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# 2.3 Contact Forces

written by erin brzusek

published on march 25, 2020

Last updated on June 14, 2020

## Essential Knowledge 3.C.4 ⛓

Contact forces result from the interaction of one object touching another object, and they arise from interatomic electric forces. These forces include tension, friction, normal, spring (Physics 1), and buoyant (Physics 2).

Key Concept: Contact Forces - forces that occur when an object or system is in direct contact with another

 Force Description Tension Occurs when an object is pulled by a rope, string, or chain Friction Occurs when two surfaces are trying to slide (static) or sliding (kinetic) Normal Occurs when an object is in contact with a surface Spring Occurs when a spring or an elastic material is compressed or extended

When creating free-body diagrams, we must understand how to correctly illustrate the direction of these contact forces.

• Tension - The direction the rope, string, or chain is attached

• Friction - opposes the direction of motion

• Normal - Perpendicular to the surface

• Spring - Opposes the direction of the extension or compression Image Courtesy of studyphysics.ca

The image above shows the free-body diagram of an object on an inclined plane, Weight is always drawn directly downwards. Friction is drawn opposite the direction of motion, and normal force is perpendicular to the surface.

🎥Watch: AP Physics 1 - Introduction to Forces Part 2

Key Concept: Hooke’s Law - the amount of stretching or elongation of a string when a mass is attached to it is directly proportional to the applied weight.

Equation: F = -kx

• Where k is the spring constant in units of Newtons per Meter (N/m) and x is the stretching or elongation of the spring beyond its original length.

 Example Problem:How much force is needed to pull a spring with a spring constant of 10 N/m a distance of 20 m?Equation: F = kxSince we’re looking for the force required to pull the spring apart, we don’t need the minus sign!k = 10 N/mx = 20 mF = 10 N/mᆞ20mAnswer: The force needed is 200N

🎥Watch: AP Physics 1 - Unit 2 Streams

Key Concept: Friction: A key factor in understanding the setup of the equations surrounding Newton’s Laws is familiarizing oneself with friction. Friction acts as the force that opposes the motion or attempted motion of an object.

The equation for friction is given by Ff = μ*n where μ (Greek letter mu) is the coefficient of either static or kinetic friction. Friction is present if a problem mentions a “rough” surface, or specifically states the coefficient of static or kinetic friction. join more students

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Equation: Ff = μ*n

 FRQ PRACTICE:Want more practice with Friction? Check out this FRQ from the 2017 AP Physics 1 exam.