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DAY 19 LETS GO! THE END OF THE QUARTER Topic 3 Forces continues Exam next class END OF QUARTER EXAM Purpose: A summary of the class since the last exam Major ideas from the ungraded work, HW, labs, demos Occurring: next class Topics: All on website: Videos as well I LOVE THIS QUESTION (because…) All that matters is the time in air. Which direction controls time in air? Y! So which shot has less Y velocity? The one to B, because it did not go as high. REMEMBER Remind yourself of each of Newton's Laws AND what they mean. LETS MAKE SURE…DEMO IT INERTIA MEET THE FORCES The force of EARTH pulling: MEET THE FORCES The force of the SURFACE pushing: MEET THE FORCES The force of the SURFACE pushing opposite motion: Coefficents: STATIC KINETIC Would the normal force change.. -If the mass of the block changed? If you pulled right as well? ELEVATORS In an elevator, you feel lighter/heavier in your knees at the start/end of the elevator ride. And it depends if you go DOWN or UP…. LETS TRY Elevator velocity: going UP at a constant LETS TRY Elevator going UP and speeding up LETS TRY Elevator going UP and slowing down LETS TRY Elevator velocity going DOWN at constant LETS TRY Elevator up going DOWN and speeding LETS TRY Elevator down going DOWN and slowing 6) An elevator lifts a panda up at an upwards acceleration of a. a) Which is larger then, the up force of the elevator cable or the weight? 6) An elevator lifts a panda up at an upwards acceleration of a. c) Draw a FBD of the panda, call the elevator force the normal force, the panda's mass M. 6) An elevator lifts a panda up at an upwards acceleration of a. d) Create a net force equation for the panda's situation. e) If the elevator had 600N of force up, how much mass was the panda? THE EXAMPLE I DO A force of 20N is needed to overcome the frictional force of 5N and accelerate a 3kg mass across the floor. What is its acceleration? YOU TRY… A 10kg mass is moving to the right at 6m/s, and you are pulling it right with a force of 20N. Friction is present. A) If you want the mass to come to a stop in 12 seconds, what acceleration would it have? YOU TRY… A 10kg mass is moving to the right at 6m/s, and you are pulling it right with a force of 20N. Friction is present. B) How much friction is required to stop it after 12 seconds even though you are pulling it. MY EXAMPLE A block of 2kg is moving at a velocity of 3m/s. The coefficient of kinetic friction is .2. If it is moving left, and you push right with a force of 6N, what is the acceleration of the object? EXAMPLE A 7kg block is on a rough surface, and is being pulled at an angle. 1) Draw a FBD with X and Y split up EXAMPLE A 7kg block is on a rough surface, and is being pulled at an angle. 2) Is the normal force more, less or equal to the weight? EXAMPLE A 7kg block is on a rough surface, and is being pulled at an angle. 3) If it was moving right, could the net force be left? FORCES AT ANGLES AGAIN A force on a surface will mean that the angled force will have an effect on the normal force. You use Sin and Cos to find the X and Y parts of the force If there are other forces in the Y direction, then the normal force does NOT equal the weight LETS DO ONE You pull a cat again at an angle. There is friction opposing your pull. 1) Draw a FBD and then another with the X and Y components Pull in Y Ff Fn Pull Y Force of person on the cat vertically Pull Force person the Pull:X: Force of of person onon the catcat horizontally Weight: Force of Earth on the cat Fn: Force of ground on cat Ff: Force of ground on cat Pull Cat Pull in X Weight Pull Y Force of person on the cat vertically Pull X: Force of person on the cat horizontally QUESTIONS 1) Which is larger, the normal force or the weight? Fn Pull in Y The weight, since the cat is in equilibrium in Y. And theFf weight is balanced by the normal force AND the Y pull. Weight: Force of Earth on the cat Fn: Force of ground on cat Ff: Force of ground on cat Cat Pull in X Weight Pull Y Force of person on the cat vertically Pull X: Force of person on the cat horizontally QUESTIONS Weight: Force of Earth on the cat 2) If you pull at a larger angle, will the weight Fn: Force of ground on cat or the normal force change? Fn Ff: Force of ground Pull in Y The weight will NOT, since it’s the Earths pull on theFf cat. The normal force WILL, since you are still in equilibrium in Y, your larger pull in Y will decrease the normal force needed from the table. In other words… on cat Cat Pull in X Weight Pull Y Force of person on the cat vertically Pull X: Force of person on the cat horizontally QUESTIONS Weight: Force of Earth on the cat 3) If the cat was moving at a constant velocity,Fn: Force of ground on cat would increasing the angle change Fn Ff: Force of ground Pull in Y on cat the friction? Ff YES! Since friction depends Cat on normal force (Ff = µFnormal) then more of a Y pull will decrease the normal force, resulting in less friction In other words… Weight Pull in X Pull Y Force of person on the cat vertically Pull X: Force of person on the cat horizontally QUESTIONS 4) Say that the coefficient of friction is µk and pulled at angle θ. Lets create Fn 2nd Law equations for X and Y Pull in Y Weight: Force of Earth on the cat Fn: Force of ground on cat Ff: Force of ground on cat Ff Cat X: Lets say the acceleration will be with the pull (my guess) so +. There is an imbalance then. X: F1x – Ff = imbalance Weight Y: No imbalance, call down – Y: -Weight + FNormal = equilibrium Pull in X QUESTIONS X: F1x – Ff = imbalance X : F1Cosθ – µFnormal = ma Y: -Weight + FNormal = equilibrium Y: -mg + FNormal = 0 SUMMARY When changing the angle of a force on a surface, the normal force changes. Which changes the friction as well. A downward push results in MORE normal force, an upward push results in LESS normal force. Friction is the coefficient of friction times the normal force. Use static for non-moving, kinetic for moving