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Transcript
Welcome to Physics 1010: The Physics of Everyday Life www.colorado.edu/pjysics/phys1010 HW 3 is available on CULearn Next week: class will be covered by Rachel Pepper You are responsible for bringing a working clicker every day. Today • Skydiving (Net force) • Crashing Cars (Ouch) • Driving in snow (Friction) We have a bunch of unregistered clickers. Register clicker today! Confusing language Regular people: acceleration: speeding up. Physicists: acceleration: the rate of speeding up or slowing down or changing direction of motion. Regular people: kg: a weight= 2.2 pounds, Physicists: kg: a mass. On earth this mass has a weight of 1kg x 9.8 m/s2 = 9.8 N Regular people: Velocity: how fast you are going = speed. Physicists: Velocity: the speed and direction of motion. Terminal velocity A 100 kg man jumps from a plane. Immediately after he jumps, a. The force of gravity on the man is 100 kg downwards, the net force on him is 100kg downwards, and his acceleration is 9.8 m/s2 downwards. b. The force of gravity on the man is 100 kg downwards, his net force is 0, and his acceleration is 0 m/s2. c. The force of gravity on the man is 980 Newtons downwards, the net force on him is 980 Newtons downwards, and his acceleration is 9.8 m/s2 downwards. d. The force of gravity on the man is 980 Newtons, the net force on the man is 0 Newtons, and his velocity increases until it is 9.8 m/s downwards and then stops increasing. e. The force of gravity on the man is 980 Newtons, the net force on the man is 0 Newtons, and his acceleration is 9.8 m/s2 downwards. A 100 kg man jumps from a plane. Immediately after he jumps, c. The force of gravity on the man is 980 Newtons downwards, the net force on him is 980 Newtons downwards, and his acceleration is 9.8 m/s2 downwards. Fgravity on man Forces always measured in Newtons! 100 kg is a mass. Force of gravity on an object = (mass of object) x (9.8 m/s2) and is straight downwards. = 100 kg x 9.8 m/s2 = 980 Newtons Sum of all forces = Fnet In this case, Fnet = Fgravity on man Fnet = mass x acceleration Since Fnet is not zero, tells us man will accelerate! Acceleration is always in the same direction as the net force! 1 As man falls for some time, he finds that he is falling at a constant velocity. At this time… As man falls for some time, he finds that he is falling at a constant velocity. At this time… a. The force of gravity on the man is 100 kg downwards, the net force on him is 100kg downwards, and his acceleration is 9.8 m/s2 downwards. Fair on man e. The force of gravity on the man is 980 Newtons, the net force on the man is 0 Newtons, and his acceleration is 0 m/s2. b. The force of gravity on the man is 100 kg downwards, his net force is 0, and his acceleration is 0 m/s2. Force of gravity on an object (on Earth) c. The force of gravity on the man is 980 Newtons downwards, the net force on him is 980 Newtons downwards, and his acceleration is 9.8 m/s2 downwards. Fgravity on man = (mass of object) x (9.8 m/s2) = 100 kg x 9.8 m/s2 = 980 Newtons and is straight downwards. Sum of all forces = Fnet In this case, Fnet = Fgravity on man + Fair on man = 0 !!!! d. The force of gravity on the man is 0 Newtons, the net force on the man is 0 Newtons, and his acceleration is 0 m/s2. Fnet = mass x acceleration e. The force of gravity on the man is 980 Newtons, the net force on the man is 0 Newtons, and his acceleration is 0 m/s2. Since Fnet is zero, tells us man will not accelerate! Acceleration will be zero, Velocity will be constant ! Crashing little cars: Fair on man Fnet Fgravity on man Velocity Before reaching terminal velocity, forces don’t quite cancel. Still small acceleration. Case A: hit hard end of ramp time Case B: hit sponge at end of ramp Sketch your predictions of the velocity vs. time, acceleration vs time, net force, and measured force (by probe) for this motion. Starting when we let go and ending after crash. FOR CASE A AND B on same graph. Since Fnet is not zero, tells us man will accelerate! Acceleration will be smaller than 9.8 m/s2 because net force is smaller than force of gravity on man. Velocity will be changing ! Speeding up in downwards direction SPONGE Crashing little cars time time time time time time Velocity Velocity Net force time Meas Acceleration Force Velocity Net Force Meas. Acceleration Force time C Net force B Meas Acceleration Force Case A: hit hard end of ramp A Case B: hit sponge at end of ramp time time time D. Same as a, but with all curves on negative side of 0 instead of positive. E. None of these choices. time Crashing little cars: Case 1: hit hard end of ramp SPONGE Case 2: hit sponge at end of ramp Just before crash: Car in case 1 is at same velocity as Car in case 2 After crash: Both car in 1 and 2 are at rest. Velocity = 0 m/s Same in both Case 1 and 2 Acceleration = change in velocity during crash time elapsed during crash Shorter in Case 1 Than Case 2 time time So, during crash acceleration of 1 greater than acceleration of 2. Force in 1 greater … ouch! A Velocity Fnet = mass x acceleration Net Force In this case, Fnet = Fgravity on man + Fair on man time Meas. Acceleration Force Sum of all forces = Fnet Measure Net Force Acceleration Force SPONGE time time time time Wall exerts force on car. 2 Force sensor plus metal weight has mass of 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 9.8 Force sensor + metal weight PULL Positive direction Answer: b Force sensor + metal weight B time C Applied Force Applied Force ?N time Starts moving here Applied Force Friction 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.) Friction 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. Applied Force A PULL time -? N Force sensor + metal weight time Force sensor + metal weight PULL A PULL time Forces Forces Starts moving here Applied Force (by string) A Applied Force Starts moving here Friction Force Static friction Net force = 0 (velocity =0) time Friction Force Sliding friction Net force = 0 (const. velocity) Balancing 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 5 Increase 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. Static friction Sliding friction Answer 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! Wheels and friction. (and using physics to drive better) 1. Why do you stop faster in car on snow if don’t skid? 2. How to keep from getting stuck in snow. Answer is c. Larger mass, larger sliding friction force …. The heavier the box, the more force it takes to push it across the floor. Friction allows cars to speed up or slow down. Car tires exert force on road … road exerts force on tires. Drive … engine rotates tire. Tire pushes backwards on road. 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 fast enough that air resistance important. ) Force of road on tire (car) Force of tire on road Friction force- Always opposes change in motion. No motion, no friction. No change in speed, little friction. 3 Wheels and friction. (and using physics to drive better) 1. Why do you stop faster in car on snow if don’t skid? 2. How to keep from getting stuck in snow. Friction - sliding and static. (Books say static bigger, our experiments show not usually true. Not real law of physics like F= ma or conservation of energy where it always works. Is true on snow, probably more heating and melting if slide.) atoms of same color all hooked together by forces like tiny springs friction atoms (never perfectly smooth) block Drive … engine rotates tire. Tire pushes backwards on road. table Force of road on tire (car) Force of tire on road Friction force- Always opposes change in motion. No motion, no friction. No change in speed, little friction. many intro physics books give simple formulas describing this, but they are wrong. atoms block More force pushing surfaces together. Squash atoms together more, more friction. Also type of atom, amount of surface area, … block move friction force opposing motion table table As surface dragged across- start atoms vibrating= heat! Heat energy = work= Ffriction x distance moved. When not sliding, can get embedded and stuck more than when sliding. So static friction force bigger than sliding frict. force. desk leg floor 4 Wheels- no sliding surfaces across each other! So no work, no heating, no wasted energy. atoms tire Add lubricant slimy stuff that does not stick to either surface and flows out of the way of the atoms, but keeps surfaces apart. road 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 5