The use of springs when modeling complicated real-world structures is a useful tool. Where, k is the spring constant, L is the length after deflection, and L o is the length before deflection. Springs exert contact forces in a mechanical device. Frictional forces, however, can be neglected if the surface is smooth. A contact force can usually be split into two components: a normal force perpendicular to the plane of contact and a frictional force parallel to the plane of contact. Unless stated otherwise, we will always assume that ropes and cables are straight and their force is constant along the length.Ī pulley serves to change the direction of the force of a rope/cable, but the tension will be the same on both sides if we assume a mass-less pulley (not true in dynamic problems).Ĭontact forces result from contact between the surfaces of two objects. Ropes and cables exert tension forces that only pull on an object and are therefore always represented as forces that are directed away from the object. Where m is the object's mass and g is the gravitational constant (g = 9.81 m/s 2 = 32.2 ft/s 2 on Earth's surface). The weight of an object in a gravitational field can be expressed The force of gravity is an important factor when significant masses are involved. They may also act across the object's surface or at any point in its body. On a rigid body, the force can be applied to any point on the object along the line of action of the force.įorces may be either internal (such as the force of a piston in an engine) or external (such as the force of a baseball bat against the ball). The vector sum of the external forces acting on an object in equilibrium is zero.Ī force acts through a line of action, which is a vector representation of a force's magnitude and direction. Therefore, an object in equilibrium is not experiencing any accelerations, so Newton's 2nd Law can be written as This may be done before or after making the diagram.Īn object is in equilibrium when each point inside the object has the same constant velocity (generally, velocity is zero for static's problems). However, it is best to choose a coordinate system that will simplify future calculations. The coordinate system is arbitrary when constructing a free-body diagrams.
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