Topics: Forces, Apparent Weight, & Friction Physics 151 Week 9 Day 2 Topics: Forces, Apparent Weight, & Friction Applying Newton’s 2nd Law Apparent Weight Friction Static Friction Kinetic Friction Coefficient of Friction Normal forces and Friction Applications

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

Net Force Model Graph Clicker Questions A worker is pushing a cart along the floor. The force exerted by the worker on the cart is purely horizontal. The direction the worker is going is +. (Note: time axes are to the same scale, but ordinates {"y axes"} are not.) At first, the worker has to push hard in order to get the cart moving. Then the cart is easier to push as it moves with constant speed. Finally, the worker has to pull back on the cart in order to bring it to a stop before it hits the wall. Match the graphs with the given variable. (graphs can be used > 1 time) 1. Friction Force 2. Force exerted by worker 3. vx 4. ax 5. Fnet

Brainstorm: What do we know about Friction Force? Seems to relate to pushing force (Drag Force?) - Increases with more contact surface - Not dependent on surface area, but surface type Car needs friction for acceleration including steering Can determine acceleration along with other forces 2 types static and kinetic, once object starts moving crosses from Ffs to Ffk / also rotational/rolling Friction is smallest friction Caused by electron repulsion between two objects - Friction opposes (in opposite direction of) applied force - Direction of Friction force is Opposite of direction of motion / acceleration - Parallel to motion / Resists force of acceleration Friction Force is a contact force that acts on an object by its surface Converts Kinetic Energy to Thermal Energy Friction Force depends on what is in contact (involves a coefficient ) Friction is resistance between 2 surfaces in contact / resists change Friction does not depend on mass / related to weight It slows things down and makes them stop (can be small/negligible) Can occur doing motion or without motion It acts two ways It usually accompanies a normal force / perpendicular to normal force It is in the negative y direction, parallel to the surface of an object Ffs > Ffk - initially starting object moving is harder than keeping it moving

Static & Kinetic Friction - Part I Demonstration Explain the graph, i.e. why does it look this way? Slide 4-19

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. 
 Label times a-f that match the free-body diagrams in the previous problem. If the mass of the box is 3.0 kg, the maximum Ffs is 10 N, and Ffk has an average of 6.0 N, find the coefficients of static and kinetic friction. Slide 4-19

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

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

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

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 0.30. 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 0.25. 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

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

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