Makerspace Builds: A Gravity Lamp and a Static Can Racer

Model how the distance between two interacting objects changes the energy stored in their system.

Answer Key

Build 1: the gravity lamp

Read the build notes and study Table 1. Then answer the questions.

Build Notes: Energy From a Gap

1On Thursday evenings, teens bring projects to the makerspace at the Linden Street Library. This month mentor Ms. Rania Holloway challenged eighth graders Tobias Engqvist and Marisol Fenwick to build devices that run on potential energy. She explained that potential energy is stored in a system of two objects that pull or push on each other, even without touching, and that the amount depends on how the objects are arranged.

2Their first build was a gravity lamp. A 2.0 kg bag of sand hangs from a cord. The cord runs up over a pulley at the top of a wooden frame and down to a small geared generator, which is wired to an LED. When the bag is let go, it sinks slowly toward the floor and turns the generator, and the LED glows until the bag reaches the floor. Tobias pulled the bag up to a mark on the frame, measured from the floor, and let it go. An energy meter added up the electrical energy that reached the LED, and Marisol timed how long the LED stayed on. Each height was tested three times, and Table 1 shows the averages. Ms. Holloway pointed out that much of the energy turns into heat and sound in the gears, so the LED gets only part of it, about the same share in every trial.

3Their second build was a static can racer. An empty aluminum soda can lies on its side on a smooth, level table. Marisol rubbed a balloon 10 times with a wool cloth and clamped it at one end of the table, just above the tabletop. The charged balloon and the can attract each other. Tobias held the can still at a measured gap from the balloon and let it go, and the can rolled toward the balloon. A slow-motion video measured the can's speed just before it touched the balloon. Marisol rubbed the balloon again before every trial, and they ran three trials for each gap (Table 2).

4At the end of the night, Marisol charged two balloons the same way and hung them side by side on threads. They pushed each other apart and hung a few centimeters from each other without touching.

Table 1: Gravity Lamp (2.0 kg bag, average of 3 trials)
Starting height (m)	Energy to LED (J)	LED on (s)
0.5	2	39
1	3.9	78
1.5	5.9	118
2	7.8	157

Which two objects make up the system whose stored energy lights the LED?

A. The bag of sand and Earth
B. The bag of sand and the cord it hangs from
C. The pulley and the wooden frame
D. The generator and the LED it is wired to

Which statement best describes the pattern in Table 1?

A. Each extra 0.5 m sent less energy to the LED than the 0.5 m before it.
B. Doubling the height from 1.0 m to 2.0 m made the energy 4 times as large.
C. Each extra 0.5 m of height sent about the same 2 J more to the LED.
D. The LED stayed on for about 40 s from each starting height.

Use Table 1. Which bar model best shows the relative amounts of energy stored in the bag–Earth system with the bag at 0.5 m, 1.0 m and 2.0 m above the floor?

Use Table 1. In one trial the bag starts at 2.0 m, but its cord snags and the bag stops at the 1.0 m mark.

About 3.9 J of energy reached the LED in that trial.

Build 2: the static can racer

Study Table 2 and use paragraphs 3 and 4 and Table 1 to answer the questions.

Table 2: Can Racer (balloon rubbed 10 times)
Gap at release (cm)	Speed at balloon (m/s)
2	0.1
4	0.16
6	0.19
8	0.2

Use Table 2 and the can's speed. At which gap was the most energy stored in the balloon–can system before Tobias let go of the can?

A. 6 cm
B. 2 cm
C. 4 cm
D. 8 cm

Use paragraphs 2 and 3 and both tables. Fill in each blank with a word from the word bank. Three words are not used.

Word bank: attract · repel · increases · decreases · motion · mass

In both builds, the two objects in the system attract each other. Moving the two objects farther apart increases the energy stored in their system. When the objects are let go and move closer together, the stored energy changes into energy of motion.

Circle the two changes that store more energy in the system named.

Balloon–can system: Tobias moves the can from 2 cm to 6 cm away from the charged balloon.Bag–Earth system: the bag sinks from the 1.5 m mark to the 0.5 m mark.Bag–Earth system: Tobias lifts the bag from the floor to the 1.0 m mark.Balloon–can system: the can rolls from 8 cm away to 2 cm away from the balloon.Bag–Earth system: Marisol carries the bag across the room at a height of 1.0 m.

Use paragraph 4. The two charged balloons push each other apart. How could Marisol store more energy in the system of the two balloons?

A. Move both balloons across the room while keeping the gap between them
B. Push the two balloons closer together, against their push on each other
C. Pull the two balloons farther apart, the way Tobias lifted the bag higher
D. Hang the balloons on longer threads at the same gap from each other

In Table 2 the can's speed rose by 0.06 m/s from the 2 cm gap to the 4 cm gap, but by only 0.01 m/s from 6 cm to 8 cm. Which explanation fits the model best?

A. The can gained mass as it rolled the longer distance, so it sped up more slowly.
B. The balloon pulls weakly on a can that far away, so the extra gap stores little more energy.
C. The table sloped uphill toward the balloon, which slowed the can on the longer rolls.
D. The balloon lost most of its charge before the 8 cm trials, since it was rubbed once at the start.

10.

Tobias says, "The balloon–can system stores the most energy when the can sits right next to the balloon, because that is where the pull is strongest." Do you agree? Write a claim, give evidence from Table 2 and Table 1, and explain your reasoning using the idea of a system.

Claim: I disagree. The balloon–can system stores the least energy when the can is right next to the balloon and more energy when they are farther apart. Evidence: A can released 2 cm from the balloon reached only 0.10 m/s, but a can released 8 cm away reached 0.20 m/s. The faster can had more energy of motion, so more energy had been stored in the system before it was released. The gravity lamp follows the same rule: with the bag at 0.5 m only 2.0 J reached the LED, but with the bag at 2.0 m, 7.8 J did. Reasoning: The energy is stored in the system of two objects that attract each other, the balloon and the can, or the bag and Earth, and it depends on the distance between them. Pulling attracting objects apart stores energy, and letting them come together turns that stored energy into motion. The pull is strongest up close, but a can released up close has only a short distance to speed up, so little energy was stored there.