Physics 111. Free-Body diagrams - block. Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in CCLIR 468.

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ics day, ember 21, 2004 Ch 5: Free-Body Diagrams Newton s Laws Newton s 2nd Law - force & acceleration Newton s 1st Law - Inertia Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in

1 ics day, ember 21, 2004 Ch 5: Free-Body Diagrams Newton s Laws Newton s 2nd Law - force & acceleration Newton s 1st Law - Inertia Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in CCLIR 468 Exam 1 Distribution µ = 70.1 σ = Number Don t forget to read over the lab write-up and be ready for the quiz Score Free-body Diagrams provide a useful methodology to understanding physics problems. The physics of a problem is contained in constructing the free-body-diagram. If you can master the construction of free-bodydiagrams, you will be able to solve some very difficult physics problems. Let s start with the very simple of the block on the desk top. 1

2 Identify the system of interest by circling it. Everything else in the picture is in the surrounding environment. The block! Identify points at which the environment comes into contact with the system. The desk top touches the block. Name the force at the points of contact. Normal force of desk on block. Identify any long-range forces acting on the system. The Earth s Gravity on the block (a.k.a. Weight )! Draw a separate picture with ONLY the system. Add arrows for each force indicating the direction of the force and relative magnitude. ical The Free- Body Diagram N desk on block W Earth on block 2

3 Draw a Representation for the following situation: A car is being towed up a hill in San Francisco. Class Worksheet #1a: Pict. Hill Now draw the Free-Body Diagram. A car is being towed up a hill in San Francisco. Class Worksheet #1b: FBD Hill Last time we identified a number of forces. What effects do these forces have? The harder he pulls, the greater the reading on our force meter, the more rapidly the block accelerates. #1 - Pull object - keep force constant (student volunteer) Class Worksheet #2: student W2: Force acceleration - defined We mark on our force meter the readings corresponding to accelerations of 1 m/s 2, 2 m/s 2, 3 m/s 2 If our green block is made to match an international standard object known as one kilogram, then the markings on our force meter are called Newtons. Defining a Newton 3

4 We now replace our green block with a red block. We take data observing the acceleration and the reading on the force meter... Applied Acceleration Acceleration Force of Green Block of Red Block (m/s/s) (m/s/s) Sketch Acceleration versus Force. Class Worksheet #3: Graph We observe: Acceleration is proportional to Force. We also observe that the proportionality constant seems to be a property of the object to which the force is applied. We denote the proportionality constant 1/m where m is the inertial mass, or mass for short, then we have (Newton s Second Law) a = F net m This equation may be one of only two that you remember after this course is over. You may be more familiar with this form: Net (Newton s Second Law) F = m a Last time we identified a number of forces. What effects do these forces have? [Force] = (kg) (m/s 2 ) = 1 Newton ( N for short) #2 - What happens to object after application of force ends? units of force 1st Law via demos 4

5 Law I: Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. What does this really mean? We leave the room and return 10 minutes later. Where will the keys be? We leave again and return 10 minutes later. Where will the keys be? Unless there s SOME outside force that acts upon the keys, they re not going anywhere!! nope, still there Use Representations and Free-Body Diagram to answer the following questions: We re in outer space far away from any planet, or galaxy, or star... Our spaceship is moving with constant velocity toward the distant planet GRADUATION... The engines are off! What happens to our velocity? Class Worksheet #4: up elevator An elevator in the Sears Tower goes up the shaft at a steady speed. 1) Tension > Weight 2) Tension = Weight 3) Tension < Weight Spaceship to graduation W4: Up elevator 5

6 Use Representations and Free-Body Diagram to answer the following questions: Class Worksheet #5: down elevator An elevator in the Empire State Building goes down the shaft at a steady speed. 1) Tension > Weight 2) Tension = Weight 3) Tension < Weight Let s look at a couple of demos involving blocks sliding across the floor. Again, use FBD s to help you understand the motion. Class Worksheet #6: Block Predictions Block across the floor at constant speed. Why is it not accelerating? Block across the floor after push. Why does it stop? Let s go back to a balled tossed vertically upwards. Once I let go, I m not touching it anymore. Why is it moving upwards after I let go? Newton s First Law - Inertia. Although gravity is the only force and it acts downward, the ball had initial velocity upward. 6

Physics 111. Tuesday, September 21, 2004

Physics 111. Tuesday, September 21, 2004 ics Tuesday, ember 21, 2004 Ch 5: Free-Body Diagrams Newton s Laws Newton s 2nd Law - force & acceleration Newton s 1st Law - Inertia Gravitational vs. Inertial Mass Announcements Help this week: Wednesday,