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Chapter 4 -Newton’s 2 nd Law

Chapter 4 -Newton’s 2 nd Law. Force Causes Acceleration. We learned w/ the 1 st Law that objects do not like changes in their motion… if a net force is applied to an object the motion of an object will change… ‘changing motion’ basically means the same thing as Acceleration

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Chapter 4 -Newton’s 2 nd Law

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  1. Chapter 4 -Newton’s 2nd Law

  2. Force Causes Acceleration • We learned w/ the 1st Law that objects do not like changes in their motion… • if a net force is applied to an object the motion of an object will change… • ‘changing motion’ basically means the same thing as Acceleration • So then we can say…. ‘force causes acceleration’

  3. Friction • Force Caused by irregularities between surfaces that are in contact with each other • Always directed opposite motion • Rolling Friction<Sliding Friction < Static Friction • Air Resistance is another kind of friction

  4. Amount of Friction… • Depends on • Weight of the object • How rough or smooth the surfaces in contact are • Does not depend on • Surface area of object • Speed of the object

  5. Mass & Weight • Mass (m) • How much matter is in an object • A measure of an object’s inertia • Mass never changes depending on location • Same everywhere • Weight (Fg) • How much gravity is pulling on an object • Aka… The FORCE of gravity acting on an object • Depends on how strong gravity is • can change from place to place….

  6. How to determine an object’s weight? • Mass and weight are always directly proportional • We find weight by using the equation • Weight = mass x acc. of gravity • Or.. Fg = mg • g = 10m/s2 • an object has a mass of 1 kg has a weight of be 10Newtons. • The “Newton” is the standard unit for force, and since Weight is a force, the Newton is used here. • 1 N = 1 kg*m/s2

  7. Mass resists acceleration • All forces do not accelerate all objects equally • Objects with more mass are more difficult to accelerate, and thus require more force

  8. Newton’s 2nd Law • ‘The acceleration of an object is directly proportional to the net force on it, and inversely proportional to its mass’ • This means…. The larger the force, the larger the acceleration • AND …. The larger the mass, the smaller the acceleration • This can be summed up w/ the equation • ∑F=ma • a acceleration (m/s2); ∑FNet Force (N); mmass (kg)

  9. Newton’s Second Law • Q - When 2 kg stone is thrown it is given an acceleration of 20 m/s2. What is the net force applied to the ball? • A - Fnet = ma = (2 kg) x (20 m/s2)=40 N • Q- What is the weight of an object with a mass of 70 kg? • A- Fg = mg = 70kg x 10 m/s2= 700 N • Q- What is the mass of an object that has a weight of 1000 N? • A- Fg=mg so m = Fg/g = 1000N/ 10m/s2= 100 Kg

  10. Free Fall Acceleration – Why don’t heavier objects fall faster? • Newton’s 2nd says that the more force an object has on it the faster it should accelerate….. So then if heavier objects have a larger weight (force of gravity), why do they not accelerate faster? • Ans. True, heavier objects have more force, but they also have more mass, which means that it requires more force to accelerate as fast as lighter objects…. • Essentially…. The larger force and larger mass cancel each other out • All falling objects accelerate downward at 9.8 m/s2

  11. Air Resistance • Depends on 2 things… • Frontal area of the falling object • How fast object is moving • There is a point where a falling object is going so fast the air resistance becomes so strong that the object can no longer accelerate….. This speed is called terminal speed • Object is still falling, but no longer accelerating • In most problems we ignore any effects of air resistance Non FreeFall

  12. Heavier parachutist must fall faster in order for air resistance to cancel out his weight

  13. Kittinger- 1960 • Video • Failed attempts • Failed Skydive Attempt • Skydiver • RedBullStratos Project • Red Bull Stratus Project • http://www.space.com/7824-skydiver-plans-supersonic-jump-edge-space.html • http://spacejump.co.uk/history.htm • http://www.popularmechanics.com/technology/aviation/safety/space-dive-the-race-to-jump-from-120000-feet • Other project details • Videos Skydive / Free Fall Record

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