5/12/15. Chapter 12: Forces Key Terms. What is Inertia? 12.1 Newton s First and Second Laws. What are Newton s Laws of Motion?

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1 12.1 Newton s First and Second Laws Chapter 12: Forces Physical Science - Doerfler Key Ideas What makes an object s motions change? What is inertia? What affects how much an object speeds up or slows down? Inertia 12.1 Key Terms What are Newton s Laws of Motion? Rolling a basketball on hardwood floor will go farther than on carpet Why does this happen? Sir Isaac Newton noticed similar effects He also wondered what caused them He discovered the relationship between motion and forces Described relationships in three laws Called Newton s laws of motion What is Newton s First Law of Motion? Ball that stops after rolling along the floor Example of Newton s first law Law states that an object s motion will not change until an unbalanced force acts on the object Law also states object will not start to move until unbalanced force acts on it Rolling ball stops moving because force of friction acts on it and slows it down If no friction, ball would keep rolling What is Inertia? Another way of stating Newton s law is that matter resists any change in motion All objects resist any change in motion Not all objects resist changes in motion by the same amount Takes less force to change motion of baseball than a bowling ball Inertia tendency of an object to resist changes in motion All objects have inertia Amount of inertia depends on object s mass 1

2 Seat Belts and Car Seats When you are in a car and stop suddenly, what happens? You feel like being pushed forward, objects near you fell over Newton first law explains this When car stops, inertia of objects in car prevent them from stopping right away Seat belts help protect humans inside the car Seat belt catches you as you slide forward when car stops What is Newton s Second Law of Motion? States that the larger the unbalanced force on an object, the more the object s motion will change Also states that the larger the object s mass, the smaller its acceleration will be Acceleration is the change in motion of an object Baseball vs. Bowling ball (throwing force) Newton s 2 nd law of motion Net force = mass x acceleration F = ma Can use to calculate how much force is needed to make object accelerate; how much object will accelerate when force acts on it What are the units of force? SI unit for force is newton (N) Amount of force needed to make a one-kilogram mass accelerate at one meter per second squared 1 N = 1kg x 1m/s 2 Sometimes, people use pounds instead of newtons to measure force 1 lb = 4.45 N; 1 N = lb How Does Mass Affect Acceleration? The heavier an object, the slower the acceleration will be Compare sports car to semi-truck Compare a cheetah to an elephant Look at pictures below: which will move faster? Slower? Units of force Newtons (N) 1 N = 1kg x m/s 2 1 N = lb Pounds (lb) 1 lb = 4.45 N 1 lb = 4.45 kg x m/s 2 How Does Force Affect Acceleration? Look at the two pictures below Car has a large mass Picture on left has one person pushing, not accelerating fast Picture on right has multiple people pushing Car still has same mass. What will happen to the acceleration on the right? Calculating Force from Acceleration Example 1: Zoo keepers lift a lion on a stretcher. The total mass of the lion and the stretcher is 175 kg. the acceleration of the lion and the stretcher is m/s 2 upward. What force do the zoo keepers use to produce this acceleration? Step 1: list givens and unknowns Mass (m) = 175kg Acceleration (a) = m/s 2 Unknown: force (F) 2

3 Lion Example Problem Step 2: write the equation F = ma Step 3: insert known values and solve for unknown F = (175kg) x (0.657 m/s 2 ) F = 115 kg x m/s 2 = 115 N The zoo keepers apply 115 N of force to lift the lion and the stretcher Calculating Acceleration from Force A sailboat has a mass of 655 kg. A force of 895 N pushes the sailboat forward. What is the sailboat s acceleration? Step 1: list givens and unknowns Mass (m) = 655 kg Force (F) = 895 N Unknown = acceleration (a) Step 2: write the equation F = ma A = F/m Sailboat Example Problem Step 3: insert known values and solve for unknown A = 895 N/655 kg A = (895 kg x m/s 2 )/655 kg A = 1.37 m/s 2 The sailboat accelerates at 1.37 m/s 2 Example #1 The force of gravity between the moon and an object near its surface is much smaller than the force of gravity between Earth and the same object near Earth s surface. A bowling ball with a mass of 7.51 kg is pulled downward with an unbalanced force of 12.2 N. What is the acceleration of the falling bowling ball on the moon? Write down Step 1 Write down Step 2 Solve 1.62 m/s 2 Example #2 A freight train slows down as it approaches a train yard. If a force of N is required to provide an acceleration of 0.33 m/s 2, what is the train s mass? Write down Step 1 Write down Step 2 Solve M = 1.15 x 10 7 kg Example #3 The most massive train was put together in South Africa in 1989 and traveled 861 km. This freight train was over 7 km long and had a total mass of kg. Suppose the train s acceleration from rest to an average speed of 38 km/h was m/s 2. What then would be the size of the unbalanced force that the locomotives exerted on the cars of the train? Write down Step 1 Write down Step 2 Solve F = 1.33 x 10 7 kg x m/s 2 = 1.33 x10 7 N 3

4 12.2 Gravity Key Ideas: What is free fall? How are weight and mass related? How des gravity affect the motion of objects? Key Terms Free fall Projectile motion Terminal velocity Weight What is Gravity? What is Gravity - video Gravity is weaker on the moon than it is on Earth Gravity is a force that pulls objects together Astronauts on moon were able to jump higher than on Earth even with large space suits Sir Isaac Newton described law of universal gravitation Contains three parts 3 parts Law of Universal Gravitation Every object in the universe pulls with gravitational force on every other object The strength of the gravitational force between two objects depends on the masses of the objects The strength of the gravitational force between two objects depends on the distance between the objects How Does Mass Affect Gravitational Force? Looking at Universal Gravitation formula, masses (m 1 and m 2 ) are multiplied together Therefore, gravitational force between two objects increases as masses of the objects increase How Does Distance Affect Gravitational Force? Look at formula again Distance is denominator in equation As distance between two objects increases, G-force decreases Gravity, elephant, and cat G-force between Earth and elephant is greater than Earth and cat easier to pick up cat G-force between elephant and cat exists, but because Earth is also present and has large mass, g-force between elephant and cat is very small compared to Earth s g-force on them 4

5 The Strength of Earth s Gravitational Force Earth s G-force pulls every object towards its center All other objects in the universe also pull on objects on Earth However, distance are far, masses are small, or both Earth s G-force affects objects on Earth most significantly G-force from other objects is usually small enough to ignore Moon and sun impact ocean tides Moon is responsible for waves What is Free Fall? Dropping a ball falls to ground because Earth s G-force Newton s 2 nd law: unbalanced force that acts on object causes object to accelerate Earth s gravity causes ball to accelerate toward the ground Free fall when Earth s gravity is the only force acting on an object Acceleration caused by Earth s gravity is called free-fall acceleration What is Free Fall? All objects have same free fall acceleration **RECALL** acceleration depends on force and mass Heavier object experiences greater G-force than lighter object However, harder to accelerate heavy object than lighter object due to mass Air Resistance and Terminal Velocity Free fall happens when gravity is the only force that acts on an object On Earth, objects fall through the atmosphere Produces friction on the object called air resistance Air resistance acts in opposite direction of G-force Air resistance increases as object continues to fall Air Resistance and Terminal Velocity Eventually, if allowed to fall far enough, air resistance equals G- force There will then be no unbalanced force Newton s 1 st law: if no unbalanced forces act on an object, object s motion will not change When air resistance is no longer unbalanced force, object will quit accelerating Terminal velocity when object falls at constant velocity What is Weight? Weight gravitational force on an object Weight of object is same as G-force on object Weights of astronauts on moon were less than on Earth Only weighed one-sixth as on Earth Astronauts needed less force to jump Astronauts being weightless is incorrect There is gravity throughout space Astronauts are actually in free fall, falling towards Earth with same acceleration (human and space shuttle) 5

6 Difference Between Mass and Weight Mass is the amount of matter in an object Measured in kg Weight is a force Like all forces, weight is measured in Newtons Weight will change if G-force on it changes Example astronaut with mass of 66 kg weighs about 650 N (150 lb) On moon, astronaut has same mass, but weight is smaller (110 N or 25 lb) Calculating Weight Can use Newton s 2 nd law formula to calculate weight F = ma Free fall is gravity (9.8 m/s 2 ) Weight is force on object because gravity Formula rewritten: Weight = mass x free fall acceleration W = mg Therefore, weight is mass in kg multiplied by 9.8 m/s 2 On moon, free fall acceleration is 1.6 m/s 2 How Does Gravity Affect Moving Objects? Throwing a baseball You want it to travel straight This does not happen for very long Follows a curved path through the air until it hits the ground Gravity pulls baseball towards the ground Projectile Motion Projectile motion curved path followed by any object that is thrown or launched near Earth s surface All objects in projectile motion are moving in two directions Horizontally and vertically These two motions combine to form curved path Horizontal and vertical motions do not affect each other Orbits Orbit is a circular or oval-shaped path that one object follows as it moves around another object in space Gravity controls all objects in the universe Space shuttle is example of projectile motion Shuttle moves forward or horizontally Earth s gravity is pulling shuttle towards it or vertically Two motions combine to form curved orbit 6

7 12.3 Newton s Third Law Key Ideas: What happens when one object exerts a force on another object? How can you calculate the momentum of an object? How does momentum change after a collision? Key Terms Momentum What is Newton s Third Law of Motion? If you kick a soccer ball, the ball moves in a different direction Newton s first law: ball s motion could not have changed unless a force acted on it There must be a force acting on it It came from your foot You probably also felt a force on your foot when you kicked it Newton s Third Law Your foot exerts a force on the ball when kicking it This is called action force At the same time, the ball exerts force on your foot This is called reaction force Newton s Third Law States that action forces always produce reaction forces Also states that action forces and reaction forces are always equal in size, but act in opposite directions Force Pairs Action force and reaction force that results are called force pair Why do they not cancel each other out when they are equal and happening at the same time? The answer is forces that act on different objects When swimming, the action force acts on the water swimmer pushes the water backward The reaction force acts on the swimmer water is pushing swimmer forward Equal Forces, Unequal Effects When you drop a soccer ball Earth s G-force pulls ball to ground action force At the same time, ball exerts equal G-force on Earth reaction force Action force and reaction force are the same size but opposite in direction Why can t we notice the effect of reaction force Earth being pulled upward? Answer: these two equal forces act on objects with very different masses 7

8 Equal Forces, Unequal Effects Newton s Second Law: a large mass will accelerate less than a small mass when you apply the same amount of force Same amount of force (G-force) acts on ball and Earth, but Earth s mass is much larger Earth s acceleration is much smaller than ball Earth does move upward, but this acceleration is so small it is impossible to measure Summary of Newton s 3 rd Law Forces occur in pairs made up of an action force and a reaction force Action and reaction forces are equal in size, but opposite in direction Action and reaction forces act on different objects Equal forces acting on different objects may have different effects Determining the Effects on Forces You can use 3 rd Law to determine how an object will move Staying with the soccer ball (why not, best sport) You can measure its weight (equal to G-force) action force Once you now the weight of the ball, you know the size of the reaction force acting on Earth If you know Earth s mass, you can calculate Earth s acceleration using Newton s 2 nd law If you don t know Earth s mass, you can still predict the object s motion using quantity called momentum What is Momentum? Momentum property of all moving objects Momentum of an object moving in a straight line is equal to its mass multiplied by its velocity Momentum is represented by the variable p Momentum = mass x velocity P = mv SI unit for momentum is kg x m/s Momentum has both size and direction Object s velocity and momentum are in same direction Effects of Velocity and Mass on Momentum How does velocity of object affect its momentum? Mass and velocity are multiplied together to calculate momentum As object s velocity increases, momentum also increases Fast moving car has more momentum than slow moving car How does mass affect momentum? If velocity stays the same, increasing mass also increases momentum Tractor trailer has more momentum than sports car at same velocity Calculating Momentum Bowling ball rolling toward bowling pins. The ball has a mass of 6.00 kg. Its velocity is 10.0 m/s down the alley. What is the ball s momentum? Step 1: list given and unknown Mass (m) = 6.00 kg Velocity (v) = 10.0 m/s down the alley Unknown = momentum, (p) Step 2: write equation for momentum P = mv Step 3: insert known values, solve for unknown P = 6.00 kg x 10.0 m/s down the alley P = 60.0 kg x m/s down the alley 8

9 Force and Momentum Changes 1 st law: object s motion will not change until unbalanced for acts on it Applying force to moving object, motion changes If velocity changes, momentum also changes If driver applies brakes, car slows down Velocity changes Therefore, momentum also changes Force and Momentum Changes If you give an object more time to change its momentum, you will need to use less force to make that change If you move your arm back when you catch a baseball, ball s momentum has more time to change Your hand uses less force to change ball s momentum Hand feels less of a sting What Happens to Momentum When Objects Collide? You can use law of conservation of momentum to predict how objects will move after they collide Law states that the total amount of momentum in an isolated system is conserved Isolated system is a group of objects that does not gain or lose mass or energy to its environment Cue ball rolling across table to other billiard balls All the pool balls are almost an isolated system They do not gain or lose mass to environment May lose a small amount of energy to friction, but the amount is generally small enough to ignore Objects Colliding Conserved means kept the same Amount of momentum in isolated system stays the same no matter what happens within the system When cue ball hits other balls they move Before collision, the other balls are not moving (no momentum) Therefore, the total momentum in the system is equal to the momentum of the cue ball When cue ball hits another ball, it passes its momentum to the other ball After collision, all the other balls have different momentum However, their total momentum is the same as it was before the collision Math Day Thoroughbred horses are among the fastest horses in the world and are used in famous racing events such as the Kentucky Derby. The mass of a thoroughbred is about kg. If a horse with this mass is galloping with a momentum of kg m/s, what is its speed? Math Day Although larger than the Atlantic walrus, the Pacific walrus can swim with a speed of about 9.7 m/s. If the momentum of a swimming walrus is kg m/s, what is its mass? 9

10 Math Day The Shinkansen, Japan s high-speed bullet train, consists of several different versions of trains. The 100-series trains consist of 16 steel cars that have a combined mass of kg. The top speed of a regular 100-series train is 220 km/h. What would be the momentum of one of these trains? 10