# Newton's 3rd Law Demonstration

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## Objective

Students will be able to observe Newton's 3rd Law of Motion as it applies to their physical world.

#### Big Idea

Newton's 3rd Law is not always intuitive to students. Providing a real world example helps them to understand the physical environment they live in.

## NGSS Background

This lesson is based on California's Middle School Integrated Model of NGSS.

MS-PS2 Motion and Stability: Forces and Interactions

PE: MS-PS2-1 - Apply Newton's Third Law to design a solution to a problem involving the motion of two colliding objects.

DCI: PS2.A - For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction (Newton's 2nd Law).

Science and Engineering Practices 2:  Developing and Using Models - Students observe models (basketball on a cart, rubber-band rockets on fishing line, bowling ball sized Newton's cradle) that specifically target aspects of Newton's Three Laws. These models will reduce ambiguity by demonstrating the principles of Newtonian Science without introducing misconceptions that may cause students to create an incorrect mental.

Models

1. Basket ball on a cart - The basketball first represents a model of an object at rest. The cart is then propelled forward, creating an object in motion. Once the cart is stopped the basketball continues to roll forward, proving that objects in motion stay in motion.
2. Rubber band rockets - Two rubber band powered rockets of different masses are flown across the classroom, proving that when force remains constant acceleration is influenced by mass.
3. Bowling ball Newton's cradle - Bowling ball sized Newton's cradle demonstrates that when one bowling ball is pulled back, its action triggers a reaction towards the other hanging bowling balls.

Crosscutting Concept: Cause and Effect - Students will be able to characterize Newton's Laws as a cause and effect relationship. These cause and effect relationship can be used to make predictions about how the natural world functions.

This demonstration is designed to accompany a unit highlighting Newton's Laws

Demonstrations

1. Newton's 1st Law Demonstration
2. Newton's 2nd Law Demonstration
3. Newton's 3rd Law Demonstration (this lesson)

## Set-up

250 minutes

Needed Materials

NOTE: Newton did not create this contraption. It was sold in the 1960s as a desk toy and named in honor of Newton.

Directions for building a bowling ball sized Newton's Cradle in your classroom

The first order of business is to select a suitable location for your Newton's Cradle. I found a spot that was open (allowing a full range of motion) and had a strong beam above the drop down ceiling in order to anchor the rig. I used 1/4 inch steel cable cut to 4 feet lengths. I carefully installed eye bolts into five bowling balls and used locking clips. The bowling balls were purchased from a local thrift store.

I had the high school metal shop build a 1/2 inch stock metal frame with beam clamps as attachment points. The size of the frame was dictated by the spacing of the ceiling panels. Chain links were welded 8 inches apart on either side of the frame to provide the correct spacing.

Here is a close up photo of the beam clamp. I took several photos of the location and explained what I wanted to the high school metal shop teacher and he was excited to build something, especially when I told him it had to remain hidden most of the year.

If building a bowling ball sized Newton's Cradle is too much, a smaller desktop version can be purchased instead. The desktop version will demonstrate the same laws of motion as the bowling ball version will.

## Student Demonstration

15 minutes

I use this rig for another lesson I teach Newton's 3rd Law: Newton's Cradle & Expo (9 of 9).

I show the students that for every action there is an equal and opposite reaction. I raise one bowling ball, call it the action, and release it. The collision sends one bowling ball away from the center. I refer to that as the reaction. The kids can easily see that one bowling ball action causes one bowling ball reaction. I continue scaling the demonstration up, adding another bowling ball to the action. When I get to three bowling balls as the reaction the kids are stumped because there are only two bowling balls that could be involved in the reaction.

IMPORTANT: Before I release, I stop and ask students what they think will happen. This is one of my favorite reflections all year! The kids don't know what will happen. Many think that the reaction is that the remaining two bowling balls (those struck) will fly higher than anything yet observed. I then drop the three bowling balls and sure enough three bowling balls are part of the reaction (one of the bowling balls that was part of the action also becomes part of the reaction - see video below). A lot of the students will have an ah-ha moment when the see the reality of the reaction.

## Student Activity

50 minutes

Students will make a foldable pull-out that ties together these three lessons:

1. Newton's 1st Law Demonstration
2. Newton's 2nd Law Demonstration
3. Newton's 3rd Law Demonstration (this lesson)

Each lesson was completed as a separate assignment. All three activities were not finished until all three lessons were completed. This science foldable is based upon Dinah Zike's Science Notebook Foldables. Basic construction can be seen in the following video:

I've added an extra square to make this foldable expand out to three sheets of paper. My students do not pull their foldable out into one large poster, but rather open it up one sheet at a time accordion style. Each square represents one of Newton's Laws.

The students are required to have have a title for each square, use a minimum of three colors, include the actual law being addressed, labels, and an explanation of the example being documented.

Student Work Sample