Teaching force and motion…

...is about challenging real-life experiences.

Hello all. Welcome to the 113th edition of TEPS Weekly! 

Force and motion are concepts taught right from the early years and continue through school and even university. At first glance, they seem simple to teach. After all, motion is everywhere: balls roll, people walk, vehicles move, objects fall, and so on. There is no shortage of examples. But that’s exactly where the problem lies.

Motion feels familiar, so we often don’t question our understanding of it. And that can lead to deep misconceptions—not just for students, but even for teachers.

Let’s begin with a simple scenario. Two identical chairs with wheels, Chair A and Chair B, are facing the same direction. A student sits on Chair A and uses their hands to push Chair B. Chair B rolls forward. Chair A remains still.

Which of these is correct?

a) Neither the student nor Chair B applies any force on the other.

b) The student applies force on Chair B, but Chair B doesn’t apply force back.

c) Both apply force on each other, but Chair B applies more force.d) Both apply force on each other, but the student applies more force.e) Both apply equal force on each other.

Take a moment to think about your answer.

In a study, 81% of teachers chose option b. Only 19% chose e—the correct answer. So what’s going on?

Many believed that since Chair A didn’t move, no force was applied to it by Chair B. They know that motion only happens when a force is applied. And they misinterpreted this as, ‘if no motion has happened it means that no force was applied by Chair B on Chair A’. That’s incorrect. We all know and can state Newton’s third law—“Every action has an equal and opposite reaction.” Interestingly, most couldn’t apply it here.

When the student pushes Chair B, Chair B pushes back with equal force (Newton’s third law). But only Chair B moves. Why does Chair A not move? Because of friction—the force between the chair’s wheels and the floor. Chair A had a student sitting on it, making it heavier. That means more friction between it and the floor. Chair B, being lighter, had less friction to overcome. So it moved more easily. (There is also the momentum to consider, but let’s not get into that for this edition.)In classrooms, we often start teaching with definitions of force, its examples, Newton’s laws, and related formulae. But here’s the problem: Knowing the laws and applying formulae to solve word problems isn’t the same as understanding them. 

Why is it difficult to understand or teach force and motion? Many students (and even adults) form their ideas about motion through everyday experiences. These are intuitive ideas that “make sense” based on what we see happening. But they are often scientifically incorrect because they miss the invisible forces that cause or prevent motion. These ideas are known as prior mental models. For example, Newton's first law suggests that a body in motion will remain in motion till an external force is applied to it. But our day-to-day experiences have shown that a cricket ball stops after rolling in the grass for some time. In this case we have neglected to account for the ‘invisible’ frictional force acting on the ball.

These prior mental models are powerful and in general, they clash with how concepts actually work in physics. That’s why simply stating the correct rule doesn’t help. Teaching motion should involve supporting students to explicitly reconstruct these prior mental models in order to truly understand force and motion.

Let’s explore some misconceptions about motion – why students believe them, and how teachers can guide students toward the correct foundational understanding.

1. If something is not moving, there must be no force acting on it.

Think of a book lying on a table or a toy resting on the floor. Nothing seems to be happening, so it feels like no force is acting on them. Their prior mental model connects motion only with visible forces. To students, being stationary looks like the “natural” state—something that doesn’t need any pushes or pulls to stay that way. But this idea doesn’t match how forces actually work.

Even when an object is at rest, forces can still be acting on it. They are just balanced, so they cancel each other out. A balanced situation means the object has zero net force, but not zero force. Take the book-on-table example:

• Gravity pulls the book downwards.

• The table pushes the book upwards with equal strength.

These two forces are equal and opposite, so they cancel each other out. That’s why the book stays still. But both forces are real and still present.

How to alleviate such a misconception

Research shows that reshaping of mental models happens when students experience cognitive conflict—when what they expect to happen doesn’t match what they see. To help students see and explain what are the invisible forces, create situations where their existing ideas are challenged. 

Let’s go back to the book on the table. We can ask: “If we suddenly removed the table, what would happen to the book? Students will say, “It will fall.” Now ask: “Why does it fall?” Let them think and answer: “Because gravity is pulling it down.”

Prompt by saying: “So if gravity was already pulling it down, why wasn’t it falling when it was on the table?”

This helps them realise the table was pushing the book up with equal strength. That upward push is called support force (also known as normal force). It’s a real force—just not always easy to notice. Ask: “So when the book is on a table, what forces are acting on it?”

Now, to reinforce, present the same concept in different scenarios. Say: Imagine two groups of children playing tug of war. Both pull with all their strength, but the rope doesn’t move at all. The centre of the rope stays still.

Prompt by asking: “Does that mean no one is applying force?”

Most students will say, “No, they are pulling equally hard!”

Then ask: “Then why doesn’t the rope move?”

This creates the perfect example to introduce the idea of balanced forces on stationary objects. Each team is pulling but with equal force in opposite directions. The forces cancel each other out, just like the table and the book. If something is not moving, there is zero net force, not no force altogether.

2. If something is moving at the same speed, a force must still be continuously acting on it.

A toy car slows down when it’s no longer pushed. A bicycle stops moving when you stop pedalling. A rolling football eventually comes to a halt. This often leads us to believe that as long as something keeps moving, there must be a force pushing it along. If that force stops, the object should stop too. But that’s not quite right.

How to alleviate such a misconception

To help students challenge this prior model, one way to create cognitive conflict is to use Predict – Observe – Explain.

Take one glass slab and one sandpaper of the same length. Place both side by side. Tell students: “We’ll roll the same marble with the same push on both slabs. Before we do that, what do you think will happen?” Let them predict. Students who think, ‘we have to keep pushing the marble for it to keep going,’ will predict that the marble will travel the same distance on both surfaces. Or that the marble will get to the end of both surfaces.

Then, perform the demonstration. Push the marble down both surfaces, using as similar a force as possible. Let students watch what happens. The marble will travel more on the glass surface.

Now, guide them to make sense of what they’ve just seen. Ask: “We pushed the same marble with the same force. But did it go the same distance on both surfaces?” Let them share what they observed. 

Then prompt: “If nothing touched the marble after the push, what could have made it stop sooner on the sandpaper?” Allow them to reflect. 

Follow with: “What’s the difference between the glass and the sandpaper that might explain this?” Invite them to touch both surfaces and describe what they feel.

Finally, ask: “If the marble kept going longer on the smoother surface, what does that tell us about what slows it down?”

Through this prompting students will begin to see: it’s the force from the roughness of surface that stops the moving marble. In fact, the smoother the surface, the marble would keep rolling.

Motion doesn’t need a continuous force to keep moving. It only needs force to change—to start, stop, speed up, slow down, or change direction. So what are students missing in these everyday observations? They don’t consider the invisible forces that are always acting, mainly friction (from the ground) and air resistance (friction from the air). These forces oppose motion and gradually slow things down. That’s why the toy car, the football, or the bicycle stop—even though no one is pushing them anymore.

With younger students, it helps to get them to imagine being the object. This helps reinforce the understanding of the activity. Try asking: “Let’s pretend you’re a little marble rolling across two surfaces—first the glass, then the sandpaper. What do you feel? What happens to your speed? What’s making you slow down?”

Encourage them to narrate the ball’s journey: “I got a push and started rolling on a sandpaper slab. Uh-oh! It was rough. The surface started pushing back against me. That’s friction! It didn’t stop me right away, but I felt slower… and slower… until I finally stopped.  Then I moved onto the glass slab. So smooth! Nothing was trying to stop me. I rolled and rolled and didn’t slow down much at all.” By shifting the perspective and letting students imagine the forces acting on them, we help them internalise what’s really happening. This storytelling method—especially in early years—makes invisible forces included in the mental model.

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Misconceptions don’t go away just by giving the right answers. We need to:

• Create experiences that challenge students’ thinking: Let them predict outcomes. Then let them observe what actually happens and reflect on the difference.

• Prompt them to explain their reasoning about everyday experience: Ask “Why do you think that happened?” and let students explain their ideas. Then guide them gently to new understandings.

Teaching concepts like force and motion is not about explaining laws or knowing formulae. It’s about helping students rethink what they already believe. Our teaching needs to reshape those prior mental models. So the next time you’re introducing force and motion to students—whether it’s a chair that doesn’t move or a paper floating down—pause and ask: What forces are acting on them? Helping students uncover the invisible forces will help them build a foundational understanding.

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Edition: 4.20