The ambulance pictured in Get Focused is speeding to an emergency. It has multiple forces acting upon it at the same time:
All of these forces act to determine how the ambulance will accelerate according to Newton's second law. The acceleration, in turn, describes how the velocity of the ambulance will change in magnitude and direction. In order to simplify and understand how these forces act together, you will need an analytical tool in the form of a free-body diagram. What is a free-body diagram?
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With practice and careful attention to detail, making a free-body diagram is an easy process; and it will greatly improve your ability to understand complex physical situations.
To get a better picture of how free-body diagrams are done correctly, read pages 129 to 131 of your textbook starting at “Representing Forces Using Free-Body Diagrams.” This will prepare you for the following lab activity.