NCERT Solutions for Class 9th Science Chapter 6 Project work — The Journey Beyond

Book page 115 Updated on2026-09-08

Q1.

You know that the force of friction depends on the nature of the surfaces in contact. Does it also depend on how hard the surfaces press each other? Is the friction acting on an object that is about to move larger than the friction after motion begins? Is the friction which acts on a rolling object less than that on a sliding object? Find answers to these questions and create an infographic. Such observations help explain why the invention of the wheel was a major milestone in human history.

Answer

The answers are: yes; yes; and yes. Here is how to establish each one experimentally, and what you should find.

Question	How to test it	What you find
Does friction depend on how hard the surfaces press?	Pull the same wooden block with a spring balance, then stack a second identical block on top and pull again on the same surface	Yes — the reading roughly doubles when the pressing force doubles
Is friction just before motion larger than after motion starts?	Watch the spring balance carefully: note the peak reading at the instant of slipping, then the steady reading while sliding	Yes — the reading falls slightly once sliding begins (static friction > sliding friction)
Is rolling friction less than sliding friction?	Drag the block, then put it on four marbles or toy-car wheels and pull again with the same spring balance	Yes — the rolling reading is much smaller

Why the wheel changed history: rolling friction is a small fraction of sliding friction, because the rolling body does not have to be dragged over the surface irregularities — it lifts over them, and only a small region is in contact at any moment. Dragging a loaded sledge over the ground needs a large, continuous force; putting it on wheels cuts that force enormously, so one bullock or one person can move a load that would otherwise need many.

What a good infographic must contain:

A labelled diagram of the spring balance test, with the reading marked in N.
A bar chart comparing your readings: one block, two blocks; just-about-to-move, sliding; sliding, rolling.
One clear sentence of conclusion under each comparison.
A short "so what" panel: ball bearings in cycle hubs, wheels on carts and suitcases, why brakes and tyre treads need more friction.

Q2.

Take two toy cars of equal mass and stick a bar magnet on top of each (Fig. 6.32). Fix a metre scale on a smooth surface. Place the cars near the midpoint of the metre scale with the like poles touching. Release the cars and record the time taken (using two stopwatches), and distance travelled by each before coming to a rest. Repeat the experiment after adding equal masses to both cars. Did the cars travel equal distances in opposite directions? Plot a graph of distance travelled versus mass. Analyse and discuss your findings.

Answer

Yes — the two cars travel very nearly equal distances in opposite directions, and both distances shrink as you add mass.

Like poles repel, so each car pushes the other away
Newton's third law: |F on car A| = |F on car B| = F
Equal masses → a = F ÷ m is the same for both → equal speeds, opposite directions

Adding equal masses to both: F is unchanged, m is larger
→ a is smaller → each car leaves with a smaller speed → shorter distance

What the graph should look like. Plot distance travelled (y-axis) against total mass of a car (x-axis). You get a falling curve — greater mass, shorter run — not a straight line.

Trial	Mass of each car	Distance moved by car A	Distance moved by car B
1	m	largest	almost the same as A
2	2m	smaller	almost the same as A
3	3m	smallest	almost the same as A

Analysis: the equal distances confirm Newton's third law — the magnetic push on each car is the same size. The shrinking distances confirm the second law — for the same force, a heavier car gets a smaller acceleration, so it starts off slower and friction brings it to rest sooner. Small differences between the two cars in the same trial are worth discussing honestly: unequal magnet strengths, slightly different wheel friction, an uneven surface, or reaction time on the two stopwatches.

Tip: release both cars at exactly the same instant, and start both stopwatches together. Repeat every trial three times and use the average — a single run of an experiment like this is not reliable.

Q3.

Wrap a rope once around a rough tree branch or post. Attach a heavy bucket to one end and try to hold it by the other end (Fig. 6.43). Now, add one more turn of the rope and repeat. You will find that each extra turn increases the ‘grip’ between the rope and the branch, increasing the friction and reducing the force required, making it much easier to hold the same load. The reduction in effort is much larger than you might expect from just adding one turn. This shows that friction does not increase in a simple linear way, small changes in contact can lead to large changes in force. In the same way, friction between a rope and a post allows large ships to be held safely at a pier.

Answer

What you will observe: with no turn you must hold nearly the full weight of the bucket. With one turn it becomes noticeably easier; with two turns you can hold the same bucket with a small fraction of the effort; with three or four turns you can hold it with two fingers.

Number of turns	Effort needed to hold the same bucket	What is happening
0	Almost the full weight	No friction from the post; your hand alone supports the load
1	Noticeably less	Friction along the wrapped length carries part of the load
2	A small fraction	Friction from the second turn acts on an already-reduced tension
3 or more	Very little	The reduction compounds turn after turn

Why the reduction is so much bigger than expected: friction acts along the whole length of rope in contact with the post, and at every point it removes a fraction of the tension still remaining. So each new turn does not subtract a fixed amount of force — it multiplies down whatever tension survived the previous turn. Repeated multiplication by a fraction falls away very fast, which is exactly what "friction does not increase in a simple linear way" means here.

Safety and honesty in the report: use a light bucket first, keep your feet clear, and record the effort with a spring balance rather than by feel if you can. Then note where you see the same idea at work: a ship's mooring line taking a few turns round a bollard at a pier, a rock climber's belay, and the way a bullock cart's rope is wound round a peg.

Q4.

It is often instructive to examine how scientific ideas develop over time. If you are interested, explore how Newton formulated the laws of motion by reading excerpts from his original work, the Principia. Both the original text and commentaries are available online.

Answer

Method for this project: read a short excerpt in translation, compare Newton's own words with the modern statement in this chapter, and write about what changed and what did not.

Find an English translation of the Philosophiæ Naturalis Principia Mathematica (1687) and read only the "Axioms, or Laws of Motion" at the start — it is barely two pages.
Note that Newton stated his second law in terms of motion — what we now call momentum, mass × velocity — not as F = ma. Section 6.5 of this chapter mentions this fuller form in the Ready to Go Beyond box.
Trace the idea backwards: what did Galileo contribute, and what did Newton add?
Write one page comparing Newton's wording with the chapter's wording, law by law.

Sample answer (an outline you can build on):

Idea	Before Newton	Newton's contribution
Motion needs a cause	It was mistakenly believed a force was needed to keep an object moving at constant velocity	Only a change of velocity needs a force; steady motion needs none
Inertia	Galileo argued by thought experiment that a body would keep moving if all impediments were removed	Newton named the property inertia and made it the first law
Force and acceleration	No quantitative rule	The second law, stated as the rate of change of momentum being proportional to the net force
Interaction	Forces were thought of as acting on one body	The third law: forces always come in equal and opposite pairs on two different bodies

Why this is worth doing: it shows that the three laws were not obvious. They replaced a belief that had been held for roughly two thousand years, and they did so because thought experiments and careful observation contradicted it. Reading the original also shows how much of physics is careful definition — Newton spends pages defining mass, momentum and force before he states a single law.

Did you know? Newton's laws still describe motion from everyday objects to planets and stars. They need modification only very close to massive objects, at speeds near the speed of light, and at atomic scales.

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