Force Class 10 ICSE — Selina Chapter 1

Force Class 10 ICSE Physics opens with the turning effect that a force produces on a rigid body. Chapter 1 of Selina Concise Physics builds the idea step by step: moment of force, couples, equilibrium, the principle of moments, centre of gravity, and uniform circular motion. Each section adds one concept and one formula. Keep the chapter open and match every formula to its figure as you read.

Download Force Class 10 ICSE Physics Chapter 1 PDF

Download the Force Class 10 ICSE Physics chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 18 pages, 8 sections, 61 questions.

What is inside Chapter 1

18 pages · 8 sections · 2 worked examples · 61 exercise questions

• EXERCISE-1(A): 26 questions
• MULTIPLE CHOICE TYPE: 20 questions
• EXERCISE-1(B): 11 questions
• MULTIPLE CHOICE TYPE: 2 questions
• MULTIPLE CHOICE TYPE: 2 questions

1.1 TRANSLATIONAL AND ROTATIONAL MOTIONS

A force can do one of two things to a rigid body. It can make the body move, or it can make the body turn. Which one happens depends on how the body is held.

When the body is free to move, the force produces translational motion. The body moves in a straight path, in the direction of the force. Pushing a ball lying on a floor is the chapter’s example.

When the body is pivoted at a point, the force produces rotational motion. The body turns about the axis passing through the pivot. A wheel pivoted at its centre rotates when a force is applied tangentially on its rim. A door turns about the axis through its hinges.

The deciding rule is one check. Is the body free to move, or is it pivoted? That single question tells you which motion the force will produce. The distinction returns throughout the force chapter for Class 10 ICSE Physics.

1.2 MOMENT (TURNING EFFECT) OF A FORCE OR TORQUE

A pivoted body cannot move in the direction of the applied force. Instead, the force turns the body about the pivot. That turning effect is the moment of force, also called torque.

Two factors decide the turning effect. One is the magnitude of the force. The other is the perpendicular distance of its line of action from the axis of rotation. The turning effect depends on the product of both factors:

\text{Moment of force} = F \times d

The perpendicular distance matters because the whole line of action contributes to the turn, not just one point of it. For maximum turning effect with a given force, apply the force where this distance is largest. That is why a door handle sits near the free end.

The SI unit is \text{N m}. The CGS unit is dyne cm. Gravitational units are \text{kgf m} and \text{gf cm}. The conversions are 1\ \text{N m} = 10^{7}\ \text{dyne cm}, 1\ \text{kgf m} = 9.8\ \text{N m}, and 1\ \text{gf cm} = 980\ \text{dyne cm}. Never write the unit of torque as joule. Torque is a vector; work or energy is a scalar.

The anticlockwise moment is taken positive, and the clockwise moment negative.

Worked pattern. A force of 12 N acts normally at 35 cm from the hinge. Convert first: 35\ \text{cm} = 0.35\ \text{m}. Then \text{Moment} = 12 \times 0.35 = 4.2\ \text{N m}, anticlockwise.

The chapter’s examples — the door handle, the handle near the rim of a hand flour grinder, the steering wheel, the bicycle chain wheel, and a long spanner — all use this formula. It is the core formula in any set of force class 10 icse notes for numericals.

1.3 COUPLE

A single applied force does not rotate a pivoted body by itself. The pivot exerts an equal and opposite reaction force. That reaction has zero moment, because its distance from the axis of rotation is zero. The applied force and the reaction together form a couple.

So a couple is two equal and opposite parallel forces that do not act along the same line. Their resultant is zero, so they cannot translate the body. But both forces turn the body in the same direction, so their moments add.

\text{Moment of couple} = F \times d

Here d is the perpendicular distance between the two forces, called the couple arm. Each force produces its own moment about the pivot, and the two moments act the same way, which is why they add rather than cancel.

Worked pattern. Two forces, each of 7 N, act at the ends of a bar 45 cm apart. The moment of the couple is 7 \times 0.45 = 3.15\ \text{N m}. Both forces turn the bar anticlockwise, so the moments add.

Daily examples include the wrench on a car wheel nut, a water tap, the cap of an inkpot, a key in a lock, winding a clock, car steering, and the bicycle pedal.

1.4 EQUILIBRIUM OF BODIES

A body is in equilibrium when several forces acting on it produce no change in its state of rest, or of linear or rotational motion.

There are two kinds. In static equilibrium, the body remains at rest. A book on a table is one example: its weight acts downwards, and the table’s reaction balances it. A beam balance at rest in the horizontal position is another.

In dynamic equilibrium, the body moves with constant velocity. A raindrop falls at steady speed because its weight is balanced by buoyancy and air friction. An aeroplane at constant height has lift balancing weight. A stone whirled on a string, and a planet or satellite in orbit, are also in dynamic equilibrium.

Equilibrium needs two conditions. First, the resultant of all the forces must be zero. Second, the algebraic sum of all the moments about the point of rotation must be zero. That means the sum of the anticlockwise moments equals the sum of the clockwise moments.

Why both conditions? A couple has zero resultant force, yet it still rotates the body. So a zero resultant force alone is not enough. Questions 22 and 23 of EXERCISE-1(A) test exactly these two conditions.

1.5 PRINCIPLE OF MOMENTS

The principle of moments states that, in equilibrium, the sum of the anticlockwise moments equals the sum of the clockwise moments. Anticlockwise moments are taken positive, and clockwise moments negative. A physical balance, or beam balance, works on this principle.

The chapter verifies the principle with a metre rule. Suspend the rule horizontally by a thread at O. Hang two spring balances with slotted weights W_1 and W_2 on either side. Adjust the weights or their positions until the rule is horizontal. Then W_1 l_1 = W_2 l_2 holds, which verifies the principle.

Worked pattern. A uniform metre rule balances on a knife edge at the 55 cm mark when a 30 gf weight hangs at the 85 cm mark. A uniform rule’s weight acts at its mid-point, the 50 cm mark. The rule’s weight gives an anticlockwise moment about the knife edge; the hanging weight gives a clockwise moment. So:

M \times 5 = 30 \times 30

This gives M = 180\ \text{g}. The distances are measured from the knife edge: 5 cm for the rule’s own weight and 30 cm for the hanging weight. This pattern of balancing two moments is the tool you reuse in later Class 10 Physics book chapters.

The numericals inside EXERCISE-1(A) form the main practice for this section. Remember the chapter’s note: for a uniform rod, the weight acts at its mid-point.

1.6 CENTRE OF GRAVITY

A body is made of many particles, each pulled down by its own weight. These parallel forces can be replaced by one force equal to the total weight W of the body. The centre of gravity (C.G.) is the point about which the algebraic sum of the moments of the weights of all the particles is zero. The entire weight acts at this point, however the body is placed.

The position of the C.G. depends on the shape, that is, on the distribution of mass. It changes if the body is deformed. A straight uniform wire has its C.G. at the middle of its length. Bend the same wire into a circle, and the C.G. shifts to the centre of the circle. The C.G. need not lie inside the material. For a ring or a hollow sphere, it lies at the centre, where there is no material.

A freely suspended body comes to rest with its C.G. vertically below the point of suspension. This fact locates the C.G. of an irregular lamina. Make three holes near the edge. Suspend the lamina in turn from each hole, using a plumb line to draw the vertical line each time. The three lines meet at G.

Memory hook for cones. A solid cone balances at one quarter of its height above the base, and a hollow cone at one third. So the hollow cone sits higher. From the vertex, the hollow cone’s C.G. is 2h/3 down, the same point as h/3 above the base. EXERCISE-1(B) practises this section.

1.7 UNIFORM CIRCULAR MOTION

Uniform circular motion is motion along a circular path with constant speed. The particle travels equal distances along the path in equal intervals of time.

Speed is uniform, but the direction of motion changes at every point. Velocity includes direction, so the velocity is variable. A change in velocity is acceleration. Therefore uniform circular motion is accelerated motion, even though the speed never changes.

At any instant, the direction of velocity is along the tangent drawn at that point of the circle. For anticlockwise motion, the direction at successive quarter positions runs north, then west, then south, then east. The speed is the same at all four, but the velocity is different.

The common mistake is to say that constant speed means zero acceleration. That is true only for a straight path. In a circle, the direction keeps changing, so acceleration is present.

1.8 CENTRIPETAL AND CENTRIFUGAL FORCE

A change in direction needs a force. In circular motion, that force points towards the centre of the circle at every point. It is called the centripetal force — the word means centre seeking.

The source of the force changes with the situation. Electrostatic attraction holds an electron in orbit around the nucleus. Gravitational attraction does the same for a planet around the sun and the moon around the earth. Tension in a string provides it when a stone is whirled. The acceleration’s direction changes at each point, but its magnitude stays constant.

Centrifugal force acts away from the centre. Its magnitude equals that of the centripetal force, but it is not the reaction of the centripetal force, because action and reaction act on different bodies. It is a fictitious force, assumed by an observer moving in the rotating frame.

The merry-go-round experiment shows the two viewpoints. An observer on the ground sees the ball move in a circle, with the string tension supplying the centripetal force. An observer on the rotating platform sees the ball stationary in front of him. He explains this by balancing the tension against a centrifugal force.

If the string breaks, the ground observer sees the ball fly off along the tangent. The rotating observer sees it move radially away, as if a centrifugal force acted on it. Only the tension was real.

What to practise from this chapter

This force chapter for Class 10 ICSE has five practice blocks, holding 61 questions in total. Work them in this order.

• EXERCISE-1(A) — 26 questions covering sections 1.1 to 1.5, including the numericals on the principle of moments.
• MULTIPLE CHOICE TYPE — 20 questions following EXERCISE-1(A).
• EXERCISE-1(B) — 11 questions on section 1.6, centre of gravity.
• MULTIPLE CHOICE TYPE — 2 questions on the centre of gravity of regular objects.
• MULTIPLE CHOICE TYPE — 2 questions on uniform circular motion and centripetal and centrifugal force.

Start with the force class 10 icse important questions in EXERCISE-1(A): answer the short questions first, then work the numericals on the principle of moments. Move to EXERCISE-1(B) for centre of gravity, then finish with the two short multiple choice blocks at the end.

Watch for four common mistakes while you practise:

• Writing the moment unit as joule. Torque is a vector, so its unit stays \text{N m}.
• Saying that a single applied force alone rotates a body. Rotation always involves a couple.
• Calling uniform circular motion unaccelerated. The direction changes, so the velocity changes.
• Treating centrifugal force as the reaction of centripetal force. The two act on different bodies, and centrifugal force is fictitious.

Before the exam, confirm the current syllabus on the official CISCE website. You can also browse the Class 10 books hub to see the other subjects in this set.

Frequently asked questions

Reference: ICSE Class 10 Physics textbook material, chapter 1: Force — CISCE-aligned.

More from this book

• Chapter 2: Work Energy and Power
• Chapter 3: Machines
• Chapter 4: Refraction of Light At Plane Surfaces
• Chapter 5: Refraction Through a Lens
• Chapter 6: Spectrum
• Chapter 7: Sound
• Chapter 8: Current Electricity
• Chapter 9: Household Circuits
• Chapter 10: Electro Magnetism
• Chapter 11: Calorimetry
• Chapter 12: Radioactivity