Friction

topicmedium9 MCQ

Static, kinetic and rolling friction with laws and applications. (Physics › Laws of Motion, NEET UG syllabus.)

What is Friction?

A contact force that opposes the relative motion or tendency of relative motion between two surfaces in contact.

Key formula / rule: Static Friction

Define friction and differentiate between static, kinetic, and rolling friction.
State the laws of static and kinetic friction.
Calculate the force of static and kinetic friction given the coefficient of friction and normal force.
Apply the concept of friction to solve problems involving objects on horizontal and inclined planes.

Confusing static friction with limiting friction; static friction can be less than its maximum value.

Term: Friction
Meaning: A contact force that opposes the relative motion or tendency of relative motion between two surfaces in contact.

Term: Static Friction
Meaning: The friction force that opposes the tendency of relative motion between two surfaces that are at rest relative to each other.

Term: Limiting Friction
Meaning: The maximum value of static friction that can be exerted between two surfaces just before relative motion begins.

Term: Kinetic Friction
Meaning: The friction force that opposes the relative motion between two surfaces that are sliding past each other.

Term: Rolling Friction
Meaning: The friction force that opposes the rolling motion of an object over a surface.

Term: Coefficient of Static Friction (μs)
Meaning: A dimensionless constant representing the ratio of the maximum static friction force to the normal force between two surfaces.

Term: Coefficient of Kinetic Friction (μk)
Meaning: A dimensionless constant representing the ratio of the kinetic friction force to the normal force between two surfaces.

Term: Normal Force (N)
Meaning: The component of the contact force perpendicular to the surface that an object rests on or moves across.

Define friction and differentiate between static, kinetic, and rolling friction.
State the laws of static and kinetic friction.
Calculate the force of static and kinetic friction given the coefficient of friction and normal force.
Apply the concept of friction to solve problems involving objects on horizontal and inclined planes.
Understand the concept of angle of friction and angle of repose.
Identify situations where friction is beneficial or detrimental.

Name: Static Friction
Note: fs is the force of static friction, μs is the coefficient of static friction, N is the normal force. fs is self-adjusting up to its maximum value.
Expression: fs ≤ μsN

Name: Limiting Static Friction
Note: This is the maximum static friction that can be exerted before an object starts to move.
Expression: fs,max = μsN

Name: Kinetic Friction
Note: fk is the force of kinetic friction, μk is the coefficient of kinetic friction, N is the normal force. This force acts when surfaces are in relative motion.
Expression: fk = μkN

Name: Rolling Friction
Note: fr is the force of rolling friction, μr is the coefficient of rolling friction, N is the normal force. This force acts when an object rolls over a surface.
Expression: fr = μrN

Name: Angle of Friction
Note: θ is the angle of friction, which is the angle between the resultant of the normal force and the limiting friction, and the normal force.
Expression: tan θ = μs

Name: Angle of Repose
Note: α is the angle of repose, which is the maximum angle of inclination of a plane at which a body placed on it just begins to slide down.
Expression: tan α = μs

Confusing static friction with limiting friction; static friction can be less than its maximum value.
Incorrectly applying the direction of friction (it opposes relative motion, not necessarily the applied force).
Assuming friction is always constant; static friction varies, kinetic friction is constant.
Believing friction depends on the area of contact (it is largely independent for kinetic friction).
Not drawing correct Free Body Diagrams (FBDs) to identify normal force and applied forces.

What is the primary characteristic of the force of friction?…
easy
Which of the following factors does NOT significantly affect the magnitude of kinetic friction between two solid surfaces?…
easy
A block of mass M is placed on a rough horizontal surface. A horizontal force F is applied to the block, but the block remains at rest. What is the magnitude of the static friction force acting on the block?…
medium