Friction Explained — Static, Dynamic, Coefficient, Viscosity & Stoke’s Law (JAMB/WAEC Physics)
Friction is the force that opposes relative motion or the tendency of motion between two surfaces in contact. This is Part 1 of our Friction series, covering static and dynamic friction, the coefficient of friction, and how these ideas extend to fluids through viscosity and Stoke’s Law.
This topic is a regular feature in JAMB and WAEC Physics, usually tested through definitions, calculations involving the coefficient of friction, and questions on viscous forces. This lesson breaks all of these down clearly.
Quick takeaways
- Friction is the force that opposes relative motion or the tendency of motion between two surfaces in contact.
- Static friction acts on an object at rest, preventing it from starting to move, while dynamic (kinetic) friction acts on an object already in motion.
- The coefficient of friction (μ) is the ratio of the frictional force to the normal force between two surfaces: μ = F/N.
- Viscosity is a measure of a fluid’s resistance to flow, essentially friction occurring within a fluid or between a fluid and a moving object.
- Stoke’s Law describes the viscous drag force on a small sphere moving through a fluid, and it’s used to calculate the terminal velocity of objects falling through fluids.
What is friction?
Friction is the force that opposes relative motion, or the tendency toward relative motion, between two surfaces in contact with each other. It acts parallel to the surfaces involved and always in the direction opposite to the motion (or attempted motion), which is why friction is responsible for slowing down moving objects and requiring extra force to start moving a stationary one.
What is the difference between static and dynamic friction?
Friction is classified into two main types depending on whether the object is moving:
- Static friction — acts on an object at rest, resisting the initial force applied to try to move it; static friction increases to match the applied force up to a maximum value, beyond which the object begins to move
- Dynamic (kinetic) friction — acts on an object that is already moving, opposing its motion as it slides across a surface
Generally, the maximum static friction is slightly greater than dynamic friction, which is why it typically takes more force to start an object moving than to keep it moving once it’s already in motion.
What is the coefficient of friction?
The coefficient of friction (μ) is a value representing how much frictional resistance exists between two specific surfaces, calculated as the ratio of the frictional force to the normal force pressing the two surfaces together:
μ = F/N
Where F is the frictional force and N is the normal force. The coefficient of friction depends on the nature of the two surfaces in contact (e.g., rubber on concrete has a much higher coefficient than ice on ice) and is generally treated as constant for a given pair of surfaces.
What is viscosity?
Viscosity is a measure of a fluid’s resistance to flow, essentially representing internal friction within the fluid itself, or friction between the fluid and an object moving through it. A highly viscous fluid, like honey, flows slowly and resists motion strongly, while a low-viscosity fluid, like water, flows easily with much less internal resistance.
What is Stoke’s Law?
Stoke’s Law describes the viscous drag force acting on a small sphere moving through a fluid at low speed, given by:
F = 6πηrv
Where F is the viscous drag force, η (eta) is the fluid’s viscosity, r is the radius of the sphere, and v is its velocity. Stoke’s Law is commonly used to calculate the terminal velocity of a small object falling through a fluid — the constant speed reached once the viscous drag force and gravitational force acting on the object are balanced.
Common mistakes students make with friction
- Confusing static friction (preventing motion from starting) with dynamic friction (opposing motion already happening)
- Forgetting that the coefficient of friction depends on the specific pair of surfaces involved, not a single universal value
- Treating viscosity as unrelated to friction, rather than understanding it as friction occurring within or against a fluid
- Misapplying Stoke’s Law formula, particularly mixing up which variable represents viscosity versus radius
Frequently asked questions
Q: What is the difference between static and dynamic friction? Static friction acts on an object at rest, resisting the start of motion, while dynamic (kinetic) friction acts on an object that is already moving, opposing its ongoing motion.
Q: How is the coefficient of friction calculated? The coefficient of friction is calculated as the ratio of the frictional force to the normal force between two surfaces: μ = F/N.
Q: What is viscosity? Viscosity is a measure of a fluid’s resistance to flow, representing internal friction within the fluid or friction between the fluid and an object moving through it.
Q: What does Stoke’s Law describe? Stoke’s Law describes the viscous drag force acting on a small sphere moving through a fluid, and it’s commonly used to calculate the terminal velocity of an object falling through that fluid.
Want to see how friction connects to gravity and motion? Read our Gravitational Field lesson →
This is one topic from our full JAMB & WAEC Physics course — see everything covered →
