Types of Forces and Force Analysis

• Forces are an interaction between two objects--you can't touch without being touched.
• There are many types of forces between objects that are differentiated by the way in which two objects interact.
• When two surfaces touch each other, forces perpendicular to the surfaces are called normal forces and forces parallel to the surfaces in contact are frictional.
• The Friction force that allows us to step forward or keeps car wheels from spinning can be called traction.
• When we touch things a combination of both normal and frictional forces are present.
• Extended or linked materials such as a string or chain exert tension forces on an object.
• When an object interacts with a fluid, such as water or air, propelling forces are called thrust, resistive forces are called drag, floating forces are called buoyant, and steering (or Bernoulli's) forces are called lift.
• When two objects interact without touching, they exert forces through a force field. Earth, for example, exerts a gravitational force on the Moon even though the Earth and Moon do not touch.
• Other non-contact forces include electric and magnetic forces.
• When we label forces, we want to indicate the type of interaction between the objects, what object the force is acting on and what object the force is by. Therefore, we will use the following notation: Fkind, on victim, by agent.

In the text, the author discusses the analysis of a log being pulled by a tractor. To do this, they first sketch a diagram of the system and its surroundings, and then enclose the system within a closed boundary line. They also shrink the system to a point at the center of the coordinate axes with one axis parallel to the direction of the log's motion. They then represent all relevant forces acting on the system with a labeled vector. Finally, they indicate which forces are equal in magnitude.

• In step 1, the author introduces the problem and presents the initial situation.
• In step 2, the author introduces the system and its boundaries.
• In step 3, the author introduces the object and its shape.
• In step 4, the author introduces the forces acting on the object.
• In step 5, the author presents the solution to the problem.