MIT Massachusetts Institute of Technology

Hanging by a Thread

Francis Lenox
Physics Teacher
Science Department
East Greenwich High School
East Greenwich, RI

This lesson focuses on two elements: understanding Newton’s laws of motion, and how to use Newton’s laws to create force diagrams. This lesson also demonstrates how to incorporate requirements of the Next Generation Science Standards (NGSS) into a physics lesson. It uses a discrepant event (phenomenon) to model forces at work on an object resulting in motion. Students should be familiar with Newton’s laws and have a basic understanding of forces. This lesson utilizes basic lab equipment (masses, spring scales, and thread/string) and can be completed in one full period, depending on the length of period.

The Performance Expectations of this lesson are:

  • HS PS2-1: Motion and Stability, Forces and Interactions
  • Analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.

The Objectives of this lesson are: 

  1. Students will apply their understanding of Newton’s Laws to predict the outcome of a demonstration.
  2. They will develop a model to explain the cause and effect of two strings being pulled with unequal forces acting upon 2 identical masses.
  3. They will plan and conduct an investigation to find evidence to validate their claim.

Frank Lenox has taught secondary science courses, primarily physics, in Rhode Island for 28 years. A state finalist for the Presidential Award for Excellence in Math and Science Teaching, Lenox challenges his students to validate the laws of physics for themselves. "Once you accept the improbabability of teaching a single field of science in a year's time with significant depth, you strive to ignite a passion for science and to teach students the skills necessary for each to continue his or her own investigation of the nature of science."

Read more about Frank at his website:

This site, sponsored by the Physics Classroom, provides instructions on drawing free body diagrams, as well as practice situations for applying those instructions.

This resource, provided by The Physics Classroom, presents a comprehensive overview of Newtons’s Three Laws of Motion.

This site is a Khan Academy complete unit of study on Forces and Newton’s Laws of Motion.

This site, a New Yorker Magazine article, will help you explain the physics of curling to your students.

This resource is sponsored by The Science Classroom and provides an overview of the physics of soccer, in case your students ask.

This resource is a lecture by MIT Professor, Walter Lewin. Lec 06: Newton's First, Second, and Third Laws 8.01 Classical Mechanics, Fall 1999 (Walter Lewin) (42:44)

Frank Lenox also recommends the following book to enrich your physics lessons: Ehrlich, Robert, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, 1990. The demonstration presented in this lesson can be found on page 30.

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