There are a few common forces that are often used in problem solving in physics:

Gravity: created between any 2 objects with mass. The more mass, the more gravitational force is present. The largest mass near us is the Earth, so we feel its gravity most strongly. The force of gravity on Earth always points down (toward the center of Earth).

Normal: The force created by a surface to oppose a force pushing on it. The word “normal” in math means “at a 90° angle to.” The normal force has its name because it always acts at a 90° angle to the surface that is creating it. The normal force always perfectly matches whatever net force is pushing the object into the surface.

Friction: The force created by a surface on an object that opposes the object’s motion. Friction always acts parallel to the surface and in the opposite direction of the object’s motion. Its strength depends on the material of the surface and the object itself. 2 types of friction: Static friction: prevents an object from moving. Balances out forces acting parallel to the surface.
Kinetic friction: acts against a moving object’s velocity. Always points in opposite direction of velocity.

Applied: The force of a push or pull from a person or another object.

Tension: The force created by the pull of a rope or string. Tension always pulls in the exact direction of the rope or string itself. Tension pulls with equal force and opposite direction on both ends of the rope.

Spring force: The force exerted on an object by a spring. The more the spring is stretched or compressed, the more force acts on the object. A spring does not apply any force if it is at its natural length (not stretched or compressed).

Air resistance: When an object is moving through air, the air applies a force in the opposite direction. For example, when you stick your hand out the window of a moving car, you can feel the force of air pushing against it. An object experiences more air resistance if it is moving faster or if it has more surface area. A falling object reaches “terminal velocity” or its maximum velocity when the force of air resistance up equals the force of gravity down.

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