Forces

• Normal


  FN(_, surface). The force that one object exerts on another by pushing on it. Ex: Someone pushes in a chair. Force is present whenever two objects (or surfaces) are touching. It is perpendicular to and away from to the surface exerting it.
• Tension


  FT(_, string). Force exerted by a string or cable pulling on the object. Ex: Pendulum hanging from string. Force is present whenever a string or cable is attached and it is taut. It is parallel to the string and away from the object being pulled.
• Spring (or elastic)


  Fspr(_, spring). The force exerted by a compressed or stretched spring. Ex: Someone bungee jumping. Force is present whenever a spring is stretched or compressed from its relaxed state. It is stretched when spring is pulled, and compressed when spring is pushed.
• Buoyant


  Fbuoy(_, fluid). The force a fluid exerts on the object in it. Ex: A boat floating in the water. Force is present whenever an object is partially or fully immersed in a fluid, and both are in the gravitational field. It is opposite the direction of the local gravitational field.
• Friction (static)


  Ffs(_, surface). The force a surface exerts on an object/surface to prevent sliding. Ex: A person going down a slide. Force is present whenever there is a non-zero normal force and when the objects would slide without it. It is parallel to the surfaces in contact, opposite to the direction it would be sliding.
• Friction (kinetic)


  Ffk(_, surface). The force that objects/surfaces exert on each other as they slide across each other. Ex: A person riding a bike. Force is present whenever there is a nonzero normal force and a nonzero coefficient of friction. It is parallel to the surfaces in contact and opposite to the relative motion of each object.
• Drag


  Fdrag(_, air). The force that air (or another gas) exerts to oppose the motion of an object moving relative to it. Ex: A person skydiving. Force is present whenever an object moves through the air. It is opposite the relative motion of the object and the air.
• Gravitational


  Fg(_, earth). The attractive force between all objects in the universe. Ex: A ball being dropped on the earth. This force is present whenever two objects have mass. It is towards the center of mass of the objects, down for the earth.
• Electric


  Felec(_, charge). The forces that objects exert on each other due to their electric charges. Ex: Lightning striking the earth. This force is present whenever the two objects have a non-zero net charge. It is along the center of the objects.
• Magnetic


  FB(_, magnet). The forces that magnetic poles exert on each other. Ex: A magnet being held to a compass. This force is present when magnets exert magnetic force on other magnets, and some metallic materials. The opposites attract, like repel.
• Applied


  Fa(_, object). The force when 1 object contacts another changing its motion or shape. Ex: Human applies force and pops a balloon. This force is present when an object touches another object and applies force on it.