1 / 22

Forces: Equilibrium Examples

Exam I. Physics 101: Lecture 02. Forces: Equilibrium Examples. Today’s lecture will cover Textbook Sections 2.1-2.7. Phys 101 URL: http://online.physics.uiuc.edu/courses/phys101/spring10 Read the course description & FAQ! Office hours have started; see web page for locations & times.

fedor
Télécharger la présentation

Forces: Equilibrium Examples

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Exam I Physics 101: Lecture 02 Forces: Equilibrium Examples • Today’s lecture will cover Textbook Sections 2.1-2.7 Phys 101 URL: http://online.physics.uiuc.edu/courses/phys101/spring10 Read the course description & FAQ! Office hours have started; see web page for locations & times.

  2. The international Genetically Engineered Machines (iGEM) competition is the Olympics of undergraduate scientific research. Universities from around the world assemble teams of undergraduates to pursue a project in areas such as bioremediation, energyconservation, health and medicine, and more. The University of Illinois is looking for representatives from its undergraduate scientific community to create and design, and execute a project in Synthetic Biology to compete in the 2010 iGEM competition at the Massachusetts Institute of Technology in November. General Meetings: Tuesday January 26, 2010 - 161 Noyes @ 6PM Wednesday January 27, 2010 - 161 Noyes @ 6PM ***Food will be served!!!*** iGEM: http://www.igem.org Application: http://openwetware.org/wiki/IllinoisSyntheticBio/Application Contact Us: IllinoisiGEM@gmail.com

  3. Phi Delta Epsilon Spring 2010 Recruitment Dates Information Sessions January 26th at 7 P.M in 217 Noyes Laboratory January 27th at 7 P.M in 217 Noyes Laboratory Prospective Member Meet & Greet January 28th at 7 P.M in 112 Chem Annex February 3rd at 7 P.M in 112 Chem Annex February 4th at 7 P.M in 112 Chem Annex Application Due Date Friday, February 5th at 4 P.M in RSO Cubicle #5 in the RSO Complex Interview Dates & Locations Regularly Scheduled Interviews: Sunday, February 7th from 10 A.M-2 P.M in the 3rd floor of David Kinley Hall Conflict Interviews: Tuesday, February 9th at 9 P.M in 3rd floor of DKH

  4. Phi Delta Epsilon Pre-Med Student? Join U of I’s only Medical Fraternity! Why? PhiDE Members have an 93% acceptance rate into medical school! Info Nights: January 26th at 7 P.M in 217 Noyes Laboratory January 27th at 7 P.M in 217 Noyes Laboratory Contact: phidemebership@gmail.com Website: https://netfiles.uiuc.edu/ro/www/PhiDeltaEpsilon/

  5. SAPHA: South Asian Pre-Health Association-General Body Meeting Who: Pre-Health Students (Pre-Med, Pre-Dent, Pre-Pharm , etc.) When: Thursday January 28, 2010 at 7:00 P.M. Where: Mechanical Engineering Building Room 253 Why: Career Center's Pre-Health advisor, Andrea Wynn, is coming to speak to discuss beneficial pre-health opportunities around campus as well as information about testing for health-related graduate schools.  Afterwards, join us for a meet-and-greet at Cocomero! Meet new people and have a great time!

  6. Overview • Last Week • Newton’s Laws of Motion • Inertia • SF=ma • Pairs • Free Body Diagrams • Draw coordinate axis, each direction is independent. • Simple Picture • Identify/draw all forces • Friction: kinetic f = mkN; static f≤ms N • Gravity W = m g • Today • Contact Force---Springs • Contact Force---Tension • 2-D Examples

  7. Normal friction hand Gravity y x Free Body Diagrams • Choose Object (book) • Label coordinate axis • Identify All Forces • Hand (to right) • Gravity (down) • Normal (table, up) • Friction (table, left) Physics

  8. y x Book Pushed Across Table Constant Speed => SF=0 • Calculate force of hand to keep the book sliding at a constant speed, if the mass of the book is 1 Kg, ms = .84 and mk=.75. x-direction: SF=0 Fhand-Ffriction = 0 Fhand=Ffriction Fhand=mk FNormal Combine: Fhand=mk FNormal Fhand=0.75 x 9.8 N Fhand=7.3 Newtons y-direction: SF=0 FNormal-FGravity = 0 FNormal = FGravity FNormal=1x 9.8=9.8 N Normal friction hand Physics Gravity

  9. F1 F2 Fgravity y x Contact Force: Springs • Force exerted by a spring is directly proportional to its displacement (stretched or compressed). Fspring= -k x • Example:When a 5 kg mass is suspended from a spring, the spring stretches 8 cm. If it is hung by two identical springs, they will stretch A) 4 cm B) 8 cm C) 16 cm F1 +F2 - Fgravity = 0 F1 +F2 = Fgravity k1x1 + k2x2= m g 2 k x = mg x = m g / (2k) = (½ m) g / k We know mg/k = 8 cm. So: ½ mg /k = 4 cm.

  10. y x Contact Force: Springs • Force exerted by a spring is directly proportional to its displacement (stretched or compressed). Fspring= -k x • Example:When a 5 Kg mass is suspended from a spring, the spring stretches 8 cm. If it is hung by two identical springs, they will stretch A) 4 cm B) 8 cm C) 16 cm

  11. Contact Force: Tension • Tension in an Ideal String: • Magnitude of tension is equal everywhere. • Direction is parallel to string (only pulls) • Example : Determine force applied to string to suspend 45 kg mass hanging over pulley: Answer: • FBD • SF = ma From Prelecture F = mg = 440 Newtons

  12. T T 1 m1g 2 m2g Pulley ACT • Two boxes are connected by a string over a frictionless pulley. In equilibrium, box 2 is lower than box 1. Compare the weight of the two boxes. A) Box 1 is heavier B) Box 2 is heavier C) They have the same weight 1 SF = m a 1) T - m1 g = 0 2) T – m2 g = 0 => m1 = m2 2

  13. T T W y x Tension Example: • Determine the force exerted by the hand to suspend the 45 kg mass as shown in the picture. A) 220 N B) 440 N C) 660 N D) 880 N E) 1100 N SF = m a T + T – W = 0 2 T = W T = m g / 2 = (45kgx (9.8m/s2)/ 2 =220 N Remember the magnitude of the tension is the same everywhere along the rope!

  14. Fc T y x Tension ACT II • Determine the force exerted by the ceiling to suspend pulley holding the 45 kg mass as shown in the picture. A) 220 N B) 440 N C) 660 N D) 880 N E) 1100 N SF = m a Fc -T - T – T = 0 Fc = 3 T Fc = 3 x 220 N = 660 N Remember the magnitude of the tension is the same everywhere along the rope!

  15. Springs Preflight What does scale 1 read? (89% got correct!) A) 225 N B) 550 N C) 1100 N The net force of this is equal to 0. This means that “Ft-Fg=0.”; Ft=Fg. Since Fg is equal to 550N, Ft is equal to 550N,also. The magnitude of force of tension should be same everywhere. Therefore, the force of tension in the string is 550N. 1

  16. Springs ACT • Scale 1 reads 550 Newtons. What is the reading on scale 2? A) 225 N B) 550 N C) 1100 N In both cases the NET FORCE on the spring is zero, and the force to the right is 550N. Therefore the force to the left is also 550 N. 1 2

  17. N T y x W Forces in 2 Dimensions: Ramp • Calculate tension in the rope necessary to keep the 5 kg block from sliding down a frictionless incline of 20 degrees. 1) Draw FBD 2) Label Axis Note, weight is not in x or y direction! Need to decompose it! q

  18. N T y x W W Forces in 2 Dimensions: Ramp • Calculate force necessary to keep the 5 kg block from sliding down a frictionless incline of 20 degrees. W sin q q W cos q q

  19. N T y x W Forces in 2 Dimensions: Ramp • Calculate force necessary to keep the 5 kg block from sliding down a frictionless incline of 20 degrees. x- direction W sinq – T = 0 T = W sinq = m g sinq = 16.8Newtons q

  20. W sin q N T q W cos q y x W W Normal Force ACT What is the normal force of ramp on block? A) FN > mg B) FN = mg C) FN < mg In “y” direction SF = ma N – W cos q = 0 N = W cos q N = m g cos q q

  21. Normal Pushing q q Friction y Weight x Force at Angle Example • A person is pushing a 15 kg block across a floor with mk= 0.4 at a constant speed. If she is pushing down at an angle of 25 degrees, what is the magnitude of her force on the block? x- direction: SFx = max Fpush cos(q) – Ffriction = 0 Fpush cos(q) – m FNormal = 0 FNormal = Fpush cos(q) / m Combine: (Fpush cos(q) / m)–mg – FPush sin(q) = 0 Fpush ( cos(q) / m - sin(q)) = mg Fpush = m g / ( cos(q)/m – sin(q)) y- direction: SFy = may FNormal –Fweight – FPush sin(q) = 0 FNormal –mg – FPush sin(q) = 0 Fpush = 80 N

  22. Summary • Contact Force: Spring • Can push or pull, force proportional to displacement • F = k x • Contact Force: Tension • Always Pulls, tension equal everywhere • Force parallel to string • Two Dimensional Examples • Choose coordinate system • Analyze each direction is independent

More Related