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
Occurs when an object is pulled by a rope, string, or chain
Occurs when two surfaces are trying to slide (static) or sliding (kinetic)
Occurs when an object is in contact with a surface
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.
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
How much force is needed to pull a spring with a spring constant of 10 N/m a distance of 20 m?
Since we’re looking for the force required to pull the spring apart, we don’t need the minus sign!
F = 10 N/mᆞ20m
Answer: 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.
Equation: Ff = μ*n
Want more practice with Friction? Check out this FRQ
from the 2017 AP Physics 1 exam.