9/13/2017. Friction, Springs and Scales. Mid term exams. Summary. Investigating friction. Physics 1010: Dr. Eleanor Hodby

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1 Day 6: Friction s Friction, s and Scales Physics 1010: Dr. Eleanor Hodby Reminders: Homework 3 due Monday, 10pm Regular office hours Th, Fri, Mon. Finish up/review lecture Tuesday Midterm 1 on Thursday - Hour eams in Duane G1B30 on Sept 21, Oct 19, Nov 16 (In class) - 40 multiple choice questions, worth 80 points. -There will be no early or late eams given and no make-up eams. - Eam will be closed book. Mid term eams - ONE 3 by 5 inch formula card. You can WRITE anything on it BY HAND. - Calculator. - Basic scientific calculator only. NO graphical calculators. If in doubt, check with me this week. - Calculator cannot connect to outside world. No calculators on cell phones or laptops allowed. - No sharing of calculators. - No spare calculators available - Your lowest midterm score will be dropped. - There will be no makeups and it is NOT possible for you to miss 2 midterms. - Eam grades and solutions will be posted after the eam on D2L. Midterm preparation Prepare by applying the principles we have learned practice. You CANNOT memorize answers to specific questions. Make a formula card now with the important equations. Go over homeworks, class clicker questions, questions in the book. - Not sure how to get the answer take it to the helproom. At least half of Tuesday s lecture will be a review lecture - Lots of clicker questions - Do your revision BEFORE Tuesday and treat Tuesday as a practice eam. No HW on eam week (week 4). Office hours will be rescheduled as review sessions in week 4. Upcoming office hours this week and net EH: pm Thursday 14 th (Helproom) IH: 12-2pm Friday 15 th (Lounge) IL: 11-1pm, Monday 18 th (Helproom) AJ: 2-4pm, Monday 18 th (Lounge) AJ: 2-4pm Tuesday 19 th (Lounge) IL: 1-3pm Wednesday 20 th (Helproom) IH, pm Wednesday 20 th (Lounge) EH: am Thursday 21 st (Helproom) NO office hours on Friday 22 nd, Monday 25 th Regular hours for HW3 Eam review Details about office hours and review sessions etc are always posted on the website. Summary Investigating friction Last time Gravity Net force Today All about force of friction - How big is it? - What causes it - How to reduce/increase it All about springs - How is spring force related to etension of spring - How to make a spring scale Block has a mass of 2.5 kg, It weighs appro. how many N? (How much force needed to lift it?) a. 2.5 b. 25 c. 1.5/2.5 d. (1.5/2.5) 9.8 Force sensor Block 1

2 Applied Force Force 9/13/2017 Investigating friction Block weight = 25N Block weight = 25N Friction between and. Predict graph of force which we must apply by pulling on sensor in order to move along at a constant speed prediction should include force from before starting to pull until is moving at constant speed across the. (Make guess as to specific value as well as shape.)? N 0 time -? N time Block weight = 25N Block weight = 25N F F slidef pull Sliding (kinetic) friction How big is the force of sliding friction? - Often calculated as μ*weight of moving object. - μ is the coefficient of sliding friction, describes roughness of surfaces - A good guess for μ, if both surfaces are reasonably smooth is But this is just an approimation..higher for rough surfaces and lower if smooth - Make sense.much easier to push say heavy bo across wooden floor than lift it. Using data from our previous eperiment, what is the value of μ for a wooden sliding along the lab bench? a) 0.05 b) 0.2 c) 0.4 d) 0.6 e) 0.8 Sliding (kinetic) friction Increase the mass of the to 5 kg, what pulling force (appro) is needed to keep moving at a constant speed? a. 0 N, b. 5N c. 10N d. 49N c. 100N It takes a pulling force of ~5N to keep the moving in a straight line across the at a constant speed of 0.2 m/s. Now I double the speed to a steady 0.4 m/s. What constant pulling force is required now? a. 0N b. less than 5N c. about 5N d. more than 5N 2

3 Microscopic details of static and sliding friction Atoms of same material (color) all hooked together by forces like tiny springs Viewed at atomic level surfaces are never perfectly smooth The atoms from the two surfaces catch and drag against each other producing a force that opposes motion called friction friction force on opposing motion Motion of Dragging surfaces across each other causes atoms to start vibrating= heat! Heat energy produced = work done = F friction distance moved. Eamples: -Rubbing hands together to keep warm - Rubbing 2 sticks to start a fire - Spinning tires on car Friction and heat Motion of Why is maimum static friction force greater than sliding friction force? When stationary, atoms at surfaces can get embedded and stuck more than when sliding. So maimum static friction force bigger than sliding friction force. What if you increase the weight of an object? Frictional force increases proportional to weight More force pushing surfaces together atoms at surface mesh together more more friction Size of frictional force also depends on material in each surface Increased weight force How does a lubricant affect friction? A lubricant is: A layer of slimy stuff between surfaces Doesn t stick to either surface, flows out of way of surface atoms Keeps surfaces apart e.g. oil, snow, water Reduces friction between the 2 surfaces Physics that might save your life Why are you supposed to pump your brakes when stopping on wet or icy road? a. to avoid brakes overheating and wearing out faster b. to keep tires rolling so do not skid and wear out tires c. to make brake lights flash on and off to get attention of drivers behind you. d. to keep tires rolling so will slow down more quickly 3

4 Physics that might save your life Rolling wheels Why are you supposed to pump your brakes when stopping on wet or icy road? a. to avoid brakes overheating and wearing out faster b. to keep tires rolling so do not skid and wear out tires c.to make brake lights flash on and off to get attention of drivers behind you. d. to keep tires rolling so will slow down more quickly Why does keeping tires rolling allow you to slow down more quickly? Seems counterintuitive. All about static and sliding friction. (rolling tires also enable you to steer handy, but not so interesting for this course) v rotation What is the velocity of the point of the wheel in contact with the ground? a. 0 b. c. d. something else There are 2 components to the velocity of the wheel at this point 1. V car forwards 2. V rotation backwards Add the 2 component vectors to get final velocity They have the same magnitude and eactly cancel! Point on wheel touching ground is instantaneously stationary! If not then skidding/spinning tires Back to wheels and friction Friction allows cars to speed up or slow down. - Car tires eert frictional force on road - Road eerts frictional force on tires and hence car which changes velocity - If rolling, tire in contact with road is stationary use static friction force - If skidding tire is moving relative to road use sliding friction force Ma static frictional force > sliding friction force. If wheel rolling and using static friction you can acc/decelerate more rapidly s another kind of force Speeding up: Force of tire on road Slowing down: Force of road on tire (and car) Drive engine rotates tire. Tire pushes backwards on road. Force of road on tire (and car) Braking tire rotation slows down Tire pushes forwards on road Force of tire on road Everything you need to know about springs spring Identical stretched spring (Hooke s Law) Hand holding spring at etension Hand pulling to right pulling to left In equilibrium: F hand = F spring F spring F hand From eperiment we know: F spring Etension (m) Force of spring (N) F spring = - k Minus sign: force opposes etension Vectors in opposite directions constant Postive number Units: N/m Pull down on spring with 1N of force measured by probe Force Probe stretches 0.07m How much would it stretch if pull down on spring with force of 0.5 N? a m b m c. 0.07m d. 0.12m e. 0.20m Investigating springs 4

5 Weight (N) 9/13/2017 Investigating springs Scales Attach 0.1 kg mass 0.1 kg mass stretches??? meters. How much would it stretch if we hang a 0.1 kg mass on the spring? a. same distance as for 1 N force b. ½ as far c. 2 times as far d. more than 2 times as far e. Less than ½ as far Scale relates to weight (N) k Scales (eg bathroom scales) are just calibrated springs. On scales, mass is stationary and in eqm. Net force = 0 force balances weight force In equilibrium: F net = 0 F net = k = 0 = k directly related to weight force,. So if you have a spring and measure the value of k (calibrate it), then you can hang any weight on it, and from can calculate the weight () and (if on earth) the mass (m) Eample I hang a 2 kg mass from a spring and it stretches 2cm. What is the spring constant (k) of the spring? (assume g = 10m/s 2 ) a) 10 N/m b) 100 N/m c) 1000N/m d) N/m e) None of these. : Zero etension Stretched spring k Eample I hang a 2 kg mass from a spring and it stretches 2cm. This implies that k = 1000N/m. I remove the first mass and hang a different, unknown mass off the same spring. It stretches 8cm from its natural length. What is the second mass in kg? a) 2 kg b) 2/3 kg c) 4kg d) 8kg e) Not enough information given : Zero etension Stretched spring k More spring questions More spring questions X? Now hang 0.2 kg mass (2N of force) off 2 different springs. They are both initially the same length but one is made of thick stiff wire and the other is made of thin bendy wire. Which spring will stretch more? a. They will stretch the same distance b. Thick wire spring stretches less c. Thick wire spring stretches more X X? k What about squashing the spring? When I hang a 0.2 kg mass on the spring it stretches an amount hang How much would it compress if we placed a 0.2 kg mass on top of it? a. squash smaller than hang b. They are about the same c. squash bigger than hang d. I still haven t figured out the buttons on my clicker. 5

6 s in ropes what s good choice of rope For climbing, best to use a. rope with soft spring (lots of stretch) b. rope with stiff spring (not much stretch) c. doesn t matter any spring is good. Important note about vectors in diagrams and equations In diagrams: Always define ve direction Arrow represents direction (sign of vector) Letter is the MAGNITUDE so always represents a POSITIVE number F spring F hand In equilibirum: Net force = F hand F spring = 0 F hand = F spring In equations: Arrow in diagram relates to sign infront of letter in equation (±) Letter represents a positive number 6