The Smart Helmet Project: Who Pulls Whom Across the Gap?

Use buckle and wheel-sensor tests to show that fields act between objects that do not touch.

Test A: the magnetic buckle

Read the club notes and study Table A. Then answer the questions.

Club Notes: Magnets in a Smart Helmet

1The bike-safety club at Juniper Hill Middle School is designing a smart helmet for younger riders. Two of its features use magnets. The chin buckle has a disc magnet inside one half and a steel plate inside the other, so the halves pull together and click shut. The rear light flashes faster as the bike speeds up. To measure speed, a small magnet clipped to a spoke of the front wheel passes a sensor on the bike's fork once on every turn of the wheel. The magnet and the sensor never touch, and the bike's computer passes the count on to the light. Club advisor Ms. Aurelie Fonseca explained that a magnet is surrounded by a magnetic field, and that a piece of steel or another magnet in that field feels a force.

2In Test A, Mirela Danso set the magnet half of the buckle on a digital kitchen scale with a plastic platform and pressed zero. Cyrus Petrakis clamped the steel half in a wooden stand directly above it and lowered it to each gap in Table A, never letting the halves touch. When something pulls up on the half on the scale, the reading drops below zero; a reading of −70 g means an upward pull equal to the weight of 70 g. Then they swapped the halves: the steel half sat on the zeroed scale, and the magnet half was clamped above it at the same gaps. They took three readings for every setup and averaged them. In a check run, a plain plastic half with no magnet or steel held 2 mm above the magnet half left the scale at 0 g.

3In Test B, the club turned the bike upside down on a stand. Where the speed sensor normally sits, they taped a phone running a magnetic sensor app, which reads in microteslas (µT). They subtracted the reading the phone gave with no magnet nearby, which comes from Earth's field. Cyrus set the spoke magnet to pass 6 mm from the phone at its closest point and turned the wheel slowly by hand. Table B shows readings with the magnet at points along its path, before (−) and after (+) the closest point. Next, they changed how far the magnet passed from the phone and recorded only the highest reading during each pass (Table C). They repeated every gap in Table C with a piece of plastic mudguard and then with a folded rubber glove filling the gap, and the highest readings matched the readings with only air in the gap. The club's sensor counts a pass only when the reading is 150 µT or more.

4Last, Mirela wiped the helmet's clear plastic visor with a dry cloth to clean it. When she held it 2 cm above some small bits of tissue paper on the table, the bits jumped up onto the visor. A visor that had not been wiped, held at the same height, did not lift any bits.

Table A: Buckle Halves on the Scale (average of 3 readings)
Gap (mm)	Magnet half on scale (g)	Steel half on scale (g)
2	−120	−120
4	−70	−71
6	−40	−40
8	−24	−23
10	−15	−15

Which result is the best evidence that the buckle halves exert a force on each other without touching?

A. The halves pulled on each other harder at 10 mm than at 2 mm.
B. The scale read −70 g with the steel half 4 mm above, but 0 g with a plain plastic half above.
C. The scale stayed at 0 g with the steel half 4 mm above the magnet half.
D. The club took three readings at each gap and averaged the results.

Use the magnet-half column of Table A. Compare the sizes of the pulls, without the minus signs.

The pull at a 2 mm gap was times as strong as the pull at an 8 mm gap.

Compare the two columns of Table A. What do they show?

A. The pull is stronger when the magnet half is the one sitting on the scale.
B. The magnet pulls on the steel, but the steel is pulled without pulling back on the magnet.
C. The steel pulls harder at each gap, since the steel plate has more mass than the magnet.
D. The magnet and the steel pull on each other about equally hard at each gap.

Mirela lays the two buckle halves on a smooth, level table, 6 mm apart. She holds the magnet half still and lets go of the steel half. Then she sets them 6 mm apart again, holds the steel half still and lets go of the magnet half. Based on Table A, what most likely happens?

A. Neither half moves, since a 6 mm gap is too wide for the halves to pull.
B. Both times, the half she lets go moves across the gap toward the half she holds.
C. The magnet half moves toward the steel, but the steel half stays where she let go.
D. The steel half moves toward the magnet, but the magnet half stays where she let go.

Test B: the spoke magnet and the sensor

Study Tables B and C and use paragraphs 3 and 4 to answer the questions.

Table B: Readings Along the Magnet's Path (passes 6 mm from the phone)
Position on path (cm)	Reading (µT)
−6	5
−4	20
−2	110
0 (closest)	420
+2	110
+4	20
+6	5

Table C: Highest Reading During One Pass
Closest gap (mm)	Highest reading (µT)
4	800
6	420
8	250
12	100
16	45

Which statement best describes Table B?

A. The reading was highest at the closest point and fell off on both sides of it.
B. The reading rose as the magnet came near and stayed at 420 µT after it passed.
C. The reading fell by about the same amount for each 2 cm the magnet moved.
D. The reading was 420 µT from −2 cm to +2 cm and near 5 µT outside that.

Use Table C.

The highest reading with a 4 mm gap was times the highest reading with a 12 mm gap.

Use Table C and paragraph 3. The sensor counts a pass only when the reading is 150 µT or more. Sort each setup.

A 6 mm gap with only air betweenA 12 mm gap with only air betweenAn 8 mm gap filled by the plastic mudguard pieceA 16 mm gap filled by the folded rubber gloveA 4 mm gap filled by the folded rubber gloveA 12 mm gap filled by the plastic mudguard piece

The pass is counted	The pass is missed

Which explanation of the sensor results in paragraph 3 fits the evidence best?

A. A magnetic field fills the space around the magnet and reaches the sensor across the gap.
B. The plastic and rubber in the gap make the magnet's field stronger near the sensor.
C. The magnet brushes the sensor for a split second on each turn of the wheel.
D. The spinning spoke pushes a puff of air onto the sensor each time the magnet goes by.

Use paragraph 4. Which conclusion about the visor fits the evidence best?

A. The wiped visor became a magnet, and its magnetic field lifted the bits.
B. The cloth left sticky threads on the visor that reached down and grabbed the bits.
C. Wiping charged the visor, and its electric field pulled the bits across the gap.
D. The tissue bits were pushed up by air moving under the visor as it was wiped.

10.

Cyrus says, "In the buckle, the magnet does all the pulling. The steel plate just gets pulled, and there is no force between the halves until they touch." Do the club's results support his idea? Write a claim, give evidence from Table A and from Table B or C, and explain your reasoning using the idea of a field.