How to Learn Junior High Physics Motion and Forces: Draw Arrows, Test at Home, Then Calculate

Knowing the motion and force formulas, still guessing the direction wrong

Ask a student who has just finished a test on motion and forces one simple question: once the ball leaves the foot, what force keeps pushing it forward? Many will say, with confidence, that some of the kick's force travels along with the ball. It sounds reasonable, because in daily life anything that moves seems to need a push. In Newtonian physics, though, the foot's force acts only while the foot touches the ball. Once contact ends, the ball is left with gravity pulling down, plus friction and air resistance. The belief that a moving object carries a store of force has been studied for decades. Ibrahim Halloun and David Hestenes published two papers in the American Journal of Physics in 1985 mapping common-sense beliefs about motion. They found that these intuitive beliefs strongly shape how students perform in physics, and that formula-centred teaching changes them very little. Seven years later, Hestenes, Wells and Swackhamer published the Force Concept Inventory, a conceptual test of force that is still used to measure understanding without any arithmetic. In Indonesia's Kurikulum Merdeka, motion and forces sit inside the Phase D science learning outcomes, the phase that usually covers grades 7 to 9. In grades 7 and 8, students meet speed, velocity, net force, friction, and Newton's three laws. This is where memorising backfires most often: the calculation questions come out right, while the word problems about bicycles and tables go wrong. So this guide reverses the usual order. The student predicts, draws, and tests first, and the formulas then arrive as tools for tidying up what is already understood.

Six steps through Newton's laws for middle school, from guess to calculation

The sequence below tests the student's intuition before any formula appears. Keep to the order, because steps five and six reuse the notes from step one. Budget one or two sittings of about 45 minutes for each step.

1. 1

   Write down force-direction predictions with the textbook closed

   The first step feels odd to students used to memorising straight away: close the textbook, take a blank sheet, and write honest answers to three questions. After the ball leaves the foot, which forces are still acting on it? When the bicycle brake lever is squeezed, which way does the force that slows the bike point? For a book sitting still on a table, is any force acting on it at all? Write each prediction as it comes, with a reason, even when unsure. Why put the guesses on paper? Halloun and Hestenes recommended treating students' beliefs as serious alternative hypotheses, and a hypothesis can only be tested once it has been stated. A belief that is never written down stays quietly in place and returns in word problems built on everyday scenes. Once it is on paper, the student has a record to hold up against the experiments later. Keep the sheet at the front of the science notebook. At the end of step six it gets read again. The predictions that changed mark the concepts that have genuinely landed, and the ones that stayed the same are worth checking again with a teacher or tutor.

   Tips

   • Use everyday wording, such as the ball keeps going because it is still being pushed, so the original belief shows clearly.
   • Mark each prediction as sure or unsure, borrowing the idea behind the Certainty of Response Index.
2. 2

   Draw a free-body diagram with one dot and a few arrows

   The free-body diagram is the plainest drawing in this topic, and that plainness is its strength. The object becomes a single dot. From that dot, draw one arrow for every external force acting on the object. The arrow's direction shows the force's direction, and its length shows its relative size. A force is a push or a pull, so every arrow should answer one question: which other object is doing the pushing or pulling? Start with the book on the table. Draw a dot, an arrow pointing down for weight (Earth pulling the book) and an arrow pointing up for the normal force (the table pushing the book). Because the book is at rest, the two arrows are the same length. Next, draw the ball a moment after the kick: an arrow down for gravity and a short arrow opposite to the motion for air resistance. No forward arrow appears, because the foot is no longer in contact. The most common slip is drawing a force that the object exerts on something else, such as the book pressing on the table. That arrow belongs in the table's diagram, so it comes out of the book's. Using a different coloured pencil for each kind of force helps the eye spot patterns quickly.

   Tips

   • Label the source at the tip of each arrow, for example table or Earth.
   • Redraw the same diagram three times in a week until it comes without looking at an example.

   Velocity arrows and force arrows get mixed up easily. A free-body diagram holds forces only, so write the direction of motion separately, outside the diagram.
3. 3

   Add the arrows into a net force before describing the motion

   With the diagram done, the next move is to combine every arrow into one net force. Forces in the same direction add up, forces in opposite directions subtract. The result decides what happens to the object's motion, and that is the heart of Newton's first law: an object at rest stays at rest, and a moving object keeps moving at a constant velocity, as long as the net external force is zero. The book on the table is the case students misread most. Those who answer that no force acts on the book are mixing up two ideas, no force and zero net force. Two forces act on the book, weight and the normal force, and they balance each other. That balance is why the book stays put. The air hockey table example in the OpenStax textbook makes the second half of the law vivid. With the air switched off, the puck stops quickly because of friction with the table. With the air on, friction becomes tiny and the puck glides with almost no change in speed. So an object that slows down is being acted on by an external force, namely friction. A more massive object is also harder to speed up or slow down. That tendency is called inertia, and the greater the mass, the greater the inertia.

   Tips

   • Under every balanced diagram, write the sentence net force zero, so the motion does not change.
   • Push an empty cardboard box and then one filled with books to feel inertia directly.
4. 4

   Test friction with a sliding experiment on the floor at home

   Now the predictions get tested by hand. Mark a start line on a tiled floor with masking tape. Push a marble or toy car with as close to the same push as possible, for example by flicking it with a finger from the same distance. Measure how far it slides with the tape measure, repeat three times, and note the average. Then move to a rug or doormat and do the same. The result is almost always obvious: on the rug the object stops much sooner. The starting push was the same, so the difference comes from a larger friction force. Friction is an external force pointing against the motion. Link this finding to the bicycle brake. When the brake is applied, friction between the brake and the wheel, and between the tyre and the road, acts opposite to the bike's motion. Since acceleration always points the same way as the net force, the acceleration is opposite to the velocity, and the bicycle slows down. The materials come straight from the student's own home. Tutors do not bring or buy equipment, so get the marble, ball and tape measure ready before the session starts. A towel spread flat on the floor works as a rough surface when there is no rug.

   Tips

   • Record the results in a small table: surface, trial 1, 2, 3, and average.
   • Draw a free-body diagram of the slowing marble, with the friction arrow pointing backwards.

   Run the experiment in an open stretch of floor, away from stairs and breakable objects.
5. 5

   Measure speed yourself with a tape measure and a phone stopwatch

   The formula v = s/t makes the most sense when the student produces the numbers. Mark two lines on the floor 2 metres apart with the tape measure and masking tape. Roll the ball from behind the first line so it is already moving when it crosses, start the phone stopwatch as it passes the first line and stop it at the second. If the ball takes 4 seconds, its speed is 2 metres divided by 4 seconds, or 0.5 metres per second. Repeat with a harder push and compare the two. This is also the right moment to separate speed from velocity. Speed says only how fast, while velocity adds the direction. A ball rolling at 0.5 metres per second towards the door and another rolling at 0.5 metres per second towards the window share the same speed and have different velocities. That distinction matters in the next step, because acceleration describes any change in velocity, including a change of direction. Notice too that the ball slows down along the way. The 0.5 metres per second figure is really an average speed. A student who spots this is already a step ahead of routine textbook exercises, because they can see the friction from step four at work.

   Tips

   • Ask someone at home to work the stopwatch so the student can watch the lines.
   • Write units on every line of working, since unit slips are more common than formula slips.
6. 6

   Move to F = m·a and action-reaction pairs, then reread the predictions

   Once force directions feel natural, Newton's second law simply puts them in order: net force is the product of the object's mass and its acceleration, written ΣF = m·a. One newton is the force that accelerates a 1 kilogram mass at 1 metre per second squared. Example: a 2 kilogram box with a net force of 4 newtons to the right accelerates at 2 metres per second squared, also to the right. Those last words matter, because acceleration always points in the direction of the net force. Newton's third law follows with pictures students can imagine: a swimmer pushes the pool wall backwards, and the wall pushes the swimmer forwards. A car's tyres push the road backwards, and the road pushes the car forwards. An action-reaction pair is equal in size, opposite in direction, and acts on two different objects. Because they act on different objects, the two forces do not cancel out. With this two-object rule, students can check for themselves why weight and the normal force on a book fall outside the definition of an action-reaction pair: both act on the same book. Finally, take out the sheet from step one. Mark the predictions that have changed, rewrite the reasons in the language of Newton's laws, and bring the ones that still feel confusing to the next session.

   Tips

   • For each action-reaction pair, write two sentences with the subjects swapped: A pushes B, B pushes A.
   • Attempt calculation problems only after the free-body diagram is finished.

Common guesses about net force and friction compared with Newton's laws

The guesses column follows misconception patterns recorded in studies at junior high schools in Surabaya and South Tangerang. The home checks use household items and can be done on the floor.

Free-body diagram and speed vs velocity checklist before the test

Junior high physics tutoring for motion and forces: from the first session onward

A first EduPoint physics session for junior high usually opens with something the student already knows, such as a bicycle braking hard on a downhill road. The student predicts which way the friction force points, draws the arrow on paper, and only then does the tutor ask what reasoning sits behind that prediction. The answer shows the tutor where older common-sense ideas about motion are still at work, and the tutor picks the home experiment best suited to testing that prediction. The tutor prepares the learning material, and the experiment runs on things the student already has at home. Session length shapes what a lesson can hold. A 60-minute session fits one short experiment and one free-body diagram discussed thoroughly. A 90 or 120-minute session leaves room for the experiment, the diagram, and then net force or speed calculations built on what was just observed. Lessons can be scheduled any time between 06.00 and 23.00, at the student's home or online. On cost, EduPoint physics tutoring starts from Rp 69,000 per hour. Students choose anywhere from 4 to 28 lessons a month, and the registration form works out the total straight away, registration fee included. Every common way to pay is accepted, from QRIS and bank transfer to paying in cash at an Indomaret or Alfamart store. A parent enrolling their child can mention the chapter currently running at school during the free consultation, so the first lesson connects directly to classwork. The sequence is short: a consultation, agreeing the schedule and learning needs, EduPoint finding a suitable tutor, and then lessons begin. If the tutor turns out to be a poor fit, the replacement costs nothing. Every trace of the work, from lesson notes and recordings to self-study material and progress reports, can be reopened at app.edupoint.id, the EduPoint learner app. The habit of drawing force arrows built in this chapter carries straight into work and energy, pressure, heat and temperature, basic electricity, and optics and light. Time spent straightening out intuitions about motion now makes the later junior high physics chapters lighter.