Make a quick list of the forces acting on you right now. How many are there? A)1 B)2 C)3-4 D)5-6 E)7+ 4
Free-Body Diagrams FBDs let you visualize how forces are interacting. Creating a free-body diagram (FBD): 1.Each FBD is for a single “thing” 2.Represent the object with a drawing or a dot. 3.Draw all the forces on the object as arrows. (don't draw anything else on the FBD) 4.Label each force with an appropriate name. 5.Larger forces → longer arrows. 6.For each force you should know: Object exerting it, object it acts on, what kind of force. 5 Draw the arrows outward from the center of the object.
FBDs – Do’s and Don’ts FBDs are not drawings of the situation, they won’t look like the full picture… they are idealized diagrams all about force. Before or after you draw the FBD, draw an arrow (off to the side) that represents the net force. Once your FBD is done you are ready to use Newton’s 2nd Law! 6
Consider the following situations for a ball: 1.Held at rest in your hand. 2.Falling downward after being dropped. 3.Moving upward just after being thrown upward. Which situation matches the force diagram shown? A)Only 1 B)Only 2 C)Only 3 D)Both 1 & 3 E)All three 7 Ignore Air Resistance
Sample Problem A boy throws a water balloon at and angle of 23° above horizontal and hits his friend who is 3.6 meters away. It is in the air for 1.1 seconds. How far did the balloon fall on its way? What was the horizontal speed as it hit the friend? Other Qs: At what speed was the ball thrown? At what angle did it hit the friend? 9
Circlular & Relative Motion Moving in a circle requires centripetal acceleration Relative motion 10
Quiz/Exam Four Question Types True or False: Choose T or F Multiple choice: Choose one, no work needed … but you should draw/calculate something Short answer: Answer in (readable) sentences and/or diagrams. Your explanation counts more than your answer. Problems: Traditional physics problems with occasional conceptual pieces. 11
Coming up… Right now → Quiz! Thursday (9/11) → 4.4 – 4.6 Warm-Up due Wednesday by 10:00 PM Homework #4 due Tuesday the 16 th Homework #5 due Saturday the 27 th Exam 1 is on Tuesday the 30 th 12