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Transcript
Chapter 3 – Newton’s Second Law of Motion-Force and Acceleration The BIG IDEA- ** 3.1 What is Acceleration? Besides developing the concepts of speed and velocity, Galileo also developed the concept of ** He found that balls rolling down inclines roll ** Also found that the speed increased by a constant amount each second – this is called ** Acceleration – def- ** Formula: ** a= V/t acceleration equals change in velocity divided by the change in time ( - delta symbol that always means ‘change in’) Acceleration can be more than an increase in speed acceleration does not just apply to speeding up. It can also apply to ** It can also be a change in speed, a change in direction or a change in both. So essentially- acceleration is a change in ** Example: in one second, we steadily increase our velocity form 30 km/h to 35 km/h. In the next second, we go from 35 km/h to 40 km/h and so on. So we are changing our velocity by 5 km/h each second. What is our acceleration? a = Δ v/ Δt a = 5 km/h 1s = 5 km/h/s or km/h s or acceleration equals five kilometers per hour per second Usually, scientists use m/s as velocity so acceleration would be m/s/s or m/s2 Note: the unit for time is entered twice- once for velocity and once for change in time. Free fall- ** The amount of acceleration is the same ** Falling objects gains speed at a rate of 10 m/s each second. a = change in speed = 10 m/s = 10 m/s2 time interval 1s Acceleration due to free falling is 10 meters per second squared. *****(It actually works out to 9.8 m/s2 which is the rate we use for gravity on this planet) Falling objects gain speed because of ** If you throw an object straight upward it will move upward for a while and then come back down. It loses 10 m/s for each second going upward and gains 10 m/s for each second moving downward. 3.2 Force Causes Acceleration Any object that is accelerating is acted upon by a force- sometimes more than one force. Acceleration depends on **. It is directly proportional to net force Acceleration ~ Net Force This means that ** The direction of acceleration is always ** 3.3 Mass is the Measure of Inertia The greater an object’s mass, the greater its ** That means it will require more force to accelerate something heavier than something lighter. If an object contains a lot of matter- heavy atoms or lots of lighter atoms- it has a lot of mass. Mass is not Volume Volume is a ** meters, liters. - it is measured in mL, cubic centimeters, cubic Mass is the ** It is measured in g, kg, etc. If an object has a large mass, it may or may not have a large volume. An extra large bag of feather will have much less mass than the same size bag of rocks! Mass is Not Weight Mass is the ** Weight depends on **. It is the force due to gravity acting on that mass. If the gravity changes so does the weight. But the mass ** Although mass and weight are different from each other, they are directly proportional to each other. Objects with large mass have large weight; objects with small mass have small weight. Kilogram vs. Newton SI unit for mass: ** SI unit for force: ** A one kilogram bag of something on this planet has a weight of 9.8N. Where the gravity is different, however, the weight will be different Kilogram vs pound: 1 kg weighs 2.2 lb. 3.4 Mass and Acceleration are Related acceleration is inversely proportional to mass. Acceleration ~ 1/mass This means that when one gets larger, the other gets smaller More mass means ** acceleration because there is more ** to changes in motion. 3.5 Newton’s Second Law ** equation: acceleration = net force/mass acceleration is equal to the net force divided by the mass a = F/m a = acceleration F = net force m = mass Example: a 1000 kg car is being pulled by a cable with 2000 N of force. What is the acceleration of the car? a = 2000 N/ 1000 kg Since 1 N = 1 kg *m/s2 a = 2000 kgm/s2 / 1000 kg = 2 m/s2 3.6 Friction def- ** Friction is a force that arises when an objects rubs against something else. Friction occurs for ** When a boat is propelled by its motor in one direction, water friction impedes its progress. When an object falls downward through the air, the force of friction (called air drag) acts ** The amount of friction between two surfaces depends on ** Read about friction in your fingerprints on page 47. There will be something on the test about this. 3.7 Objects in Free Fall accelerate at the same rate Acceleration does not depend on mass for balls rolling down an incline. They all have the same acceleration. Acceleration of free fall doesn’t depend on mass. Objects** The explanation comes from ** 3.8 Why objects in free fall accelerate at the same rate Newton’s second law tells us to consider not only the weight of the object (its force) but also its mass. All freely falling objects have the same force/mass ratio and undergo the same acceleration The acceleration due to gravity is represented by the symbol ** Consider a 10 kg boulder and a 1 kg stone The weight of a 1 kg stone (or 1 kg of anything) is 10 N. (conversion factor: 1 kg = 10 N) Therefore the weight of a 10 kg boulder is 100N Acceleration due to gravity is equal to an object’s weight divided by its mass a (g) = F / m gravity (a) of stone = 10 N / 1 kg = 10 m/s2 gravity (a) of boulder = 100 N / 10 kg = 10 m/s2 They accelerate (fall) at the same rate even though there is a difference in their size and mass What about a feather and a coin We know a feather will take longer to fall than a coin because ** BUT, if we put them both in a vacuum where air resistance isn’t a factor, they will both fall at the same rate. 3.9 Air Drag Read this section on your own and know the two factors that affect air drag as well as the definitions for terminal speed and terminal velocity We will be discussing air drag later this year Gliding pg 52 Read this page and be able to explain how gliding works and what some of the special structures are that animals have evolved that enable them to glide.