 Contact force

A contact force is an interaction transmitted between bodies through a region where their material surfaces meet. A support pushing on a resting object and friction acting along a sliding interface are examples. This macroscopic classification does not mean there is a new fundamental interaction of exactly zero range; the underlying material response involves electromagnetic and quantum interactions.

At a surface, resolve the force into a normal contribution and a tangential contribution:For a stipulated nonadhesive contact, $N\ge 0$ describes compression. In a dry static friction model, $\Vert {\mathbf{F}}_t\Vert \le {\mu }_{s}N$ is a bound, not a claim that friction always equals its maximum. The contact force adjusts to the force and moment balance while that bound and the other model constraints can be met.^[ref]

For an original inclined-plane example, a $2\mathrm{kg}$ block rests on a plane at $20^{\circ }$. With $g=9.81\mathrm{m}{\mathrm{s}}^{-2}$, the required normal and tangential forces areA chosen ${\mu }_s=0.40$ permits up to $7.38\mathrm{N}$, so this static balance is feasible in the stated model. The actual friction is $6.71\mathrm{N}$, not $7.38\mathrm{N}$. The resultant contact force balances gravitational weight, although its normal part alone does not.

Nonpenetration constraints describe a further feature of rigid contact. If the gap is $g_n\ge 0$, a simple unilateral model imposes $N\ge 0$ and $g_{n}N=0$: separated surfaces do not push on one another, while touching surfaces may carry a compressive force. Adhesion, finite deformation and fluid films require different or extended assumptions.

A force sensor between two bodies can measure the transmitted interaction while motion tracking tests the resultant-force balance. The equal and opposite contact force on the other body belongs on that other body's free-body diagram. Those two forces do not cancel within the force sum for just one chosen body.

Contact force refers to the force that acts between two objects that are in physical contact with each other. This can include a variety of types of forces that arise from interaction, such as: 1. **Frictional Force**: The force that opposes the relative motion or tendency of such motion of two surfaces in contact. It can be static (preventing motion) or kinetic (resisting sliding).