In a laboratory experiment on friction, a $135$-N block resting on a rough horizontal table is pulled by a horizontal wire. The pull gradually increases until the block begins to move and continues to increase thereafter. Figure E$5.26$ shows a graph of the friction force on this block as a function of the pull. Identify the regions of the graph where static friction and kinetic friction occur.

Verified step by step guidance

1. Examine the graph provided. The x-axis represents the applied force (F) in Newtons, and the y-axis represents the frictional force (f) in Newtons. The graph shows a red line that increases linearly, peaks, and then becomes constant.
2. Identify the region of static friction. Static friction occurs when the block is stationary and resists the applied force. In the graph, this is the region where the frictional force increases linearly with the applied force, up to the peak at approximately 40 N.
3. Determine the transition point. The peak of the graph (around 40 N) represents the maximum static friction force, also known as the limiting friction. Beyond this point, the block begins to move, and static friction transitions to kinetic friction.
4. Identify the region of kinetic friction. Kinetic friction occurs once the block is in motion. In the graph, this is the region where the frictional force becomes constant (approximately 20 N) after the peak.
5. Summarize the behavior. Static friction resists motion and increases with the applied force until the maximum static friction is reached. Once the block starts moving, kinetic friction takes over, which is typically lower and remains constant regardless of the applied force.

Key Concepts

Static Friction

Static friction is the force that resists the initiation of sliding motion between two surfaces in contact. It acts when an object is at rest and is being pulled but has not yet started to move. The maximum static friction force increases with the applied force until it reaches a threshold, beyond which the object begins to slide.

Kinetic Friction

Kinetic friction, also known as dynamic friction, is the force that opposes the motion of two surfaces sliding past each other. Once the object starts moving, the frictional force decreases to a constant value, which is typically lower than the maximum static friction. This force remains relatively constant regardless of the speed of the sliding object.

Friction Force Graph

A friction force graph plots the frictional force against the applied pull force. In the graph, the initial slope represents static friction, which increases until it reaches a peak. After the object starts moving, the graph levels off, indicating the constant kinetic friction force. Understanding this graph helps identify the transition point between static and kinetic friction.