This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

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The purpose of this assessment probe is to elicit students’ ideas about circular motion. The probe is designed to determine whether students recognize that an object will move in a straight line unless acted upon by an outside force.

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circular motion, force, inertia, Newton’s first law

The best answer is Keira’s: The marble leaving the track will travel in a straight line. This behavior is true of all objects: If no outside forces act on an object, the object will travel in a straight line at a constant speed. As the marble rolls down the marble tower’s spiral track, a force toward the center of the spiral (a centripetal force) caused by the outside wall keeps the marble rolling in a spiral path. When the marble leaves the end of the track, it is no longer in contact with the walls of the track. Without the track pushing on it, the marble no longer has a center-directed force acting on it that causes it to roll in a curved path. According to Newton’s first law, an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. Since there is no longer a center-directed, external force exerted by the walls of the circular track pushing on the marble, the marble rolls off the end of the track in a straight-line path across the floor. It will continue to do so until an outside force causes it either to change direction or slow down and stop.

Elementary Students

Students at the elementary school level may have played with curved marble towers and winding chutes or toy cars moving down curved tracks. Even though their observations may show that a moving object leaves a curved path in a straight line, students tend to revert to their intuitions that an object will continue to move in a curved path. At this level, their experiences are observational, forming a foundation to later develop explanations in middle school that are based on Newton’s laws.

Middle School Students

Force and motion relationships are developed more fully at the middle school level. Students learn about Newton’s first law of motion and a variety of phenomena that can be explained by it. The idea of inertia is conceptually developed at this level. Students have various experiences observing and explaining a variety of motions, including circular motion.

High School Students

Students build on their previous experience with Newton’s first law at the high school level, adding mathematical relationships to their understandings. As they move from qualitative to quantitative views of forces and motion, students begin to understand the mathematical consequences that lead to Newton’s laws of motion. Students are now able to use Newton’s second law to solve circular motion scenarios from new perspectives and make quantitative predictions with confidence. However, this ability to perform mathematical operations should not be overemphasized in place of conceptual understanding. Students still strongly retain many incorrect ideas about circular motion, and being able to perform mathematical calculations or restate Newton’s laws from memory is not a certain indication of understanding.

Make sure that students understand what is meant by “a spirally curving marble track.” Consider bringing in a prop, such as a child’s curved marble tower or a winding car track, or curved tube, such as a hose or flexible pipe, so that students understand the context of the probe. The prop can then be used to test students’ predictions after they commit to an outcome.

• Students often expect that objects moving in a curved path because of a wall or constraint will continue to do so when the wall or constraint is removed. This belief that the wall or constraint “trains” the object to follow a curved path is deeply rooted in students and persists even with targeted instruction (Arons 1997).
• Students have difficulty perceiving the direction of motion in a straight line when they encounter situations like an object set in motion inside a curved hollow tube. In this case many students, including those in high school, think the object continues to travel in a curved path when it comes out of the tube (Gunstone and Watts 1985).
• Students’ experiences with whirling objects on a string may contribute to their confusion about the direction of the force the string is exerting on the object. Students may think that they are exerting force along the circular path of the object’s motion, rather than perpendicular to it, toward the center (Arons 1997).
• Many students think that objects in circular motion are being “thrown outward.” This is likely because of the sensation that they feel when traveling around curves in vehicles themselves (Arons 1997, p. 121).

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1, “laws of motion map,” 62–63. New York: Oxford University Press.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

Robertson, W. 2002. Force and motion, Stop Faking It! Finally Understanding Science So You Can Teach It. Arlington, VA: NSTA Press.

Stein, M. 1998. Toying with science. Science and Children (Sept.): 35–39.

Harris, J. 2004. Science 101: Are there different types of force and motion? Science and Children (Mar.): 19.

• Encourage students to investigate the phenomenon described in the probe themselves, or help them to do so with a demonstration setup. Students who do so should predict the path of the ball’s motion and discuss their predictions before performing or viewing the experiment. Several different arrangements can help with this. Try removing a section of an embroidery hoop and rolling a marble along its inside edge, or twirling a soft ball on a string and viewing its motion after the ball is released. Especially with a ball on a string, the motion can be difficult to view, so students should take turns or use a video camera to record and review the motion (Arons 1997, p. 120).
• Ask students about their experiences with marble towers—many are likely to own or have seen a similar setup. Some tracks exist that are flexible enough to reproduce the scenario portrayed in the probe, so students can predict and see the results of the probe itself. Show students the setup before administering the probe, and perform the experiment after discussing students’ ideas. Provide an opportunity for students to revise their explanations after viewing the results.
• Talk with students about traveling in a car, on a merry-go-round, or on other amusement park rides. Discuss the feelings they have and relate these to the objects exerting force on them. Use examples from straightline motion to help students see that, just as they are pushed forward (rather than thrown backward) when a vehicle accelerates, they are pulled inward by the vehicle’s wall when going around a corner rather than thrown outward.
• If a playground merry-go-round is available, have a student riding on the merry-go-round release a ball after reaching a certain point. Have other students note the motion of the ball: Does it travel in a straight or curved line?
• Attach a string to the side of a wind-up toy that travels in a straight line. Show students that pulling on the string pulls the toy only sideways, and have them pull on the string and watch the toy travel in a curved path. Then have them release the string and watch the motion of the toy as they do so. After they do this several times in a row, have students compare this slow-motion example and the forces acting on the toy to the motion in previous examples that occur more quickly.
• If students have had experiences with hoses, ask them what direction the water flows in when they turn on a spigot and the water comes out of a coiled-up hose. Many students have seen the water shoot out in a straight line, rather than a curved path. Since this phenomenon involves a liquid rather than a solid, make sure students understand that Newton’s first law applies to liquids as well as solids.