NEWTON S LAWS OF MOTION. Chapter 2: pages Review questions 1, 5-10, 14, 17, 21-24, 30

Transcription

1 NEWTON S LAWS OF MOTION Chapter 2: pages Review questions 1, 5-10, 14, 17, 21-24, 30

2 Sir Isaac Newton Born began individual studies Proved universal gravitation Invented the Calculus Reflector telescope 1672 Mathematical Principles of Natural Philosophy ( Principia )

3 First Law of Motion Every object continues in its state t of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. CONTINUES = INERTIA

4 Mass How much matter Measure of inertia

5 Inertia

6 Bird Inertia

7 Coin Inertia

8 Second Law of Motion The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. Force Means acceleration ~ mass ~ (say is proportional to )

9 Second Law F a = m Acceleration is directly Acceleration is inversely proportional to force proportional to mass

10 Increase forces Force on brick creates acceleration Twice force on brick creates more acceleration Double mass of bricks requires double force to push

11 Acceleration equation a = F m If there is double the force Also need to double the mass To maintain i constant t acceleration Direct proportion constant ratio

12 Acceleration Δv t Change in velocity over time Definition of acceleration

13 Acceleration Change in velocity over time a = F m a = Δv t Force over mass AND velocity over time? How can this be?

14 Acceleration Force causes acceleration Force over mass F a = m Solve for the force: F = ma

15 Freely falling objects Acceleration of gravity is 9.81 m 2 s Use g, a constant a = F m m 9.81 F 2 s g = For lecture calculations m round dto 10 m 2 s

16 Weight is a Force Function of g (acceleration of gravity) Proper units: Gravity m 2 s Mass Force kg kg m 2 s Different planet different weight for the same mass F g = m mg = F w

17 Weight is a Force Wih Weight Wiht Weight = mg g = m

18 Increasing mass (Neglect air resistance in this example)

19 Galileo s investigation of motion Surface area changes air resistance Objects reach terminal velocity due to air resistance In vacuum, this is not a factor

20 Freely falling objects Boulder vs. feather F a = m a = F m m Boulder has more inertia, but not more acceleration so how come it falls faster?

21 Air Resistance Acceleration is less than g due to air resistance Friction of air against falling object Air resistance depends on Speed Frontal area exposed to air

22 Air Resistance Falling object has constant t mass, constant t weight Terminal velocity reached when air resistance matches weight Air resistance function of speed and area Falling object has variable frontal area if you deploy a parachute

23 Air Resistance Parachute increases frontal area, increases air resistanc Increased air resistance balanced with slower speed

24 Air Resistance Greater air resistance for elephant, because it is larger Greater weight because it is more massive More speed required to gain air resistance to overcome the greater weight

25 Air Resistance Force of air resistance balances greater mass at greater speed Heavier skydiver has greater terminal velocity than lightweight skydiver

26 Zero Acceleration One case: Motionless objects (no change in velocity) Downward force created by ygravity Upward force created by surface

27 Zero Acceleration Another case: Cart crossing room at constant velocity (no change in velocity) Net force is zero Force applied pushing force = frictional force

28 Zero Acceleration Push down on spring Spring pushes up on you Each molecule of table acts like microscopic spring pushinguponobject up object

29 Friction Works against forces Opposite direction Not dependent on speed Not dependent on area of contact Only dependent on weight

30 Third Law of Motion Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Force is an interaction between objects Action reaction pairs

31 Force interaction F = a F a = m m F = a m Forces in action-reaction pair are equal Different masses must have different accelerations

32 Action reaction pairs Force on object moves it Force by object acts on other things Always equal

33 Action reaction pair Hammer exerts force on nail Nail exerts equal force on hammer

34 Action reaction pairs Consider firing a cannon F Force on cannon and on cannonball the same cannonball has less mass than cannon Cannonball has greater acceleration F m =a F m =

35 Action reaction pairs Rifle has less acceleration than the bullet Because it has greater mass Forces are the same

36 Action reaction pairs Rocket accelerates Rocket accelerates upward Recoil from exhaust gas

37 The force that propels a rocket is provided by 1. gravity. 2. its exhaust gases. 3. Newton s laws of motion. 4. the atmosphere against which the rocket pushes. 57% 19% 17% 7%

38 Action reaction pairs Birds push down on air Air pushes up on bird Fish pushes backward on water Water pushes forward on fish

39 Action reaction pairs Forces of atoms within objects are in action reaction pairs No net acceleration due to these molecular forces External force needed to move object

40 Summary of laws of motion Newton s First Law of Motion Object at rest tends to remain at rest Objects in motion tend to remain moving Law of Inertia Function of mass of object Changesinmotionoccurduetopresence Changes in motion occur due to presence of net force acting on object

41 Summary of laws of motion Newton s Second Law of Motion Acceleration proportional to net force F a m

42 Summary of laws of motion Newton s Third Law of Motion Objects exert equal and opposite forces upon one another Action reaction pairs have no net force