________
F. Close this portion of the Investigation by explaining to students that they will continue to investigate acceleration and forces in the lab. As they do so, they should keep in mind the following question:
-
-
-
- How does gravity cause acceleration?
- Can acceleration due to gravity be measured?
-
-