Mass Versus Weight: Travel to Other Planets
Lesson 10 of 14
Objective: SWBAT compare the concepts of mass and weight to understand how these properties of matter are similar and different.
Space travel in science class? Well, virtually! By virtually visiting other planets, students use mathematical computations (SP5) to explore the scientific relationship between mass and gravitational acceleration (MS-PS2-4) by using algebraic expressions and equations (CCC) to calculate their weight on other planets. This lesson links physics to chemistry by exposing how forces interact with matter - and that properties of matter (like mass) have an effect on the magnitude of those forces (MS-PS1-2).
While students finalize their understanding of Forces and Motion and Matter and Its Interactions Core Disciplinary Ideas, they analyze and interpret data to provide evidence for phenomena (SP4) in order to construct explanations that include qualitative or quantitative relationships between variables that describe phenomena using models or representations (SP6).
This lesson presupposes students have background knowledge about mass (Measurement: Mass), forces (Measurement: Forces) and Newton's 2nd Law (Newton's 2nd and 3rd Laws of Motion: Bumper Boats Investigation). Using this background knowledge, students explore how mass and weight are similar and different.
In order to ENGAGE students in this lesson, students view this diagram with the instructions to make two observations and two inferences (Collecting Data: Observation and Inference) about mass and weight:
Any observation students make can lead to a fruitful discussion about mass and weight. The point of this discussion is to lead students to the conclusion that mass is constant in the universe and weight changes depending on the gravitational acceleration due to close proximity to a massive object (like a planet). Depending on the level of understanding, this discussion may also lead to other conclusions such as:
- Physical and chemical changes can change mass, but simply changing location does not change mass.
- Gravity is the attraction between at least two objects that have mass.
- Weight is a force that depends on mass and gravitational acceleration.
- Weight depends on gravity and mass, but it is not the same thing as gravity and mass.
- Gravity depends on the masses of the objects and the distance between the objects.
These concepts are challenging, so focus your discussion on the chosen learning objective. As a way to pull all of these concepts together, students create simple diagrams to show these complex relationships: Concept Diagram Example.
The EXPLORE stage of the lesson is to get students involved in the topic so that they start to build their own understanding. To help students explore, students use Mass Versus Weight Student Handout to complete the "Background Information" section. The question students are working to answer is:
Are Mass and Weight the Same?
In order to answer this question, students use the Mass Versus Weight Research Notes, a textbook or online resources to create a Venn Diagram comparing two properties of matter: mass and weight. Upon completion of this background research, we review the Venn Diagram together to check for accuracy and precision. This review of Mass Versus Weight Student Handout Notes is a great opportunity for formative assessment. To check for understanding, questions like these will provide insight:
What are some differences between mass and weight?
What are some similarities between mass and weight?
What is the relationship between mass and weight?
How does weight depend on mass?
If mass is increased, what happens to the weight of the object?
How can you "lose" weight?
Is the "Biggest Loser" TV show really a weight loss show?
The EXTEND stage allows students to apply new knowledge to a novel situation. The novel situation in this case is to use the graph of gravity factors to help you calculate your weight and mass on other planets. To get everyone excited, play the Star Wars Theme while viewing the Stars From Space Station video:
Students follow the given procedure:
1) Find your Mass on Earth by using a scale calibrated to pounds.
2) Multiply your Mass on Earth by the conversion factor of .45 Kg/pound. Record in data table. Multiply the result by the conversion factor of 10 Newtons/Kg to calculate your Weight on Earth. Record in data table.
3) Decide which planet you would like to travel to first. Record Planet Name in data table.
4) Record your Mass on “Planet X”. X = Planet Name.
5) Calculate your Weight on Planet X. Look at the “Gravity on Planets” Graph. On this graph, the acceleration of gravity on Earth = 1. You can find out the force of gravity related to Earth’s gravity by looking at the graph for other planet’s “gravity factor” (for example, Jupiter’s gravitational pull is 2.5 times that of Earth). Multiply your Mass on Planet X by the gravity factor. Record in data table.
This math is difficult for middle school students since it involves conversion factors, algebraic computation, decimals and multiple steps. To help students reason through the math, model the process and use this example if needed:
_____100_______ X _____.45 ______ = ______45___________
Mass on Earth Kg/pound Mass on Earth*
____45____ X 10 = __450___ X __ 2.5 (Jupiter)_ = ____1125____
Mass on Earth Gravitational Weight on Earth* Gravity Factor Weight on Planet X*
(Kg) Acceleration (Newtons) (Newtons)
Providing students with this process as a blank template will help students who need additional mathematical support. As students compute, ask them to explain the process and press them to provide details about the story the math is telling about mass and weight.
Teacher Note: Students may feel uncomfortable finding their mass on Earth using a scale. If students want to use a different number (the mass of their cat, for example), let them!
Teacher Note: Be sure to clarify that gravitational acceleration on Earth is 9.80665 m/s/s. Using 10 m/s/s is a way to simplify the math. The misconception that heavier objects fall faster is related to this lesson, for a fun online demonstration representing Galileo's famous mental experiment with the feather and the ball, use this site: Galileo Drops the Ball.
For a student explanation of this process, view this video:
The EXPLAIN stage provides students with an opportunity to communicate what they have learned so far and figure out what it means. This stage of the lesson presents a great place for a quick formative assessment as well! Students complete three short written arguments to summarize their understanding. For information about the PACER strategy, check out the Writing Arguments from Evidence.
Explain why your weight is different on other planets. What factors contribute to having more or less weight? Refer to the background information text in the lab notebook for additional help. (Mini-PACER)
Do objects with large masses always have large weights? Explain. (Mini-PACER)
Are the properties of mass and weight useful when trying to identify different types of matter? Why or why not? (Mini-PACER)
Follow up discussion of these questions can highlight any missing links students may have in their understanding.
The EVALUATION stage is for both students and teachers to determine how much learning and understanding has taken place. The primary ways I evaluate student learning are two-fold:
1) Review student understanding as shown in their three analysis questions. An example of student responses can be viewed here: Mass Versus Weight Student Work.
2) Give students an assessment that focuses on the "remember", "understand" and "apply" levels of understanding: Mass versus Weight versus Density Quiz or Measurement Mass versus Weight Checkout Quiz. Prior to giving the assessment, students participate in several days of review as part of their daily warm-up to evaluate the level of understanding.