Physics 151 Week 10 Day 2 Topics: Apparent Weight & Conservation of Energy  Apparent Weight  More Friction

Scales and Elevators (Apparent Weight) Slide 4-19 Consider a person with a mass of 60 kg is in an elevator standing on a scale. The elevator is accelerating upward. Draw a system schema and 2 force diagrams: One for the person and one for the scale

Scales and Elevators (Apparent Weight) Slide 4-19 Consider a person with a mass of 60 kg is in an elevator standing on a scale. The elevator is accelerating upward. Draw a system schema and 2 force diagrams: One for the person and one for the scale What does the scale read?

Scales and Elevators (Apparent Weight) Slide 4-19 Consider a person with a mass of 60 kg is in an elevator standing on a scale. The elevator is accelerating upward. Draw a system schema and 2 force diagrams: One for the person and one for the scale What does the scale read? Use Newton’s 2nd law to determine what the scale reads (This is apparent weight)

Scales and Elevators (Apparent Weight) Slide 4-19 Suppose a person with a mass of 60 kg is in an elevator standing on a scale. Use the system schema and force diagrams of the scale and the person to determine what the scale would read for the following situations: A. If the elevator is descending at 4.9 m/s. B. If the elevator has a downward acceleration of 4.9 m/s/s. C. If the elevator has an upward acceleration of 4.9 m/s/s.

Apparent Weight Slide 5-24

A 50 kg student gets in a 1000 kg elevator at rest. As the elevator begins to move, she has an apparent weight of 600 N for the first 3 s. How far has the elevator moved, and in which direction, at the end of 3 s? Example Problem Slide 5-25

Clicker Question The apparent weight of an object is A.the pull of gravity on the object. B.the object’s mass times the acceleration of gravity. C.the magnitude of the contact force that supports the object. D.the pull of gravity on an object that is accelerating upward. Slide 5-7

Answer 2.The apparent weight of an object is A.the pull of gravity on the object. B.the object’s mass times the acceleration of gravity. C.the magnitude of the contact force that supports the object. D.the pull of gravity on an object that is accelerating upward. Slide 5-8

Clicker Question The coefficient of static friction is A.smaller than the coefficient of kinetic friction. B.equal to the coefficient of kinetic friction. C.larger than the coefficient of kinetic friction. D.equal to or larger than the coefficient of kinetic friction E.not discussed in this chapter. Slide 5-9

Answer The coefficient of static friction is A.smaller than the coefficient of kinetic friction. B.equal to the coefficient of kinetic friction. C.larger than the coefficient of kinetic friction. D.equal to or larger than the coefficient of kinetic friction E.not discussed in this chapter. Slide 5-10

Coefficients of Friction Slide 4-19 What can you deduce/generalize about friction forces from this table? Describe 3-4 real world situations that can be explained by this table

Brainstorm: What do we know about Friction Force? 1.Force created by the surface – against the motion Resistance to movement (opposite of direction of motion) Always opposes motion Resists motion Applied in the opposite direction of motion 2.It is parallel to the surface Perpendicular to normal force 3.Kinetic friction acts on object in motion Kinetic Friction – friction during motion Static Friction acts on the object not in motion Object moves when static friction needed to hold surfaces in place exceeds threshold 4.It depends on the Normal F on the object and  (types of surfaces in contact) 5.F f =  6.Contact Force, Vector 7.When 2 objects are in contact 8.Generates heat / Can produce heat 9.Force of movement relative to another object 10.Slows things down 11.Resistance

What do we know about Friction Force? Does our model of friction depend on area? Does real-world friction depend on area?

Static & Kinetic Friction - Part II Describe what is happening to the forces on the box and the effect of the forces on the motion of the box from the pictures. Slide 4-19

Static & Kinetic Friction - Part III Below is graph of the friction force exerted by the table on the box. A.Label times a-f that match the free-body diagrams in the previous problem. B.If the mass of the box is 3.0 kg, the maximum F fs is 10 N, and F fk has an average of 6.0 N, find the coefficients of static and kinetic friction. Slide 4-19

Parking on a Hill Slide 4-19 A.If you park on a hill with a 10 degree slope with the car held by the parking brake, what is the magnitude of the frictional force that holds your car in place? B.The coefficient of static friction between your car's wheels and the road when wet is What is the largest angle slope on which you can park your car in the rain so that it will not slide down the hill? C.The coefficient of kinetic friction between your wheels and the wet road surface is If someone gave your your car a push on the wet hill and it started sliding down, what would its acceleration be?

Penguin in a box A loaded penguin in a box, together weighing 60 N, rests on a plane inclined at 20° to the horizontal. Between the box and the plane, the coefficient of static friction is 0.26 and the coefficient of kinetic friction is What is the minimum magnitude of the applied force F, parallel to the plane, that will prevent the sled from slipping down the plane? 2.What is the minimum magnitude of the applied force F that will start the sled moving up the plane? 3.What value of the applied force F is required to move the block up the plane at constant velocity? Hint: Draw a system schema and then draw a separate force diagram for each part of the problem

Haul the Crate A 10 kg wooden crate is placed on wood slats in the back of a pick-up truck with no tail gate. How fast can the truck accelerate before the crate falls off? Assume the coefficient for static friction for the crate on the wood slats is 0.40 and the coefficient for kinetic friction is 0.20.