NCERT Class 9 Science Chapter 6 Activities — How Forces Affect Motion
NCERT Class 9 Science — How Forces Affect Motion, Activities 6.1 to 6.7 on pages 98 to 109.
Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.
Seven activities, following the chapter’s argument one law at a time.
- Activities 6.1 and 6.2 are about friction — first that it depends on the surface, then that it can be measured.
- Activities 6.3 and 6.4 build to Newton’s second law, the first varying the force at fixed mass, the second varying the mass at fixed force.
- Activities 6.5, 6.6 and 6.7 are all Newton’s third law: felt on a wheeled chair, measured with spring balances, and then used to launch a balloon.
Key insight. Activities 6.3 and 6.4 are a matched pair and are nearly useless apart. One changes the force and holds the mass fixed; the other changes the mass and holds the force fixed. Newton’s second law is a statement about both dependences at once, and a class that does only one of the two has seen half the evidence for it.
Activity 6.1
Tape four ₹10 coins into a stack. Find horizontal surfaces of different materials — a wooden table top, a cemented floor, a laminated top, polished marble or tiles — and check they are level. Hold a large rubber band stretched by the same amount each time, release it against the stack, and measure how far the stack slides on each surface.
What you should observe. The same push sends the coins much further on some surfaces than others.
The stack travels different distances on different surfaces for the same stretch of the rubber band. The force of friction depends on the nature of the surfaces in contact — the smoother the surface, the further the stack slides.
Why the same stretch every time. That is the whole experimental design. Stretching the band by a fixed amount gives the same launching force each run, so the only thing changing between runs is the surface, and the difference in distance can be attributed to friction alone. Mark the stretch on the band or against a ruler.
Why the chapter is interested in the distance rather than the speed. A larger friction force brings the stack to rest sooner, so the distance travelled is a direct, easily measured stand-in for how much friction there was. No timing equipment is needed.
What goes wrong. Surfaces that are not level. A floor with the slightest slope adds or subtracts a component of gravity, and the comparison becomes meaningless. The instruction to check that the surfaces are level is not a formality.
Activity 6.2
Take a spring balance and a wooden block. Lay the spring balance horizontally on one of the surfaces from Activity 6.1 and check that it reads zero. Attach the block, pull it slowly and steadily, and read the balance while the block moves at a constant speed. Repeat on the other surfaces.
What you should observe. A number, this time, rather than a comparison.
While the block moves at constant speed, the spring balance reading equals the force of friction — because at constant velocity the net force is zero, so the pull exactly balances the friction. The reading is different on each surface.
Why constant speed is the condition. If the block is accelerating, the pull exceeds the friction and the reading tells you about the net force instead. It is only when the velocity is steady that the two forces are equal and opposite, and the balance is reading friction. This is the first real use of “balanced forces” in the chapter.
Why zero the balance first. A spring balance held horizontally does not necessarily read zero, and any offset goes straight into every measurement.
What goes wrong. Jerky pulling. The needle swings wildly and there is no steady reading to take. Pull smoothly, and take the reading while the block is already moving rather than at the moment it breaks away — starting friction is larger, which is a real effect but a different one.
Activity 6.3
This is a demonstration. Build a cart from four ball-bearing wheels, two pencils and an empty cardboard box. Run a thread from the cart over a pulley at the table edge to a cup hanging below. Put objects in the cup so it pulls the cart along. Release the cart and record, from a slow-motion recording, the time it takes to cross a fixed distance.
What you should observe. A heavier cup pulls the cart across in less time.
For a fixed mass of cart, a larger force gives a larger acceleration. The chapter sets this up as a hypothesis first — “for the same object, a larger force results in larger acceleration” — and the activity tests it.
Why a hanging cup rather than a hand. The weight of the cup and its contents is a force you can double exactly, by doubling the contents. A hand cannot pull twice as hard on demand, and the whole point is that the force is known to have doubled.
Why slow motion. The cart crosses the table in well under a second, which no one can time with a stopwatch to useful accuracy. A phone recording played back frame by frame turns an impossible measurement into an easy one — and it is the piece of apparatus most likely to already be in the room.
What goes wrong. Friction in the pulley or stiff wheels, which eat the applied force and flatten the difference between runs. Ball-bearing wheels are specified for exactly this reason.
Activity 6.4
Repeat Activity 6.3 with one change: keep the pulling force the same, and double the mass of the cart. Record the times T₁ and T₂ for the same distance in the two cases.
What you should observe. The heavier cart takes longer over the same distance.
For a fixed force, a larger mass gives a smaller acceleration. Both runs start from rest ($u = 0$) and cover the same distance $s$, so
$$s = \tfrac{1}{2}a_1T_1^{\,2} \quad\text{and}\quad s = \tfrac{1}{2}a_2T_2^{\,2} \qquad\Longrightarrow\qquad \frac{a_2}{a_1} = \frac{T_1^{\,2}}{T_2^{\,2}}$$
Together with Activity 6.3: $a \propto F$ and $a \propto 1/m$, which is Newton’s second law, $F = ma$.
Why the same distance matters so much. It is what makes the two runs comparable without measuring acceleration directly. Since $s$ is identical, it cancels between the two equations, and the ratio of accelerations comes straight from the ratio of the squares of the times — two stopwatch numbers and nothing else.
Why the times are squared. From rest, distance grows as the square of the time. So a cart that takes 1.4 times as long is accelerating at about half the rate, not at 1/1.4 of it. This is where the arithmetic most often goes wrong.
What goes wrong. Changing the mass of the cup instead of the cart. That changes the force, which is Activity 6.3 again. The mass added must go on the cart, and the cup and its contents must be left exactly as they were.
Activity 6.5
Find a chair with wheels and a large heavy table. Sit on the chair with your legs raised off the floor. Using both hands, push the table away from you. What happens to you? Then try pulling the table towards you.
What you should observe. You move, not the table.
Push the table away and you and the chair move backwards; pull it towards you and you move forwards. The table pushes back on you exactly as hard as you push on it — every action has an equal and opposite reaction.
Why your feet must be off the floor. With your feet down, friction from the floor holds you in place and absorbs the reaction, and nothing appears to happen. Lifting them removes that anchor so the reaction has nothing to act against but you. This single instruction is what makes the activity work.
Why the table barely moves and you move a lot. The forces are equal; the masses are not. A heavy table divided by the same force gives a tiny acceleration, while you on a wheeled chair get a large one. Equal forces do not mean equal effects — which is the most common confusion about the third law, and this activity pre-empts it.
What goes wrong. A chair with stiff castors, or a carpeted floor. It needs to roll freely, or friction hides the effect just as surely as feet on the floor would.
Activity 6.6
Take two identical spring balances, lay them horizontally on a table and connect them by their hooks. Fix the free end of one to something immovable or hold it. Predict what the two scales will read when you pull the free end of the other. Then pull, and repeat several times with different strengths of pull.
What you should observe. Both balances read the same, every time, whatever the pull.
Both spring balances show the same reading. The chapter puts it plainly: “it was found experimentally that the magnitudes of such two forces are equal.”
Why prediction comes before pulling. Most students expect the balance they are pulling to read more. It does not. Committing to a prediction first is what makes the result land — and the instruction to repeat it “by varying the magnitude of the force” is there to stop anyone dismissing a single agreement as coincidence.
Why this is the quantitative version of Activity 6.5. The chair told you the reaction exists and acts in the opposite direction. The spring balances tell you its magnitude — equal, exactly, at every size of pull.
What goes wrong. Balances that are not identical, or not zeroed horizontally. Any difference in calibration shows up as a difference in reading and looks like a real result. Swap them round and repeat — if the discrepancy follows the instrument, it is the instrument.
Activity 6.7
Collect a large balloon, a drinking straw, adhesive tape, a long thread and two nails or hooks on opposite walls. Inflate the balloon and tie its neck. Tape the straw along the balloon with one end pointing towards the neck. Thread the string through the straw and fix it taut between the two hooks. Then untie the neck and let go.
What you should observe. The balloon shoots along the string in the direction opposite to the escaping air.
Air rushes backwards out of the neck, and the balloon moves forwards along the thread. The balloon pushes the air out; the air pushes the balloon along with an equal and opposite force.
Why the straw and thread are there. They constrain the balloon to one dimension. Released freely, a balloon flies about the room unpredictably because the neck flaps and the thrust direction wanders — entertaining, but it shows nothing. On a taut thread the motion is along one line and the cause is unmistakable.
Why this is how a rocket works. The chapter makes the comparison directly. A rocket carries its fuel, burns it, and throws the exhaust backwards at high speed; the reaction on the rocket drives it forwards. It does not push against the air or the ground — which is why a rocket works in space, where there is nothing to push against.
What goes wrong. A slack thread, which lets the balloon sag and drag. Pull it genuinely taut between the hooks. Tape the straw parallel to the balloon’s axis too — angled, the thrust fights the thread instead of driving along it.
Common mistakes
- Not keeping the rubber band stretch constant in Activity 6.1. If the launching force varies between runs, the different sliding distances say nothing about the surfaces.
- Reading the spring balance in Activity 6.2 while the block accelerates. The reading equals friction only at constant speed, when the forces are balanced.
- Doing only one of Activities 6.3 and 6.4. They test different halves of $F = ma$. One alone establishes a proportionality, not the law.
- Forgetting that the times in Activity 6.4 are squared. From rest, $s = \tfrac12 aT^2$, so acceleration goes as $1/T^2$ — a common source of wrong numbers.
- Keeping your feet on the floor in Activity 6.5. Friction then absorbs the reaction and nothing moves, which looks like the third law failing.
- Expecting equal forces to produce equal motion. The table and you feel the same force; you accelerate far more because your mass is far less. This is the third law’s most misunderstood consequence.
- Letting the balloon in Activity 6.7 fly free. It needs the straw and taut thread, or the motion is too chaotic to show anything.
Practise next
- The end-of-chapter questions, Revise, Reflect, Refine, where $F = ma$ from Activities 6.3 and 6.4 is applied numerically and the third law is used on recoil and rocket problems.
- The in-text Pause and Ponder questions of this chapter, ten across four boxes, which follow the same one-law-at-a-time order as these activities.
- The activities of Chapter 4, where Activity 4.5’s marble leaving the ring along a tangent is explained by the force that was keeping it on the circle.
- Chapter 7, Work, Energy, and Simple Machines, which asks what these forces accomplish when they act over a distance.
Doubts are answered by Shiwam, usually within a day. Ask about this question specifically — a general question about the chapter is better asked in class.