Objectives Describe examples of force and identify appropriate

Objectives � Describe examples of force and identify appropriate SI units used to measure force � Explain how the motion of an object is affected when balanced and unbalanced forces act on it � Compare and contrast the four kinds of friction � Describe how Earth’s gravity and air resistance affect falling objects

Force � Definition - push or pull that acts on an object - can accelerate or decelerate on object ex. kicking a soccer ball at rest wind pushing against you as you walk � Measured - units: Newtons (N) = 1 kg m/s 2 � Represented - force vectors: arrow = direction length = strength or magnitude

Forces Cont. � Combining Forces - forces acting in the same directions: add 3 N + 5 N = 8 N - force in opposite directions: subtract 6 N – 2 N = 4 N � Net Force - the overall force acting on an object after all the forces are combined � Two Types - balanced - unbalanced

Balance vs. Unbalanced Forces � Balanced forces - forces that combine to produce no net force ex. tug of war with no winner 2 N - 2 N=0 - no net force, no movement � Unbalanced forces - force that results when the net force acting on an object is not equal to 0 ex. 2 N - 4 N = 2 N - object will move in direction of net force

Practice Problems � You push a car with a force of 50 N, your friends pulls with a force of 25 N. Draw force diagrams, and calculate the net force acting upon the car. Will the car move? 50 N + 25 N = 75 N � You push a box towards your friends with a force of 80 N while one friend pushes the box against you with a 55 N, the other 25 N. Draw force diagrams and calculate the net force acting upon the car. Will the box move? 80 N 55 N + 25 = 0

Common Forces � Two common forces - gravity: force that acts between any two masses - attractive force that can act over large distance - Earth’s gravity acts downward toward the center of Earth = 9. 8 m/s/s = 10 m/s/s ex: Earth’s gravity holds you on the ground

Common Forces Cont. � Friction - force that opposes the motion of objects that touch as they move past one another - four types: static, sliding, rolling, fluid - static: acts on objects that are not moving ex. Keeps a giant potted plant from being easily pushed - sliding: force that opposes the direction of motion of an object as it slides over a surface ex. You and a friend push the plant hard enough to over come static friction and move it across the floor!

Common Forces Cont. - rolling friction: force that acts on rolling objects ex. soccer ball rolling across the floor - fluid friction: force that opposes the motion of an object through fluid * any mixture of gases is considered a fluid ex. airplane flying through the air

Gravity �A force that acts between 2 mases � An ATTRACTIVE force, pulls object together � Does not require objects to be touching! � Earth’s gravity acts as a DOWNward force, balanced by Upward force �As a result, the most common motion is downward motion

Falling objects � Affected by both gravity and air resistance � Gravity causes object to fall downward, air resistance acts in the direction opposite motion (up) and slows acceleration

Falling objects As objects fall to the ground they accelerate and gain speed � INC speed = INC air resistance � If a objects falls long enough, upward force and downward force will become BALANCED and the acceleration = 0, CONSTANT VELOCITY �

Projectile Motion � The motion of a falling object after it is given initial forward velocity � Air resistance and gravity are only forces acting on object

Objectives 12. 2 � Compare and contrast Aristotle Galileo’s and Newton’s ideas � Define inertia � Explain Newton’s 1 st law of motion, and apply them to physical situations

Galileo Vs. Newton: Inertia � Galileo - introduced the idea of inertia - maintained that motion of an object requires a force � Newton - grasped the significance of inertia - motion of an object requires and initial force, not continual - law of inertia defines natural motion and tells us what kinds of motion are the result of applied forces

Newton’s First Law of Motion - Newton redefined Galileo’s idea of inertia Inertia: tendency of things to resist a change in it’s motion � Law 1 - Every material object continues in its state of rest or uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it - Key Word: continues

Newton’s 1 st Law Cont. ex. stamp our feet to remove snow shake a garment to rid dust or dirt table cloth and dishes stunt * Holds true whether your are at rest or moving at a constant velocity ex. when we jump straight up, we land in our footsteps rather than at a location equal to the distance the earth moves during our jump?

Inertia Problems Q: A hockey puck sliding across the ice finally comes to rest. How would Aristotle interpret this behavior? How would Galileo and Newton interpret it? A: Aristotle: a constant force was not applied to the puck therefore it would come to a stop. Galileo & Newton: The forces acting against the puck become greater than that of force acting on the puck and therefore it will come to a stop

Objectives � Be able to understand apply Newton’s 2 nd Law of motion � Using Newton’s 2 nd: calculate force, mass, and acceleration � Recognize that the free fall acceleration near Earth’s surface is independent of the mass of the falling object � Explain the difference between mass and weight

Newton’s 2 nd Law � The acceleration of an object is directly proportional to the net force acting on the object, in the direction of the net force, and is inversely proportional to the mass of the object - in short: A = net force or F = ma mass - symbols: a = acceleration Fnet = force m = mass

Practice Problems A boy pushed forward a cart of groceries with a total mass of 40. 0 kg. What is the acceleration of the cart if the net force on the cart is 60. 0 N? � An automobile with a mass of 1200 kg accelerates at a rate of 3. 0 m/ s 2 in the forward direction. What is the net force acting on the automobile? � A 25 N force accelerates a boy in a wheelchair at 0. 5 m/s 2. What is the mass of the boy and the wheelchair? �

Practice Problems Cont. �A = F/m 60. 0 N = 1. 50 m/s 2 40. 0 kg � F = ma 1200 kg x 3. 0 m/s 2 = 3600 N � M = F/a 25 N = 50 kg 0. 50 m/s 2

Inertia Problems Q: Would it be easier to lift a huge truck on the Earth or on the Moon? A: On the moon, when you lift an object you are dealing with weight, since weight is the gravitational pull on the most massive body, and the moon has less gravity than the earth

Acceleration � Acceleration can be: �Equal to 0: Net Forces = 0 static motion - object is not moving or accelerating - all forces are balanced ex. a book at rest on the table dynamic motion - object is moving but not accelerating - all forces are balanced ex. a book sliding across the table at a constant velocity

Acceleration Cont. �< gravity: net forces >0 - object is moving & experiencing friction - forces are unbalanced ex. a feather falling to the ground *important to remember NET FORCES ex. air resistance is neglected: net force is the objects weight ex. presences of air resistance: net force is less than the weight objects weight – air resistance

Acceleration Cont. - Air resistance of an object depends on 2 factors 1. The frontal area of an object - greater the area greater the air resistance 2. The speed of the object - greater the speed greater the air resistance

Acceleration Cont. � Terminal Speed - When acceleration of an object equals zero - If concerned with direction; use terminal velocity � Why? - Velocity indicates a direction and speed

Acceleration Cont. Ex. Skydiving - as you fall you gain speed, air resistance therefore builds until finally it equals your weight. If this happens, the net force is equal to zero and you no longer accelerate, reaching terminal speed feather few ~ centimeters per second sky diver ~ 200 kilometers per hour

Acceleration Cont. �= gravity (free fall) - object is falling at 9. 8 m/s 2 - air resistance can be neglected ex. Any object falling in a vacuum Why does the object with double the mass not accelerate greater?

Acceleration Problem Q: A jumbo jet cruises at a constant velocity of 1000 km/hr when the thrusting force of it’s engine is a constant 100, 000 N. What is the acceleration of the jet? What is the force of air friction (air resistance) on the jet? A: The jet is not accelerating, it is at a constant velocity and therefore the net force must be 0 meaning the thrust is canceled out by the air resistance, therefore the air resistance must be 100, 000 N

Newton’s 3 rd Law � Action Reaction Forces - whenever on object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. ex. produces motion: car tires push backwards against the road pushes forward against the car tires ex. no motion: hand pushes against the wall pushes against your hand

- rd Law Cont. Newton’s 3 if we drop a sheet of tissue paper in front on the heavy weight champion of the world and challenge him to hit it in midair with 50 lbs of force, his best punch couldn’t even come close Why? - a force is not a thing in itself but makes up an interaction between itself and another (system) - the action force and the reaction force - net force only equals 0 when force are equal and opposite ex.