# Topics: Forces, Apparent Weight, & Friction

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Topics: Forces, Apparent Weight, & Friction
Physics 151 Week 9 Day 3 Topics: Forces, Apparent Weight, & Friction Apparent Weight Friction Static Friction Kinetic Friction Coefficient of Friction Normal forces and Friction Applications

Energy Model Types of Energy Kinetic Energy KE = 1/2 mv2
Potential Energy: Gravitational Potential Energy PEg = mgy Spring Potential Energy Pes = 1/2 kL2 Conservation of Energy (Closed System) Visualizations: Energy Bar Charts

General Force Model Newton 0th Law
Objects are dumb - They have no memory of the past and cannot predict the future. Objects only know what is acting directly on them right now Newton's 1st Law An object that is at rest will remain at rest and an object that is moving will continue to move in a straight line with constant speed, if and only if the sum of the forces acting on that object is zero. Newton's 3rd Law Recall that a force is an interaction between two objects. If object A exerts a force on object B then object B exerts a force on object that is in the opposite direction, equal in magnitude, and of the same type. Visualizations: Force Diagrams System Schema

Net Force Model Newton's 2nd Law
acceleration of an object = sum of forces acting on that object / the mass of the object Note: Solve Newton’s 2nd Law equations in component form Visualizations: Force Diagram System Schema Motion visualizations as needed Remainder of week: Friction Model Apparent Weight Slide 4-19

Demonstrations: What do we know about Friction Force?
2 - Increases with more contact surface - Not dependent on surface area, but surface type Does our model of friction depend on area? Does real-world friction depend on area?

Parking on a Hill 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? 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? 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? Slide 4-19

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 0.15. What is the minimum magnitude of the applied force F, parallel to the plane, that will prevent the sled from slipping down the plane? What is the minimum magnitude of the applied force F that will start the sled moving up the plane? 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.

Scales and Elevators (Apparent Weight)
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) Slide 4-19

Scales and Elevators (Apparent Weight)
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. Slide 4-19

Apparent Weight Slide 5-24

Example Problem 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? Slide 5-25

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

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

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