When a body moves across a surface, the friction it experiences is called?

Dynamic (kinetic) friction. Friction experienced by a body already in motion across a surface is called dynamic, or kinetic, friction. Read more: Friction, Torque and Rotational Motion.

Friction, Torque and Rotational Motion

Stranded on frictionless ice, running won’t move you an inch. A single sneeze will. This article covers friction, torque, and rotational motion.

MCQ Questions

General Science

Friction, Torque and Rotational Motion

Friction is a nuisance, but without it, walking would be impossible

Friction

A contact force

Static friction acts before motion — its max value is “limiting friction”

Torque

Rotational equivalent of force

A door handle sits far from the hinge to maximise torque

Moment of Inertia

Resistance to rotation

Depends on mass distribution, not just total mass

Angular Momentum

Conserved quantity

Constant when net torque is zero, like linear momentum

📑 Contents

1Friction, Torque, and Levers
2Moment of Inertia and Angular Momentum
3Test Yourself
4Friction and Rotation in Everyday Devices
5Exam Point of View
6Related Topics

✊ Must Know

Friction, Torque, and Levers

FrictionFriction is the force resisting relative motion between two surfaces in contact. It’s a contact force, unlike gravity or electrostatic force.
Static FrictionStatic friction acts before motion begins. Its maximum value, just before slipping starts, is called limiting friction.
Sliding vs RollingSliding friction acts once an object is already sliding, and is generally weaker than static friction. Rolling friction is weaker still, which is why wheels and ball bearings move so much more easily than a dragged object.
Reducing FrictionLubricants like oil or grease, and devices like ball bearings, reduce friction between moving parts. Friction is also why walking, writing, and holding objects are possible at all, so it is only reduced where it isn’t wanted, never eliminated everywhere.
Icy GroundTaking smaller steps on ice reduces slipping, since it lowers the sideways force needed, staying within the available friction.
TorqueTorque is the rotational equivalent of force. A door handle sits far from the hinge to maximise torque for a given push.
LeversA lever works on the principle of balanced torques. A wheelbarrow is a classic example of a lever.
Without FrictionWithout any friction at all, running, walking, or even rolling produces zero forward push, since there’s no surface grip to react against.

📘 Good to Know

Moment of Inertia and Angular Momentum

Moment of InertiaMoment of inertia measures a body’s resistance to rotational change. It depends on both mass and how that mass is distributed relative to the axis.
Unstable EquilibriumA ball balanced precisely on a vertical rod is in unstable equilibrium — any tiny disturbance causes it to fall.
Angular MomentumWhen net torque on a system is zero, its angular momentum stays constant, just as linear momentum is conserved when net force is zero.

Test Yourself

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🌟 Great to Know

Friction and Rotation in Everyday Devices

Friction Is EssentialFriction is often treated as a nuisance, but it’s essential: without it, walking, driving, and even holding objects would be impossible.
Sphere vs. DiscA solid sphere and a solid disc of equal mass and radius have different moments of inertia, since mass distribution relative to the axis matters, not just total mass.
Everyday DevicesDoor handles, washing-machine spin cycles, and wheelbarrows all apply the same underlying rotational physics principles.
The Sneeze TrickOn a truly frictionless surface, a person can still move by sneezing. The rapidly expelled air pushes the body backward against it, exactly like a tiny rocket, needing no friction at all.

📝 Exam Point of View

Exam Point of View

CDS-II 2017, Q16Stuck on frictionless ground, running, jumping, and rolling all fail — only sneezing works, since it moves the body through momentum conservation, not surface grip. View this question on the full CDS-II 2017 paper →
CDS-II 2017, Q59When gravity and the normal reaction cancel out on a frictionless plane, the net force is zero, so Newton’s First Law keeps the object moving at constant velocity — not accelerating or slowing. View this question on the full CDS-II 2017 paper →
Exam Wisdom: Static vs. KineticWatch for questions that swap static friction (before motion starts) with kinetic friction (once already sliding) — the two have different maximum values, and mixing them up is a common trap.
Exam Wisdom: Torque DirectionTorque depends on the perpendicular distance from the pivot to where the force is applied, not just the force’s size — pushing a door near its hinge takes far more force than pushing near the handle.