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### Contact forces are those types of forces which result when the two interacting objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces.

Definition provided by Physics Classroom

Contact Forces:

A contact force is literally that, a Force that requires contact to Accelerate something. For example, if I were to push you, I'd need to contact you to accelerate your body in the push.

Reactionary Forces:

A term used to describe Forces in which the Magnitude of the force is in response to a force applied

Tension:

As tension is the Magnitude of a Force, it is measured in Newtons and is always measured parallel to the string on which it applies.

Normal:

The normal force is the support Forces exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the Weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other.

Static Friction:

Static friction is the Forces between two or more solid objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface. Coefficient of static friction, typically denoted as μs, is usually higher than the Coefficient of kinetic friction.

Kinetic Friction:

Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The Coefficient of kinetic friction is typically denoted as μk, and is usually less than the Coefficient of static friction for the same materials.

Coefficient of Friction:

The coefficient of friction, often symbolized by the Greek letter µ, is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction.

Hooke's Law:

Hooke's law of Elasticity is an approximation that states that the Extension of a spring is in direct proportion with the load applied to it. Many materials obey this law as long as the load does not exceed the material's elastic limit. Materials for which Hooke's law is a useful approximation are known as linear-elastic or "Hookean" materials.

Spring Constant:

The Forces in a spring is proportional to the stretch or compression of the spring. The spring constant is the proportionality constant.

Air Drag:

The air resistance is a special type of Frictional force that acts upon objects as they travel through the air. The Forces of air resistance is often observed to oppose the motion of an object.

Terminal Velocity:

As the object Accelerate (usually downwards due to the force of gravity), the drag Forces acting on the object increases, causing the Acceleration to decrease. At a particular Speed, the drag force produced will equal the object's Weight. At this point the object ceases to Accelerate altogether and continues falling at a constant speed called terminal velocity (also called settling velocity). An object moving downward with greater than terminal velocity (for example because it was thrown downwards or it fell from a thinner part of the atmosphere or it changed shape) will slow down until it reaches terminal velocity

Applied Forces :

An applied force is a Force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

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