Physics 111. Tuesday, September 21, 2004

Transcription

1 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

2 Announcements Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in CCLIR 468

3 Announcements Don t forget to read over the lab write-up and be ready for the quiz.

4 Announcements Exam 1 Distribution µ = 70.1 σ = Number Score

5 Ch 5: Newton s Laws 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.

6 Ch 5: Newton s Laws Let s start with the very simple example of the block on the desk top. Pictorial Representation: Free-Body diagrams - block

7 Ch 5: Newton s Laws Identify the system of interest by circling it. Pictorial Representation: The block! Free-Body diagrams - block

8 Ch 5: Newton s Laws Everything else in the picture is in the surrounding environment. Pictorial Representation: Free-Body diagrams - block

9 Ch 5: Newton s Laws Identify points at which the environment comes into contact with the system. Pictorial Representation: The desk top touches the block. Free-Body diagrams - block

10 Ch 5: Newton s Laws Name the force at the points of contact. Pictorial Representation: Normal force of desk on block. Free-Body diagrams - block

11 Ch 5: Newton s Laws Identify any long-range forces acting on the system. Pictorial Representation: The Earth s Gravity on the block (a.k.a. Weight )! Free-Body diagrams - block

12 Ch 5: Newton s Laws Draw a separate picture with ONLY the system. Add arrows for each force indicating the direction of the force and relative magnitude. ical Representation: The Free- Body Diagram N desk on block W Earth on block Free-Body diagrams - block

13 Ch 5: Newton s Laws Draw a Pictorial Representation for the following situation: A car is being towed up a hill in San Francisco. Class Worksheet #1a: Pict. Hill W T Tension in Rope on Car Weight - force of gravity of Earth on Car N Normal Force of Road on Car f Friction of Road on Car

14 Ch 5: Newton s Laws Now draw the Free-Body Diagram. A car is being towed up a hill in San Francisco. Class Worksheet #1b: FBD Hill T rope on car W Earth on car N road on car f road on car

15 Announcements Last time we identified a number of forces. What effects do these forces have? #1 - Pull object - keep force constant (student volunteer) Class Worksheet #2: student W2: Force acceleration - defined

16 Ch 4: Newton s Laws The harder he pulls, the greater the reading on our force meter, the more rapidly the block accelerates. Force Meter Frictionless pond of ice

17 Ch 4: Newton s Laws We mark on our force meter the readings corresponding to accelerations of 1 m/s 2, 2 m/s 2, 3 m/s 2 Force Meter Frictionless pond of ice

18 Ch 4: Newton s Laws 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. Force Meter Frictionless pond of ice

19 Ch 4: Newton s Laws We now replace our green block with a red block. We take data observing the acceleration and the reading on the force meter... Force Meter Frictionless pond of ice

20 Ch 4: Newton s Laws Applied Acceleration Acceleration Force of Green Block of Red Block (m/s/s) (m/s/s) Sketch Acceleration versus Force. Class Worksheet #3: Graph

21 Ch 4: Newton s Laws acceleration (m/s 2 ) Acceleration vs Force a = F 1kg a = F 2kg Force (N)

22 Ch 4: Newton s Laws 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

23 Ch 4: Newton s Laws 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)

24 Ch 4: Newton s Laws F = m a [Force] = (kg) (m/s 2 ) = 1 Newton ( N for short)

25 Announcements We saw earlier that the application of a force leads to acceleration #2 - What happens to object after application of force ends?

26 Ch 5: Newton s Laws 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?

27 Ch 4: Newton s Laws We leave the room and return 10 minutes later. Where will the keys be?

28 Ch 4: Newton s Laws We leave again and return 10 minutes later. Where will the keys be?

29 Ch 4: Newton s Laws Unless there s SOME outside force that acts upon the keys, they re not going anywhere!!

30 Ch 4: Newton s Laws 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?

31 Ch 4: Newton s Laws Nothing! We keep going...

32 Ch 4: Newton s Laws and going...

33 Ch 4: Newton s Laws and going... Nothing outlasts ics Students! They keep going and going and going...

34 Ch 5: Newton s Laws Use a Pictorial Representation and a Free-Body Diagrams to help you answer the following: 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

35 Ch 5: Newton s Laws Use a Pictorial Representation and a Free-Body Diagrams to help you answer the following: 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