Newton's 2nd Law Demonstration

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Objective

Students will be able to observe Newton's 2nd Law of Motion as it applies to their physical world.

Big Idea

Newton's 2nd 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-PS2-2 - Plan an investigation to provide evidence that the change in an object's motion depends on the sum of the forces on the object and the mass of the object.

DCI: PS2.A - The motion of an object is determined by the sum of the forces acting on it. If the total force the object is not zero, its motion will change (Newton's 1st Law). The greater the mass of the object, the greater the force needed to achieve that same change in motion. For any given object, a larger force causes a larger change in motion (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.

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 (this lesson)
  3. Newton's 3rd Law Demonstration

Set-up

120 minutes

Materials Needed: 100 lb tess fishing line, cheap metal coat hooks (2) or eyebolts (4), rope clips (4), small pulleys (2), silicon grease, Cub Scout Space Derby Rockets (2). 

Rig Set-Up: Run 2 lines of 100 lb tess fishing line across the classroom. Cut two straws in half (lengthwise) and thread both halves on each line (see photo).

Using a rope clip, attach one end of the line to the wall. At the opposite wall, attach a small pulley, run the fishing line through the pulley and attach a weight (sand filled water bottles - see photo).

I purchased some cheap metal coat hooks and bolted them to the walls of my classroom. I kept them high enough so students could walk underneath. Another option would be to run the fishing line across the classroom and attach them to the end of desks. The weight will keep the fishing line taunt as the kids work with the lines. Use a rag or paper towel to grease the lines with silicon grease (helps reduce friction). 

Directions for building a space derby rocket.

Space Derby Rockets are balsa wood models that can be glued with ordinary white school glue and shaped with a potato peeler. There are many videos and instruction sets available online. 

I imbedded eight metal BBs (see photo below) into the red space derby rocket to increase it's mass. The other blue rocket is kept as light as possible.

By winding each rocket the same number of turns the force (F) is kept the same. According to Newton's 2nd Law when force (F) is kept the same, a decrease in mass (m) will result in an increase in acceleration (a).  

Student Demonstration

15 minutes

I use this rig for another lesson I teach. Check it out - Newton's 2nd Law: Balloon Racers - Newton's Law Expo (6 of 9).

When demonstrating Newton's 2nd law it is important to carefully develop a lesson that does not introduce misconception. I recommend staying away from an inclined plane or ramp. With an inclined plane, heavier objects (adding mass) often result in additional acceleration. Something that is at start odds with Newton's 2nd Law which describes adding mass as a decrease in acceleration. I use Cub Scout Space Derby vehicles. These rubber band powered rockets shoot across the room on fishing lines.

 

In 1687 Isaac Newton published his book Philosophiae Naturalis Principia Mathematica, or more commonly referred to as the Principia. An English translation would be the Mathematical Principles of Natural Philosophy. Newton's Laws were originally written in Latin and translated into English, thus explaining the many versions that are taught in school (something that I have found adds to the confusion). I teach one version and remind my students that they will encounter many different re-phasings that all have the same meaning.

Newton's Law (version I teach)

  1. Objects in motion stay in motion. Objects at rest stay at rest. Unless acted upon by another force.
  2. Acceleration is based on force and mass (F=ma).
  3. For every action there is an equal and opposite reaction. 

With this demonstration students can easily see that any increase in mass (red rocket) results in a decrease in acceleration when force is kept constant.

I reinforce the relationship between force (F), mass (m), and acceleration (a) with the board seen in the picture below. Place your finger in the variable you wish to keep constant, in this case force (F). Use your other finger to raise the mass (m) value and the the acceleration (a) value will tilt downward, showing the relationship of Newton's 2nd Law.

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

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