9/9/20101Welcome to Physics 1010: The Physics of Everyday Life www.colorado.edu/physics/phys1010Next week: guest host Rachel PepperTodayFriction – what’s up with that?How scales work- they use springs.How springs work- Hooke’s law, F =-kx. Springs in your life . Force sensor plus metal weight has massof 1.2 kg, It weighs approx. how many N? (How much force needed to lift it?)a. 1.2 b. 12 c. 1.2/2.2 d. (1.2/2.2) x 9.8PULL Force sensor + metal weightFriction between table and force sensor. Predict graph of force which we must apply by pulling on string in order to move sensor along table at a constant speed … prediction should include force from before starting to pull until sensor is moving at constant speed across the table. (Make guess as to specific value as well as shape.)AppliedForcetimePositive direction? N-? NAnswer: bPULL Force sensor + metal weightFriction between table and force sensor. Predict graph of force which we must apply by pulling on string in order to move sensor along table at a constant speed … prediction should include force from before starting to pull until sensor is moving at constant speed across the table. timeAppliedForcetimeAppliedForcetimeAppliedForceStarts movinghereABCPULL Force sensor + metal weighttimeForcesStarts movinghereAApplied ForceFriction ForceStatic frictionSliding frictionBalancing sliding friction: How much force is required to keep it moving along table at constant speed if weight of (force sensor + metal ) = mg = 12 N ?a. Weight x 0.3, b. Weight x 0.7, c. Weight x 1.5 d. Weight x 5Net force = 0 (const. velocity)Net force = 0 (velocity =0)PULL Force sensor + metal weighttimeForcesStarts movinghereAApplied Force (by string)Friction ForceStatic frictionSliding frictionAnswer is A. Sliding friction force =~ 1/3 of the weight. SO, applied force must be +Weight X 0.3. Makes sense that takes less force to slide box than to lift box!9/9/20102Increase the mass to 30 kg, what is force needed to keep moving at a constant speed?a. same as before, b. less than before, c. more than before.Answer is c. Larger mass, larger sliding friction force …. The heavier the box, the more force it takes to push it across the floor.Sliding friction force does not change much as speed increases. Takes about as much force to go at slow constant velocity as high constant velocity. (unless fastenough that air resistance important. )1. Why do you stop faster in car on snow if don’t skid?2. How to keep from getting stuck in snow. Friction allows cars to speed up or slow down.Car tires exert force on road … road exerts force on tires. Wheels and friction. (and using physics to drive better)Drive … engine rotates tire. Tire pushes backwards on road.Force of tire on roadForce of road on tire (car)1. Why do you stop faster in car on snow if don’t skid?2. How to keep from getting stuck in snow. Wheels and friction. (and using physics to drive better)Drive … engine rotates tire. Tire pushes backwards on road.Force of tire on roadForce of road on tire (car)Friction - sliding and static. tableblockfrictionatoms (never perfectly smooth)atoms of same color all hooked together by forces like tiny springstableblockatomsmovefriction forceopposing motiontableblockMore force pushing surfaces together.Squash atoms together more,more friction. Also type of atom, amountof surface area, …9/9/20103As surface dragged across- start atoms vibrating= heat!Heat energy = work= Ffrictionx distance moved.When not sliding, can get embedded and stuck more than when sliding. So staticfriction force bigger than slidingfrict. force.blocktableWheels- no sliding surfaces across each other! So no work,no heating, no wasted energy.tireroadDriving on normal dry pavement.Compare case of wheels rolling in usual fashion versuslocked up and skidding. Which case produces more heat and why?a. if rolling, causes more atoms to jiggle around more comparedto skidding, so more heat.b. if rolling will be going faster so rolling= more heat.c. If rolling the friction force will be more than when sliding, sorolling will produce more heat. d. if sliding, causes more atoms to be jiggling around more so more heat.e. if sliding friction force will be larger so will have more heatwhen sliding. Correct answer is d. if sliding, causes more atoms to be jiggling around more so more heat.atomsLubricantslimy stuff that does not stick to either surface and flows outof the way of the atoms, but keeps surfaces apart.Springs – another kind of force9/9/20104Force ProbeRelaxedSpringPull down on spring with 2N (0.45 lbs) of force … measured by probeSpring stretches ??? meters. How much would it stretch if pull down on spring with force of 6 N?a. same distance as for 2 N force b. 2 times as farc. Between 2 and 3 times as far d. 3 times as fare. More than 3 times as farHow much would it stretch if we hang a 0.2 kg mass on the spring?a. same distance as for 2 N force b. ½ as farc. 2 times as far d. more than 2 times as fare. Less than ½ as farAnswer is d. 3 times as far Answer is a. Same distance. Fgravity= 0.2 kg x 9.8 m/s2= 2 N Data says the larger the mass (or force down), the further the spring stretches, and these are proportional … 2 times the force, 2 times the stretch, … etc. Fgravitypulls down on massFgravity= weight = mg = 0.2 kg x 9.8 m/s2MassRelaxedSpringPull down on spring with 2N (0.45 lbs) of force … measured by probeFspringpulls up on massFspring, balances weightStretch distance = XMeans that Fspring= constant x distance stretched.Fspring= -kxMinus says that force is in opposite direction to stretch.The larger the stretch (x), the larger the force of the spring pulling up!Constant = k Is the “Spring Constant”Scales0LBS510MasskxmgIn equilibrium, Fnet = 0 Fnet = mg – kx = 0So, mg = kx When these spring force balances gravity force so that the net force is 0 and the mass is stationary, we say the mass is in equilibrium.So if you have a spring and measure the value of k, then you can hang any weight on it, and from x can calculate the weight = kx.Scale relatesx to weightRelaxedSpringNow hang 0.2 kg mass (2N of force) off shorter, fatter springHow will spring stretch change?a. Stretches same distance as before. b. Stretches further than before.c. Stretches less than before. Answer is c. Stretches less than before. Fspringmust still balance same weight, mg. Fspring= -kx But, k is larger for