Force A force is a push or a pull A force is an interaction between objects; they are exerted by one thing onto another thing…it is not something that can be possessed Force is a vector quantity

Free Body Diagrams A Free Body Diagram (FBD) is a visual representation of all the forces acting on a single object FBDs are extremely powerful problem solving tools that bridge the gap between qualitative analysis and a quantitative mathematical representation

Drawing an FBD… A simple dot can be used to represent the object in question Identify the forces acting ON that object only Draw one arrow to represent each force acting ON the object All arrows should be drawn from the dot or the center of the object The arrows point in the direction of the force. The length of the arrow represents the relative magnitude of the force…ie longer arrows = larger force

Example 1 A block of wood resting on a desktop Normal Force FN Fg Weight/Force due to Gravity

Example 2 A block of wood moving with a constant velocity across a surface FN Fg Applied Force FA Friction f

Example 3 A block of wood accelerating to the right across the surface FN Fg FA f

Example 4 A block of wood accelerating to the right when pulled by a rope at an angle FN Fg Tension T f

Example 5 A tetherball while swinging at a constant speed around a pole Fg Tension T Notice there is nothing in the original description that says the tetherball is being hit. A diagram for the ball as it is being hit would show an additional force acting on the ball.

Try it… Draw a FBD and write the 2nd law equations for a lawn mower being pushed at a constant speed