Energy, Machines, and Motion
Part One: Energy
Lesson 5: Introduction to Forces
Lesson 5 is split into two inquiries. In the first inquiry students explore elastic forces by using rubber bands. In the second inquiry students explore the difference between mass and weight and the role gravity plays in determining weight. Students will be introduced to a tool to measure forces, a spring scale.
Focus Question: What is a force? How are mass and weight (force of gravity) related?
3. Lesson Set-up and Management
EALR 2: Inquiry
Core Content: Questioning and Investigating
6-8 INQC Collecting, analyzing, and displaying data are essential aspects of all investigations.
EALR 4: Physical Science
Core Content: Balanced and Unbalanced Forces
6-8 PS1C Unbalanced forces will cause changes in the speed or direction of an objects motion.
- Students explore elastic force and gravitational force.
- Students measure force with a spring scale using the unit of Newtons.
- Students use data tables and graphs to interpret data.
3. Lesson Set-Up and Management
Materials:
- Lesson 5.1 students pull rubber bands attached to a wing nut on a pegboard. You will need to line up the meterstick so it is even with the end of the rubber band (See Illustration 5.4) Have one example laid out for students to see. Though the directions and illustration is clear it is a difficult set up for some students.
- Make sure that all of the rubber bands students are using are the same size. This will be a controlled variable between the groups. Have a few spare rubber bands ready to go, they tend to become brittle from year to year.
- For Inquiry 5.2 you will need objects of various masses. You may
consider collecting some objects of similar sizes, but different
weights such as a tennis ball and baseball. Keep these together to use
from year to year.
It is sometimes difficult for students to pull the rubber band accurately to the desired length.
Using a ruler to mark the length as shown will lead to better measurements
Student Management:
- If you have access to a document camera consider having students practice reading a spring scale. This may be a new tool for them. In this unit there are two scales, both have different increments between the numbered Newtons. Make sure students understand the difference.
- Make sure that students are connecting the increase in Newtons, with an increase in force. Students may focus on the increase Newtons rather than the concept of increased force.
- During Inquiry 5.2 students will need to make a data table to
collect their data for the mass of their washers. Here is an example
of tables used by other students:
Gravitational Force on Washers
Mass
# of washers
Weight
(N)
1
2
3
4
5
- Remind students how to set up their graph for Inquiry 5.1. The independent/manipulated variable (stretching distance) will go on the horizontal axis and the dependent/responding variable (elastic force) will go on the vertical axis.
- Make sure students are using uniform scales on the axis. Students should also include a title on their graphs.
- The last two reflection questions ask students to think more deeply about what is a force, and then define the term force. Make sure you guide students to define force as a push or a pull on an object. In these two lessons students only experience a pull, consider also giving students experiences with a push.
- One way is to ask for a student volunteer to stand in front of the class. When they are at ease give the student a small push. This will bring laughs from the class, but also give them a reference for an applied force. I have often called the applied force the student’s name. An example would be the “Tanner” force. Having this discrepant name has helped my students remember that a push is also a force.
Writing Support:
- Students will make a list of forces they know or think they know about. Consider leaving room for students to add to their list as they proceed with the unit. Also consider having a chart visible for students to see during the unit.
Reading Support:
- Prior to Inquiry 5.2 read “The Differences Between Mass and Weight” as a class. Consider having your students draw a sketch of themselves on Earth and on the moon standing on a scale. Write captions that capture how much they would weigh on each and how much their mass would be on each location. If weight is a delicate issue, I have use my (the teachers) middle school weight of 100 pounds as an example.
- Students confuse the idea of weight and mass. The following website quickly calculate a person’s weight on different objects in space: http://www.exploratorium.edu/ronh/weight/
- There are two readings that follow Lesson 5.
- “Bungee Jumping: the Forces Are With You” gives a dramatic application of elastic force. There are many video clips on the internet that students can watch to see this event.
- “Hooke and Newton: Geniuses at Work” gives a historical perspective of two of the 17th century’s greatest scientist.
Math Support:
- The metric unit for measuring forces is the Newton (N). Many people in America use pounds, ounces and tons. The lessons in Energy, Machines and Motion use the metric unit also used in the scientific community. A Newton is around 1/5th of a pound. Extension 1 in the Teacher’s Guide has the students calculate their own weight to Newtons.
- Have students practice reading their spring scales as they pull objects and as they have objects hang. You might need to re-calibrate the spring scale depending on the orientation of the scale.
- Students need practice with the increments on the spring scale in both fractional and decimal forms.
- There are two different spring scales in the unit. Students will need to practice reading the incremental differences of each scale.
Last updated 09/09/2009