Physics 151 Week 11 Day 3 Topics: Newton’s 2nd Law and Applications  Applying Newton’s 2nd Law  Apparent Weight  Free Fall  Terminal Velocity  Friction – Model vs. Brainstorm

Scales and Elevators (Apparent Weight) Slide 4-19 How many of you would like to be able to control you weight? What is weight? What does a scale measure? What is happening in this video?

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)

Apparent Weight Slide 5-24

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.

If I drop a set of key and a piece of paper Slide 4-19 If I drop a piece of paper and a set of keys, which lands first?

Free Fall, Drag force, and Terminal Velocity Slide 4-19 The hammer and the feather (on the moon) Sky Diving and terminal velocity

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?

Discussion of U5: Worksheet 3 Slide 4-19 Problem 7 A 100 N force is applied to a 50 kg crate resting on a level floor. The coefficient of kinetic friction is Is the 100 N force sufficient to cause a motionless crate to accelerate? Draw a force diagram, then explain why or why not based on your calculations.

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

Brainstorm: What do we know about Friction Force? 1.Seems to relate to pushing force (Drag Force?) 2.- Increases with more contact surface - Not dependent on surface area, but surface type 3.Car needs friction for acceleration including steering 4.Can determine acceleration along with other forces 5.2 types static and kinetic, once object starts moving crosses from F fs to F fk / also rotational/rolling Friction is smallest friction 6.Caused by electron repulsion between two objects 7.- 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 8.Friction Force is a contact force that acts on an object by its surface 9.Converts Kinetic Energy to Thermal Energy 10.Friction Force depends on what is in contact (involves a coefficient  ) 11.Friction is resistance between 2 surfaces in contact / resists change 12.Friction does not depend on mass / related to weight 13.It slows things down and makes them stop (can be small/negligible) 14.Can occur doing motion or without motion 15.It acts two ways 16.It usually accompanies a normal force / perpendicular to normal force 17.It is in the negative y direction, parallel to the surface of an object 18.F fs > F fk - initially starting object moving is harder than keeping it moving

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 friction depend on area?

Demonstration: What do we know about Friction Force? 7 - 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 Does friction always oppose motion?

Brainstorm: What do we know about Friction Force? 18. F fs > F fk - initially starting object moving is harder than keeping it moving

Example Problem A sled with a mass of 20 kg slides along frictionless ice at 4.5 m/s. It then crosses a rough patch of snow which exerts a friction force of 12 N. How far does it slide on the snow before coming to rest? Slide 5-21

Burglars are trying to haul a 1000 kg safe up a frictionless ramp to their getaway truck. The ramp is tilted at angle θ. What is the tension in the rope if the safe is at rest? If the safe is moving up the ramp at a steady 1 m/s? If the safe is accelerating up the ramp at 1 m/s 2 ? Do these answers have the expected behavior in the limit θ → 0° and θ → 90°? Example Problem Slide 5-28

Macie pulls a 40 kg rolling trunk by a strap angled at 30° from the horizontal. She pulls with a force of 40 N, and there is a 30 N rolling friction force acting on trunk. What is the trunk’s acceleration? Example Problem Slide 5-22