Lesson Plan

Differentiate between force, net force, and dynamics

Explain that force is a vector with magnitude and direction, and combine forces acting along a line

Distinguish external forces from internal forces and identify the net external force on a system

Apply a sign convention to represent one-dimensional forces and read the sign of a result as a direction

Draw and label a free-body diagram showing the external forces acting on an object

4.1

"Force is the cause of motion, and motion draws our attention." "The study of motion is called kinematics, but kinematics describes only the way objects move—their velocity and their acceleration. Dynamics considers the forces that affect the motion of moving objects and systems." "When multiple forces act on an object, the forces combine. Adding together all of the forces acting on an object gives the total force, or net force. An external force is a force that acts on an object within the system from outside the system." "Thus, a horizontal force of –3 N means pushing with 3 N to the left." "Unknown values are taken to be in the positive direction, so if your calculation of a vertical force comes out as +5.2 N, then you know it is an upward force. A negative result means the force is downward." "A free-body diagram represents the object being acted upon—that is, the free body—as a single point. Only the forces acting on the body (that is, external forces) are shown and are represented by vectors (which are drawn as arrows)." "Notice that the two arrows have equal lengths in the figure, which means that the forces of tension and weight are of equal magnitude. Because these forces of equal magnitude act in opposite directions, they are perfectly balanced, so they add together to give a net force of zero."