Force
ICSE Class 10 · Physics · Chapter 1
What this chapter covers
- Translational and rotational motion
- Moment of a force (torque)
- Clockwise and anticlockwise moments
- Equilibrium
- Centre of gravity
- Uniform circular motion
- Simple numerical problems on moments and equilibrium
Key definitions
Translational motion is the motion of a body in which every particle of the body moves through the same distance in the same direction in the same time.
Rotational motion is the motion of a body in which every particle of the body moves in a circle about a fixed line called the axis of rotation.
Equilibrium is the state of a body in which the forces acting on it produce no change in its state of rest or of uniform motion, either in a straight line or in rotation.
Uniform circular motion is the motion of a body along a circular path with constant speed.
Key points
- A force on a free body through its centre causes translational motion; a force on a pivoted body causes rotational motion.
- The moment of a force, or torque, equals force × perpendicular distance of its line of action from the axis: τ = F × d.
- The SI unit of moment is N m and the cgs unit is dyne cm; 1 N m = 107 dyne cm.
- A door handle, a large steering wheel, a long spanner and a horizontal bicycle crank all use a large moment arm to turn with a small force.
- Moments are clockwise or anticlockwise; anticlockwise is usually taken as positive.
- For equilibrium, the resultant force must be zero and the total anticlockwise moment must equal the total clockwise moment.
- The principle of moments is verified with a metre rule hung from two spring balances, with slotted weights hanging from it.
- The centre of gravity is the point through which the whole weight of a body acts; for regular uniform bodies it lies at the geometric centre and may lie outside the material.
- The centre of gravity of an irregular lamina is found where plumb lines from different points of suspension meet.
- Uniform circular motion has constant speed but changing velocity, so it is accelerated and needs a force towards the centre.
- Centripetal force is a real force towards the centre; centrifugal force is an apparent outward force felt only in the rotating frame, equal in size and opposite in direction.
Questions answered
Define the moment of a force and state its SI and cgs units.
How many dyne cm make 1 N m?
Distinguish between translational and rotational motion, with one example of each.
Why is the handle of a door fixed at the edge farthest from the hinges?
Why is it easier to turn a steering wheel of large diameter?
When does pushing on a bicycle pedal produce no turning effect?
State the two conditions for a body to be in equilibrium.
State the principle of moments.
Past board exam questions
ICSE 2027 sample paper · 1 mark
A heavy object and a light object are placed on a seesaw. Which arrangement produces the maximum turning effect about the pivot?
- Both objects near the pivot on opposite sides.
- Heavy object close to the pivot; light object at the far end on the opposite side.
- Light object close to the pivot; heavy object at the far end on the opposite side.
- Both objects at the ends of the seesaw, on opposite sides of pivot.
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ICSE 2027 sample paper · 6 marks
Complete the following by choosing the correct answers from the bracket: (a) Friction acts __________ [tangential / radial] to a car undergoing circular motion. (b) A glass prism is immersed in water. The angle of minimum deviation will __________ [decrease / increase]. (c) The CGS unit of specific heat capacity is _________ [erg g⁻¹ K⁻¹ / cal/g⁻¹ K⁻¹]. (d) The minimum distance for distinct echo formation is __________ [greater / lesser] in water as compared to air. (e) Two copper wires can have different resistivity if they have different __________ [lengths / temperatures]. (f) Nuclear __________ [fusion / fission] is possible at ordinary temperature and pressure.
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ICSE 2027 sample paper · 3 marks
A metal rod AB of length 80 cm is balanced at 45 cm from the end A with 100 gf weights suspended from the two ends. (Along the rod: A is at 0 cm, its centre C at 40 cm, the fulcrum at 45 cm and B at 80 cm; 100 gf hangs from A and 100 gf from B.) (a) If this rod is cut at the centre C, then compare the weight of AC to the weight of BC. (Use >, < or =) (b) Give a reason for your answer in (a).
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ICSE 2026 board paper · 1 mark
For a body to be in dynamic equilibrium, its:
- momentum should be zero
- acceleration should be zero
- kinetic energy should be zero
- velocity should be zero
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ICSE 2026 board paper · 6 marks
Complete the following by choosing the correct answers from the bracket: (a) A car is moving in uniform circular motion. The direction of friction between the tyres and the path is __________ [towards the centre / tangential to the path]. (b) When a ray of light passes from a denser to a rarer medium, its wavelength __________ [decreases / increases]. (c) The lid of a calorimeter minimises heat loss by __________ [convection / radiation]. (d) Quality of sound depends on its __________ [amplitude / waveform]. (e) A substance whose resistance becomes almost negligible at a temperature near absolute zero is called a __________ [semiconductor / superconductor]. (f) __________ radiation deviates minimum in a magnetic field. [Alpha / Beta].
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ICSE 2026 board paper · 2 marks
State two factors on which the position of Center of Gravity of a body depends.
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ICSE 2026 sample paper · 3 marks
A metal rod AB of length 80 cm is balanced at 45 cm from the end A with 100 gf weights suspended from the two ends. (a) If this rod is cut at the centre C, then compare the weight of AC to the weight of BC. (Use >, < or =) (b) Give a reason for your answer in (a).
Show answer
ICSE 2025 board paper · 1 mark
A body is acted upon by two equal and opposite forces, that are NOT along the same straight line. The body will:
- remain stationary
- have only rotational motion
- have only rectilinear motion
- have both rectilinear and rotational motion