Contact Forces: Friction, Normal Reaction and Tension
Simple Explanation
Contact forces only act when two objects physically touch. The normal reaction is the push a surface exerts perpendicular to itself, supporting an object resting on it. Friction is a force that opposes relative sliding (or the tendency to slide) between two touching surfaces. Tension is the pulling force transmitted through a stretched string, rope, or cable.
Why Do We Need It?
Almost every everyday force you can feel β a floor pushing up on your feet, a rope holding a swing, friction letting you walk without slipping β is one of these three contact forces.
Formula
Friction Force
F(friction) = ΞΌN
The maximum friction force between two surfaces is proportional to how hard the surfaces are pressed together (the normal reaction force); the constant of proportionality depends only on the two materials in contact.
- F(friction)
- β the friction force, in newtons (N)
- ΞΌ
- β the coefficient of friction between the two surfaces (dimensionless β no units)
- N
- β the normal reaction force pressing the surfaces together, in newtons (N)
When to use it: Whenever the maximum friction force between two surfaces needs to be found, given the coefficient of friction and the normal force.
Worked Example
Find the maximum friction force on a crate
A crate rests on a floor, pressed down with a normal force of 300 N. The coefficient of friction between the crate and floor is 0.4. Find the maximum friction force before the crate starts to slide.
Why Does This Work?
The rougher two surfaces are, and the harder they are pressed together, the more their microscopic irregularities interlock and resist sliding β which is exactly why friction scales with both the material roughness (ΞΌ) and the normal force pressing them together.
Real-Life Example
Why icy roads are dangerous
A car that stops safely on dry tarmac can skid dangerously on an icy road.
Ice has a much lower coefficient of friction than dry tarmac, so the same tyres provide far less friction force to slow the car down β this is exactly why braking distances increase so much on ice.
Practice
A box is pressed onto a surface with a normal force of 150 N. The coefficient of friction is 0.25. Find the maximum friction force.
MediumCommon mistake
Assuming friction always equals the coefficient times the object's weight β friction depends on the NORMAL force pressing the surfaces together, which only equals weight on a flat, horizontal surface with no other vertical forces.
Quick Review
- Normal reaction: perpendicular push from a surface supporting an object.
- Friction: F(friction) = ΞΌN, opposes relative sliding.
- Tension: the pulling force transmitted through a stretched rope or string.