TYPES OF FORCES AND MOMENTS
The forces that are typically observed for many free body diagrams include:
- Applied forces and moments - these are externally applied forces and moments such as pushing, pulling, and applied rotation on the body. Pushing acts on the body (arrow towards body), pulling acts away from the body (arrow away from body), and applied moments are drawn in the direction of the resulting rotation.
- Gravity - if the object is falling, then its weight will create a force acting in the direction of the gravity acceleration. The arrow typically points in the direction of the gravity acceleration, away from the body. The action of the force is through the center of mass of the body.
- Surface forces - if the body is resting or moving along a surface, that surface will generate a force on the body. The force works on the body (arrow is towards the body) and it is perpendicular (normal) to the surface. The point of application on the body should be the contact point between the body and surface.
- Tension - if the body is connected to a surface or another body via a connection (rope, chain, string, rubber band) through which tension and compression can act, then it will generate a force. The force vector is typically drawn away from the body (arrow pointing away from the body) along or parallel to the connection. The point of application of the force is the point where the connection meets the body.
- Friction - as a body rolls or slides along a surface, there is a retarding force known as friction that must be accounted as a force. In some problems the friction is small and friction is neglected (equal to zero); it is recommended that the force vector be drawn but set to zero. The vector representing the friction should be drawn in the opposite direction to the expected motion of the body and parallel to the surface. For a body that is rolling, the friction is in the direction of the body’s motion, but it is opposite to the direction of the body where it is in contact with the surface, as seen in Fig. 2↓.
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