NCERT Solutions for Class 9th Science Chapter 4 Activity 4.5: Let us investigate — Uniform circular motion

Book page 67 Updated on2026-09-08

Q1.

Take a ring, such as an adhesive tape ring and one marble.

Answer

Use a stiff, fairly wide ring — the empty cardboard core of an adhesive-tape roll is ideal — and one smooth glass marble that rolls freely inside it.

Tip: a wider ring gives a longer circular path, so the marble completes fewer revolutions per second and you can follow its direction of motion by eye. A ring with a smooth inner wall matters too: a rough wall keeps nudging the marble and the "circular" motion becomes ragged.

Q2.

Place the ring flat on a smooth surface and throw the marble inside the ring in a way that it rotates along the inner boundary of the ring (Fig. 4.24).

Answer

Rest the ring flat on a smooth table or floor and flick the marble in along the inner wall, not towards the centre, so that it hugs the boundary and circles round.

Why it happens: left to itself the marble would roll in a straight line. It goes round only because the inner wall of the ring keeps pushing it inwards, continuously bending its path. That inward push from the wall is what turns straight-line motion into circular motion — and it is also what will be missing the moment you lift the ring.

Tip: a smooth surface matters. On a rough floor friction slows the marble quickly, the speed stops being constant, and the motion is no longer uniform circular motion.

Q3.

Predict what will happen if you lift the ring while the marble is moving.

Answer

Prediction: the marble will stop curving and will roll away in a straight line — along the tangent to the circle at the point where it happened to be when the ring left it.

It will not keep circling, and it will not fly outward along a radius from the centre. Write your prediction down before you test it.

Why it happens: at every instant the marble's velocity already points along the tangent — that is the direction it is actually moving. The wall was only bending that velocity round. Remove the wall, and nothing is left to change the direction, so the marble simply carries on the way it was already going at that instant.

Q4.

Now, after one or two complete revolutions of the marble, pick up the ring without disturbing the motion of the marble. What do you observe? Does the marble continue moving in a circular motion? Or does it move in some other manner?

Answer

Observation: the marble immediately leaves the circle and rolls off in a straight line. It does not continue in circular motion.

The straight line it takes is the tangent to the circle at the exact point where the marble was when the ring was lifted — so if you repeat the activity and lift the ring at a different point, the marble goes off in a different direction.

Why it happens: once the marble is released it continues to move in the direction it was already moving at that instant, and with no wall pushing it sideways there is nothing to change that direction. This confirms two things at once: (a) the velocity in circular motion is along the tangent, and (b) circular motion needs something acting continuously to keep bending the path. You will learn the reason behind (b) in a later chapter.

Q5.

Repeat the activity multiple times to confirm the result.

Answer

Do it at least four or five times, lifting the ring at a clearly different point of the circle each time, and mark or note the direction the marble takes away.

Every trial should give the same finding: a straight-line escape, always along the tangent at the release point.

Why it happens: a single trial could be luck — the marble might have been nudged, or the surface might have sloped. Repeating with the release point deliberately changed tests the rule, not one instance of it. If the marble left along a radius, or kept curving, the tangent explanation would be wrong. It never does, which is what makes the conclusion trustworthy.

Check it yourself: if the marble slows visibly during the revolutions, your surface is not smooth enough — move to a glass tabletop or a well-polished floor, so that the circular motion stays close to uniform.

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