Unit F: Energy, Forces, and Motion

How can your weight change with motion?

1. Get Set to Explore

Vocabulary

  • acceleration: A change in speed or direction or both; accelerate is sometimes restricted to situations when a moving object speeds up; decelerate is sometimes used to refer to situations in which a moving object slows down.
  • apparent weight: The weight an object in an accelerating environment is perceived to have in that environment, although the force of gravity remains constant.
  • force: A push or a pull which is quantified by multiplying the mass of the affected object by its acceleration.

Building Background

  • Ask students if they have ever ridden in an elevator and gotten a funny feeling in their stomachs as the elevator started or stopped moving. If so, let students describe what they noticed. If not, try to arrange for the class to ride in an elevator in the building or in a nearby building, if that would be possible. Explain that the funny feeling relates to the forces acting on them as the elevator accelerates. Go over the definitions of the vocabulary words, using the elevator as the context.
  • Review the difference between weight and mass, referring students to Student Edition page F114, if necessary. Explain that elevators accelerate momentarily when they start and stop. This acceleration, in the up-down direction, changes the apparent weight of people riding in the elevator for the few seconds that the elevator is changing speed.
  • Present the Discover! question: How can your weight apparently change with motion? Let students work in pairs to formulate answers. Invite them to share ideas in a class discussion. Write their ideas on the board to refer to during the simulation.

2. Guide the Exploration

  • Students can remain in pairs and launch the Discover! Simulation. Suggest that they move the cursor over different parts of the screen to see what the controls do and to learn about the readouts. Point out that they can control the speed setting on the elevator by moving the slider to fast, medium, or slow. They can use the arrows to control the direction the elevator moves. Direct students to make a chart for recording what happens to the rider's apparent weight when the elevator is at different speed settings and when it moves in different directions.
  • Allow students ample time to work on the simulation and note relationships among the different variables. Then revisit the Discover! question and let students talk about what they've learned from the simulation so far. Encourage the class to revise, correct, and add to their ideas that are posted on the board. Have them return to the simulation afterward to look for cause-effect relationships and to clarify concepts.

3. Review/Assess

  • Ask a volunteer to read the first three sentences of Step 3's Wrap-up text aloud to the class. Let students discuss what the text implies. Guide them to realize that as an elevator accelerates downward, the rider experiences a momentary decrease in apparent weight. Next they should analyze what happens when the elevator accelerates upward. Have students revise their answers to the Discover! question based on any refinements they make to their thinking. Clarify any confusing concepts.
  • Once students understand how the apparent weight of a person momentarily changes when they ride in an accelerating elevator, ask a volunteer to read the Extension question aloud. Through class discussion, encourage students to think about how the two situations are similar and how they are different. If necessary, have students review the definition of weight, which they can find by moving the cursor over the scale in the simulation. Guide students to realize that because the train accelerates horizontally, rather than vertically, the passengers on the train are not accelerating relative to the direction that the force of gravity acts on them. So their apparent weight does not change.

If time permits, present students with the following question:

  • Critical Thinking Apply Before a descending elevator stops at a floor, the elevator brakes, momentarily accelerating upward. What effect would this have on the rider's apparent weight? Answer: This is the same situation as when an elevator begins to ascend. In such cases, the rider's apparent weight momentarily increases.

4. Reaching All Learners

English Language Learners

Before doing the simulation, go over the following word pairs with English Language Learners: rise, fall; increase, decrease; accelerate, decelerate; ascend, descend; horizontal, vertical. Point out that the syllable de- gives the word the opposite of its usual meaning. Encourage students to work in pairs to make pictures illustrating the meaning of these words.