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Agenda Warm Up Review HW Quiz 6 Friction Notes

Warm Up. AP Physics Monday 13.11.04 Standards: b2b Newton’s 2 nd Law Objective: SWBAT distinguish between the coefficient of static and kinetic friction & solve related problems. 30kg. Agenda Warm Up Review HW Quiz 6 Friction Notes. 30 °.

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Agenda Warm Up Review HW Quiz 6 Friction Notes

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  1. Warm Up AP PhysicsMonday 13.11.04Standards: b2b Newton’s 2nd LawObjective: SWBAT distinguish between the coefficient of static and kinetic friction & solve related problems. 30kg Agenda • Warm Up • Review HW • Quiz 6 • Friction Notes 30° Find the acceleration of the box down the slope if there is no friction? Homework #3g

  2. Warm Up 2. Which of the following diagrams best represents the gravitational force W. the frictional force f, and the normal force N that act on the block? AP PhysicsTuesday 13.11.05 Standards: b2b2 write down vector equation that results from Newton’s 2nd LawObjective: SWBAT Objective: SWBAT distinguish between the coefficient of static and kinetic friction & solve related problems. Agenda • Warm Up • Friction Notes • Friction Sample Problem • Friction Practice Homework #3g Homework #3g

  3. AP PhysicsWednesday 13.11.06Standards: b3a-d Systems of 2 or more objects including pulleys Objective: SWBAT be able to solve problems involving pulleys Warm Up Find the coefficient of kinetic friction of a 40 kg box that is accelerating at 1.5 m/s2under an applied Force of 8000 N. Agenda • Warm Up • N’s 3rd Law Notes • Pulleys Homework #3h

  4. Warm Up A Truck is pulling a boat. Draw a free body diagram with all of the Force pairs present. AP PhysicsThursday 13.11.07Standards: B2c analyze 2D situation with multiple forces acting on an objectObjective: SWBAT know how to solve pulley problems. Agenda • Warm Up • Pulley’s!!! Homework #3h

  5. Warm Up 11. A block of mass 3m can move without friction on a horizontal table. This block is attached to another block ofmassm by a cord that passes over a frictionless pulley, as shown above. If the masses of the cord and the pulley are negligible, what is the magnitude of the acceleration of the descending block? (A) Zero (B) g/4 (C) g/3 (D) 2g/3 (E) g 54 AP PhysicsFriday 13.11.08Standards: B2 Objective: SWBAT find the terminal velocity of an object that undergoes an air resistance of F=Bv Agenda • Warm Up • Review Homework • AP Free Response Problems Homework #3I Problems

  6. Friction Static Friction – Force of Friction on objects not sliding. Including car tires. This force must be overcome for any object to move. fs=μsN or Ffs=μsFN Static Friction is greater than kinetic friction. μs Tires cannot overcome static friction. If they could then the tires would skid. Tires only skid when brakes are slammed. The static friction makes the tire roll. Kinetic Friction, or Friction of sliding objects. . fk=μkNor Ffk=μkFn It is smaller than static friction and applied to an object only after it starts moving. μk

  7. Friction Practice Problem Mr. A pushes a 30kg box with a force 50 N but it does not move. He then pushes the box harder wit a force of 120N causing the box to move with a constant velocity. What is μs of the surface? Once it is moving the 120N force accelerates at 0.33 m/s2. What is the μk of the surface?

  8. Free Body Diagram Practice #3b For each of the following problems create a free body diagram. Where asked, answer questions. • A piano falls from the sky and air makes it reach terminal velocity • A cart from an amusement park ride is accelerated up at an angle θ. Assume friction is negligible, but air resistance is significant. • The cart from problem 2 drops at a very steep angle of ϕ. • A 70kg rock falls from the cliff and the air provides an upwards force of 30N. • A 65kg skydiver opens her parachute. Air resistance to her body is 20 N and the Lift Force is 617N. What is the Net Force acting on her? • A 25kg child rolls down a hill angled 60° to the horizontal. Friction is significant. • A 15kg chandelier is attached to the ceiling using a cable. • A 25kg child is hanging stationary from monkey bars. One arm is angled at 30° and the other arm is angled at 60°. • **A cart is sitting on a table and attached to a pulley by a string. The other side of the pulley has a mass M attached to the bottom. Both objects are stationary. Find the free body diagram of the cart and of the mass separately.

  9. What you have to remember is… mgsinθ Ff θ FN Fnet||=mgsinθ-Ff mgcosθ Fnet =FN- mgcosθ θ W=mg mgsinθ

  10. Free Response Question #3f • B2007B1. An empty sled of mass 25 kg slides down a muddy hill with a constant speed of 2.4 m/s. The slope of the hill is inclined at an angle of 15° with the horizontal as shown in the figure above. • a. Calculate the time it takes the sled to go 21 m down the slope. • b. On the dot below that represents the sled, draw/label a free-body diagram for the sled as it slides down the slope • c. Calculate the frictional force on the sled as it slides down the slope. • d. Calculate the coefficient of friction between the sled and the muddy surface of the slope. • e. The sled reaches the bottom of the slope and continues on the horizontal ground. Assume the same coefficient of friction. • i. In terms of velocity and acceleration, describe the motion of the sled as it travels on the horizontal ground. • ii. On the axes below, sketch a graph of speed v versus time t for the sled. Include both the sled's travel down the slope and across the horizontal ground. Clearly indicate with the symbol t the time at which the sled leaves the slope.

  11. Friction Problems #3g • The largest flowers in the world are the Rafflesiaarnoldii, found in Malaysia. A single flower is almost a meter across and has a mass up to 11.0 kg. Suppose you cut off a single flower and drag it along the flat ground. If the coefficient of kinetic friction between the flower the ground is 0.39, what is the magnitude of the frictional force that must be overcome? • Robert Galstyan, from Armenia, pulled two coupled railway wagons a distance of 7m using his teeth. The total mass of the wagons was about 2.20x105 kg. Of course, his job was made easier by the fact that the wheels were free to roll. Suppose the wheels are blocked and the coefficient of static friction between the rails and sliding wheels is 0.220. What would be the magnitude of the minimum force needed to move the wagons from rest? Assume that the track is horizontal. • The steepest street in the world is Baldwin Street in Dunedin, New Zealand. It has an inclination angle of 38.0° with respect to the horizontal. Suppose a wooden crate with a mass of 25.0 kg is placed on Baldwin Street. An additional force of 59 N must be applied to the crate perpendicular to the pavement in order to hold the create in place. If the coefficient of static friction between the crate and the pavement is 0.599, what is the magnitude of the frictional force? • A blue whale with a mass of 1.90x105 kg was caught in 1947. What is the magnitude of the minimum force needed to move the whale along a horizontal ramp if the coefficient of static friction between the ramp’s surface and the wale is 0.460? • The most massive car ever built was the official car of the General Secretary of the Communist Party in the former Soviet Union. Suppose the car is moving down a 10.0° slope when the driver suddenly applies the brakes. The net force acting on the car as it stops is -2.00x104 N. If the coefficient of kinetic friction between the car’s tires and the pavement is 0.797, what is the car’s mass? What is the magnitude of the normal force that the pavement exerts on the car? • Suppose a giant hamburger slides down a ramp that has a 45.0° incline. The coefficient of kinetic friction between the hamburger and the ramp is 0.597, so that the net force acting on the hamburger is 6.99x103N. What is the mass of the hamburger? What is the magnitude of the normal force that the ramp exerts on the hamburger?

  12. N’s 3rd Law & Force Pairs

  13. Pulley’s • Atwood’s Machine • Mass & Cart System

  14. 4 kg M M 8kg Pulley’s & Newton’s 3rd Law #3H • What are the force pairs present when an object is falling from the sky. Make a free body diagram for each object. • What are the force pairs present when a horse is pulling a cart. Make a free body diagram for each object. • What are the force pairs for a child pushing a sled with a rope. Make a free body diagram for each. For 4 & 5. Find M if a=-2 m/s2and a=2m/s2 For 6. Find M if a is downward at 4m/s2 and 2m/s2 as the arrow shows For 7. Find a. 5. 4. 7. a a 6. 4kg 2M M M

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