Types of forces: When answering the question, "Why do things move the way they do?" it is helpful to draw a diagram to show all the force vectors that act on an object. Take a moment to review the following types of forces: Gravity: The attractive force between two masses; the weight of an object is the gravitational force exerted on it by the planet on which the object is located. Normal force: The force exerted on an object pressed against a surface; the normal force is always perpendicular to the surface Tension: The force exerted on an object by a rope or other material suspending it Hint - unless you have a box resting on a surface and being pulled by a rope, it is unlikely that you would have BOTH tension and a normal force acting on an object. Friction: The resistive force against sliding between two surfaces in contact; the friction force is always parallel to the surface and opposite the direction of motion Vectors The graphical addition of two vectors can be done using the parallelogram method, as shown in Fig. 1. The same result can achieve by drawing the vectors "head-to-tail