Forces as Interactions (College Board AP® Physics 1: Algebra-Based)

Forces as interactions

• A force exerted on an object or system is always due to the interaction of that object with another object or system

  • A force is a push or pull

• Forces can affect an object's:

  • speed

  • direction

  • shape

  • size

Effects of forces on different objects

• The effects of forces on an object often depend on the type of force acting

  • The push force (thrust) of an engine can cause a car to speed up, whilst the force exerted by the brakes (friction) can cause it to slow down

  • The gravitational pull of the Sun on a comet causes the comet to change direction

  • When two opposing forces push on each end of a spring, the spring changes shape (it compresses)

• Forces can be described as contact or non-contact

Contact forces

• Contact forces describe the interaction of an object or system touching another object or system

• They are macroscopic effects of interatomic electric forces

  • A macroscopic effect is an effect observable by the naked eye, this means it is an effect that can be seen

Interatomic electric forces

• Electric or electrostatic forces occur between charged particles

• Like charges repel one another, and opposite charges attract one another

  • When a negatively charged particle gets close to a positively charged particle the positively charged particle exerts a pull force on the negatively charged particle (attraction)

  • When a negatively charged particle gets close to another negatively charged particle, the particles experience a push force from one another (repulsion)

Electrostatic forces acting on charged particles

• Atoms are composed of positively charged nuclei and negatively charged electrons

• When objects come into contact with each other, their atoms at the surface interact through electric forces

• These electric forces can be repulsive or attractive

  • When two atoms are close, their electrons will repel each other causing a strong electrostatic repulsive force

  • At slightly larger distances, attractive forces between atoms arising from temporary or permanent dipoles in atoms can also exist

Types of contact force

• Examples of contact forces include:

  • Normal (reaction) force

  • Tension

  • Kinetic and static friction

  • Thrust and upthrust

• Thrust

  • Thrust is a force produced by an engine that speeds up the motion of an object

  • The engine of a car exerts a thrust force and increases its speed

• Upthrust

  • When an object is fully or partially submerged in a fluid, the fluid exerts an upward-acting push force on the object

  • A boat floats on a lake due to the upthrust exerted by the water on the boat

  • A ball held underwater will shoot upwards when released due to the upthrust exerted by the water pushing it back to the surface 

Upthrust acting on a boat

Normal forces

• The normal force is the perpendicular component of the force exerted on an object by the surface with which it is in contact

  • The normal force is always perpendicular to the surface

  • Normal forces are directed away from the surface

• Normal forces arise from repulsive forces created by the repulsion of the electrons in atoms

  • The electrons in a book at rest on the surface of a table repel the electrons in the table keeping the book on the table from not falling through it

Repulsive forces between electrons

• When an object rests on a horizontal surface, the surface exerts a push force (normal force) vertically upwards on the object

  • The electrons in a football resting on a grass surface repel the electrons in the grass surface keeping the football from not falling through it

Normal force acting on a football

• When an object is traveling on a horizontal or inclined surface, the surface exerts a push force (normal force) perpendicular to the angle of the surface on the object

• The normal force is equal to the vertical weight component of the object when it is in contact with the surface i.e. not above or below the surface

Normal force on an inclined plane