PHY205H1F Summer Physics of Everyday Life Class 2 Force Causes Acceleration Friction Mass and Weight Mass Resists Acceleration Newton s Second Law of Motion Newton s 3 rd how-fraud-drives-up-rates.html ] Law Photo downloaded May 15, 2013 from the Toronto Star http://www.thestar.com/sponsored_sections/insurancehotline/2013/05/01/ Net Force Net force is the combination of all forces that change an object s state of motion. Net force is the vector sum of all the forces acting on an object. F 2 F 1 F 1 + F 2 = F net The Force of Gravity a.k.a. Weight Weight = mg g = 10 m/s 2 The direction of the weight is toward the centre of the earth. Weight is measured in newtons. The Earth exerts a gravity force on the ball. Status Light When I start asking clicker questions: Status light will flash green when your response is registered on my computer. The Clickers Power Light Status will flash red On/Off Switch if your response is not registered. Please turn on your clicker now 4 Discussion Question Chris Hadfield recently spent nearly five months in orbit around the Earth. He was living on the International Space Station, which orbits at 370 km above the surface of the Earth (low earth orbit). What was Chris Hadfield s weight while on the space station? A. Zero B. The same as his weight on earth. C. A little bit less than his weight on earth. D. Not exactly zero, but much, much less than his weight on earth. Orbit is drawn to scale International Space Station Radius of the Earth: 6400 km, g = 10 m/s 2 Altitude of Space Station: 370 km, g = 9 m/s 2 (about 10% less) Normal Force a.k.a. Support Force The Force of Friction depends on the kinds of material and how much they are pressed together. is due to tiny surface bumps and to stickiness of the atoms on a material s surface. Sliding Friction The diving board exerts a normal force on the dog. Example: Friction between a crate on a smooth wooden floor is less than that on a rough floor. The ground exerts a sliding friction force on Suleyman. 1
Static Friction The ground exerts a static friction force on the shoe. Multiple Forces on a Single Object A car is parked on flat, horizontal pavement. Which of the following forces are acting on the car? A.Gravity B.Normal C.Static friction D. All of the above E. A and B, but not C The Net Force A car is parked on flat, horizontal pavement. The net force is the vector sum of all the forces on the car. What is the direction of the net force on the car? A. Up B. Down C. The net force is zero In Class Discussion Question. Which is true? Friction A. always causes objects to slow down. B. always causes objects to speed up. C. can cause objects to speed up or slow down, depending on the situation. Mass and Weight 1 kilogram weighs 10 newtons (9.8 newtons to be precise). Relationship between kilograms and pounds: 1 kg weighs 2.2 lb = 10 N at Earth s surface 1 lb = 4.45 N 4.54 kg weighs 10 lbs Mass Resists Acceleration The same force applied to Twice the mass produces half the acceleration. 3 times the mass, produces 1/3 the acceleration. 1 Acceleration ~ mass Acceleration is inversely proportional to mass. 2 Newton s Second Law The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass. a F m net A fan attached to a cart causes it to accelerate at 2 m/s 2. Suppose the same fan is attached to a second cart with smaller mass. The mass of the second cart plus fan is half the mass of the first cart plus fan. The acceleration of the second cart is A. 16 m/s 2. B. 8 m/s 2. C. 4 m/s 2. D. 2 m/s 2. E. 1 m/s 2. Example Chapter 4, Problem 7 A rock band s tour bus of mass M is accelerating away from a stop sign at a rate of 1.2 m/s 2. Suddenly a piece of heavy metal, mass M/6, falls onto the top of the bus and remains there. What is the acceleration of the bus + metal? 2
Discussion Question A constant net force acts on an object. What about the object must be changing as a result? A. position B. velocity C. acceleration D. All of the above E. A and B, but not C The greater the mass of the object the greater its force of attraction toward the Earth. the smaller its tendency to move i.e., the greater its inertia. So, the acceleration is the same. It is equal to the acceleration due to gravity: 10 m/s 2 (precisely 9.8 m/s 2 ). When acceleration is g free fall Newton s second law provides an explanation for the equal accelerations of freely falling objects of various masses. Acceleration is equal when air resistance is negligible. Acceleration depends on force (weight) and inertia. A 600 g basketball and a 60 g tennis ball are dropped from rest at a height of 3 m above the ground. As they fall to the ground, air resistance is negligible. Which of the following statements is true for the balls as they fall? A. The force of gravity is 10 times greater on the basketball than on the tennis ball B. The force of gravity is the same on both balls C. The force of gravity is slightly larger on the basketball than on the tennis ball A 600 g basketball and a 60 g tennis ball are dropped from rest at a height of 3 m above the ground. As they fall to the ground, air resistance is negligible. Which of the following statements is true for the balls as they fall? A. The acceleration of the basketball is 10 times greater than the acceleration of the tennis ball B. The acceleration of both balls is the same C. The acceleration of the basketball is slightly larger than the acceleration of the tennis ball Non- When an object falls downward through the air it experiences: force of gravity pulling it downward. air drag force acting upward. R depends on the speed of the object relative to the air, and the size of the object Terminal Speed R increases with speed Net force goes to zero when the object is moving fast enough so that R = mg (air resistance = weight) Then no net force No acceleration Velocity does not change Non- Example A skydiver jumps from plane. Weight is the only force until air resistance acts. As falling speed increases, air resistance on diver builds up, net force is reduced, and acceleration becomes less. When air resistance equals the diver s weight, net force is zero and acceleration terminates. Diver reaches terminal velocity, then continues the fall at constant speed. Pre-Class Quiz Results 103 students wrote the pre-class quiz; the average was 3.6/4. The standard metric unit of mass is kg. The standard metric unit of weight is N (the Newton). In the apple-pulling-the-orange sequence, the force that accelerates the system across the floor is friction! Some comments: I found the apple-orange part most confusing. Internal vs External Forces. (LOTS of students said something like this) I will definitely go over this carefully today. 3
Questions via email: Student Emails Im wondering do we need to register the clicker in portal too? No you don't have to register the clicker on portal, just on http://www1.iclicker.com/register-an-iclicker Note this can be done after you start using the clicker, and you will still get your marks. is there a time limit for the pre reading quiz? No, there is no time limit for the pre-class reading quizzes, as far as I am aware. You might want to be careful that portal doesn't log you out automatically though if you don't do anything for a long time.. Note from the course administrator, April Seeley The PHY205H1F course has exclusions A student CAN NOT take PHY205H1F at the same time as PHY131H1F nor can a student have taken previously PHY131H1/PHY132H1/PHY151H1/PHY152H1/PHY110 Y1/PHY138Y1/PHY140Y1 or equivalent. If a student is found to have this exclusion they will be removed from PHY205. PHY205H1F Physics of Everyday Life Chapter 5 Forces and Interactions Newton s Third Law of Motion Vectors 3 Newton s Third Law If object 1 acts on object 2 with a force, then object 2 acts on object 1 with an equal force in the opposite direction. F 1 F on 2 2 on1 A Mack Truck drives North on the highway, and collides head-on with a mosquito. Which is true? A. The Mack Truck exerts a greater force on the mosquito than the mosquito exerts on the Mack Truck. B. The mosquito exerts a greater force on the Mack Truck than the Mack Truck exerts on the mosquito. C. The Mack Truck exerts the same force on the mosquito as the mosquito exerts on the Mack Truck. D. Impossible to determine without knowing the speeds of the truck and mosquito. E. Don t know or none of the above A Mack Truck drives North on the highway, and collides head-on with a mosquito. Which is true? A. The Mack Truck does more damage to the mosquito than the mosquito does to the Mack Truck. B. The mosquito does more damage to the Mack Truck than the Mack Truck does to the mosquito. C. The Mack Truck does the same amount of damage to the mosquito as the mosquito does to the Mack Truck. D. Impossible to determine without knowing the speeds of the truck and mosquito. E. Don t know or none of the above F = ma or a = F / m If the force is equal on the truck and the mosquito, is the acceleration equal? Acceleration is higher if m is lower ( F divided by m) Mosquito accelerates more, so it receives more damage. Forces always come in pairs Every force interaction involves two objects, and two forces. These forces are equal in strength and opposite in direction. are always the same kind of force (ie gravity, normal, friction, tension, etc.) always act on different objects. 4
Car/Earth Friction Interaction Consider an accelerating car. The tires of the car are pushing backward on the Earth (static friction). The Earth is pushing forward on the tires of the car (static friction). Rocket/Gas Pressure Interaction Consider a rocket accelerating upward. The rocket is pushing down on the expelled gas (pressure). The expelled gas is pushing up on the rocket (pressure). Man/Rope Tension Interaction This man is pulling the rope to the left. What is the other force in this interaction? A. His feet are pushing the ground to the right. B. The ground is pushing his feet to the left. C. The rope is pulling the man to the right. D. Gravity of the Earth is pulling the man down. E. Gravity of the man is pulling the Earth up. Basketball/Earth Gravity Interaction Consider a basketball in freefall. Gravity is pulling this ball down. What is the other force in this interaction? A. The thrower s feet are pushing the ground down. B. The ground is pushing the thrower s feet up. C. Gravity of Earth is pulling the thrower down. D. Gravity of the ball is pulling the Earth up. E. Air is pushing the ball up. Basketball/Earth Gravity Interaction a = F m a = F The Earth is pulling down on the ball. The ball is pulling up on the Earth. m Ride the MP Elevator! In the corner of every elevator in the tower part of this building, there is a mass hanging on a spring. If you look closely at the spring, it has a scale which reads Newtons. This is how much upward force is needed to support the hanging mass. In your tutorial next week you will be going with your team to look more carefully at this scale, and record how it changes as the elevator accelerates! Example: Chapter 5, Problem 1 A boxer punches a piece of kleenex in midair and brings it from rest up to a speed of 25 m/s in 0.05 s. (a) What acceleration does the kleenex have while being punched? (b) If the mass of the kleenex is 0.003 kg, what force does the boxer exert on it? (c) How much force does the paper exert on the boxer? Discussion Question A boxer cannot exert much force on a piece of tissue paper suspended in air because A. the time of the interaction is too brief B. there is little follow-through with the punch C. the tissue lacks enough strength D. the tissue paper has too little mass to exert much force on the boxer's fist Defining Your System Consider a single enclosed orange. Applied external force causes the orange to accelerate in accord with Newton s second law. Action and reaction pair of forces is not shown. 5
Consider the orange and the apple pulling on it. Action and reaction do not cancel (because they act on different things). External force by apple accelerates the orange. Consider a system comprised of both the orange and the apple The apple is no longer external to the system. Force pair is internal to system, which doesn t cause acceleration. Action and reaction within the system cancel. With no external forces, there is no acceleration of system. Consider the same system, but with external force of friction on it. Same internal action and reaction forces (between the orange and apple) cancel. A second pair of action-reaction forces (between the apple s feet and the floor) exists. One of these acts by the system (apple on the floor) and the other acts on the system (floor on the apple). External frictional force of floor pushes on the system, which accelerates. Second pair of action and reaction forces do not cancel. A bird flies by Newton s Third Law A. flapping its wings. B. pushing air down so that the air pushes it upward. C. hovering in midair. D. inhaling and exhaling air. Newton s Third Law Slightly tilted wings of airplanes deflect A. oncoming air downward to produce lift. B. oncoming air upward to produce lift. C. Both A and B. D. Neither A nor B. Vectors & Scalars Vector quantity has magnitude and direction. is represented by an arrow. Example: velocity, force, acceleration Scalar quantity has magnitude. Example: mass, volume, speed Vector Addition The sum of two or more vectors For vectors in the same direction, add arithmetically. For vectors in opposite directions, subtract arithmetically. Two vectors that don t act in the same or opposite direction: use parallelogram rule. Two vectors at right angles to each other use Pythagorean Theorem: R 2 = V 2 + H 2. Which figure shows A A A? 1 2 3 6
[image downloaded Jan.16 2013 from http://campbellpost.wordpress.com/2012/01/26/canoe/ ] Vector components Vertical and horizontal components of a vector are perpendicular to each other The components add to give the actual vector You run horizontally at 4 m/s in a vertically falling rain that falls at 4 m/s. Relative to you, the raindrops are falling at an angle of A. 0. B. 45. C. 53. D. 90. Vectors Example: Chapter 5, Problem 6 You are paddling a canoe at a speed of 4 km/h directly across a river that flows at 3 km/h, as shown. (a) What is your resultant speed relative to the shore (b) In approximately what direction should you paddle the canoe so that it reaches a destination directly across the river? Before Class 3 next Wednesday Please read Chapters 7 and 8, or at least watch the 20-minute pre-class video for class 3 Pre-class reading quiz on chapters 7 and 8 is due Wednesday May 22 by 10:00am Something to think about: There are two seemingly identical mouse traps sitting on the floor. They have the same mass, size, colour, shape and smell. One has been set by bending the spring back and hooking it, the other is not set. What is the physical difference between the two traps? Why is one so much scarier than the other? 7