May the (balanced and unbalanced) forces be with you

Physics is phundemental and phun. I enjoy teaching physics because the subject lends itself to many hands-on demonstrations and real-world applications. It is easy for students to see things start, stop, and change direction. As teachers, it is our challenge to get students to think about why these things occur instead of just passively observe. So we must today address the Texas objective

TEKS 8.6A demonstrate and calculate how unbalanced forces change the speed or direction of an object’s motion

Which helps build up to the higher level skills addressed in the Next Generation Science Standards

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

I have tried many different iterations of balanced/unbalanced forces lessons in the past and I am consistently foiled by that nemesis of all scientists, engineers, and ice skaters: friction. Thus, I have found it simplest to introduce the concept of balanced and unbalanced forces in the friction free world of computer simulations.

If you are not yet familiar with the PhET simulations, prepare to have your world be revolutionized. These little apps are amazing for letting students “tinker” with the world and figure things out on their own, no mess or danger involved. It even better now that PhET has started developing sims in HTML5 instead of JAVA/Flash so the sim runs on any platform (including Chromebooks).

I like to start this lesson, as with many lessons, with an assessment probe to check and see what preconceived notions my students already have. For this activity I give each group a set of papers folded in half (print double sided). The outside has a statement that the students discuss and agree or disagree with. The inside then has some examples with confirming or disconfirming evidence. For example:

For this example, the outside of the paper says “If there is motion, then a force is acting.” Most students tend to say “yes, it is true that motion is a result of force.” When they open the paper, they see the example of the astronaut floating away from the spaceship. Clearly the astronaut is moving/is in motion. Where is the force? Students must discuss and reconcile their previous answer. The complete set of assessment probes has more examples (some of which are easier for students than others).

Now we have been talking about forces without ever really clarifying what we mean by “force.” Take a moment to have students discuss and record in their notes what they mean by “force,” traditionally defined as “a push or pull.”

Download full notes and record page for this lesson here.

Explain to students that today we will be exploring the effects that forces have on objects. To accomplish this exploration we will be using the “Force and Motion: Basics” simulation (shown above). Have students access the sim via whatever technology platform you prefer and provide the instructions.

[NOTE: I usually like to give out physical paper copies of instructions because I find it easier for students to follow along that way rather than toggling back and forth between windows, but a digital copy can work too]

Students should be able to follow instructions to work at their own pace through the remainder of the lesson. Here’s a description of what they’ll be doing.

Part 1: What do forces do? – in this section students use different configurations of people pulling on a cart to see how balanced forces have no effect on motion and unbalanced forces cause changes in motion. Students are introduced to the term “net force” as meaning the “total of all the forces acting on an object.”

Part 2: How do forces change motion? – here the students turn “on” some of the different indicators in the sim to examine how to change the motion of the box. Students discover that forces cause changes in motion and objects with more mass require more force to move as well as that larger forces produce greater changes in motion (higher speed).

Part 3: Friction – How do forces interact? – in this section friction is re-introduced into the sim and students discover how external forces must overcome friction to cause motion. Here again we see the concept of balanced and unbalanced forces and examine how unbalanced forces change the motion of objects.

Part 4: Balancing Act (extension for students who finish quickly) – redirect students to the “Balancing Act” PhET sim. In this sim students play around with balanced and unbalanced forces quite literally – using a balance. This part of the activity is not essential for the lesson, but does help develop conceptual understanding (and gives those quick kiddos something to do while the rest of the groups catch up).

When all students are finished, you can come back together as a class, discuss, and synthesize your learning using the concept map:

I think the concept map is straightforward, but my students don’t, so we walk through step by step starting at the top. We know that forces act on objects (already filled in). The total force acting on objects is the “net force” (already filled in but emphasize strongly as this is not a term in common everyday use).

If the net force equals zero (right side of concept map), then we say that the forces are balanced. Balanced forces can either result in objects staying at rest (still) or causing no change in motion. The “no change” in motion part is hard for students to understand, but we will come back to this concept when discussing Newton’s first law in more specific detail and explaining inertia.

If the net force equals not zero (left side of concept map) the forces are unbalanced and always cause change in motion.

You can then do some specific example calculations with the kiddos. I also like to have them make up their own as an end of class knowledge check or later activity.

Answers: (1) Net force = 300 N down (needs a direction), unbalanced, motion down (2) Net force = 25 N Right, unbalanced, motion right (3) Net force = 0 N, Balanced, No motion… might not have introduced the unit “Newton” yet, so explain at your leisure or leave this “N” a mystery until future lessons.

We’re done with the basics! Now you can go back to that NGSS higher level thinking objective and have students “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.” That investigation can be theoretical or actual. Personally, I like just looking at the relationship between force and mass here, but I also put off the “project” part until after introducing all three of Newton’s Laws in specific, so you could simply explain the idea here and have students do some brainstorming about how they could “prove” what they just learned. Enjoy!

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