Physical Science

1. Physical Science Force and Momentum Slides subject to change

2. Issac Newton • Isaac Newton last words:"I don't know what I may seem to the world. But as to myself I seem to have been only like a boy playing on the seashore and diverting myself now and then finding a smoother pebble or a prettier shell than the ordinary, whilst the great ocean of truth lay all undiscovered before me.

3. Force

4. Net Force • A net external force on an object is required to move the object. Motion results when the forces are unbalanced. • Another small external force. • One large external force

5. Aristotle Said ... • Objects require force to keep moving. • An object’s natural state is to be at rest.

6. Newton Said ... • Objects do NOT require force to keep moving. • An object’s natural state is to keep doing what it is doing – unless an unbalanced force acts on it. A sliding object - with no friction, will keep moving in a straight line.

7. Newton’s First Law 1An object will continue to be in a state of rest or of uniform velocity (speed and straight line direction) unless acted upon by an external, unbalanced force. • Objects tend to "keep on doing what they're doing.“ • Objects resist changes in their state of motion.

8. Inertia • This tendency to resist changes in state of motion is described as inertia. • Mass is the measure of inertia. • Units kilograms.

9. Mass • Which vehicle has more inertia? 2900 kg 730 kg Mercedes-Benz Smart Car Hummer H2 SUV

10. Newton’s Second Law: Force 2 The acceleration of mass m is directly proportional to the unbalanced force F. • The greater the force, the more acceleration. Mathematically, F = ma

11. Units of Force • Units of force in the SI system are newtons (N). • 1 N = (1 kg)(1 m/s2). • One newton is the force required to accelerate one kilogram one meter/s2. acceleration units mass units

12. Force is a Vector • Direction is important. F1 = + 8.0 N (to right) F2 = – 4.0 N (to left) m=2.0 kg F2 F1 + −

13. Find the Net Force Given Formula m = 2.0kg Fnet = Sum of forces F1 = + 8.0 N (to right) F2 = – 4.0 N (to left) Net force = Fnet = +8.0 N – 4.0 N = + 4.0 N

14. Find the Acceleration Given Formula m = 2.0kg Fnet = ma Fnet= + 4.0 N From Newton Fnet = ma 4.0 = 2.0a a = 2.0 m/s2 Note, the answer is positive, the mass accelerates to the right at 2.0 m/s2.

15. Force What is the acceleration when F = + 750 N (to right) Mcar = 1,050 kg Mtrailer = 325 kg + − F

16. Force Given Formula F = + 750 N (to right) F = ma Mcar = 1,050 kg Mtrailer = 325 kg Mtotal = Mcar + Mtrailer = 1,375 kg F = Ma = + 750 = 1,375a a = 0.55 m/s2, acceleration is to the right.

17. Newton’s Third Law 3 Whenever two bodies experience an interaction, the force of the first body on the second is equal and opposite to the force of the second body on the first. • “For every action there is an equal but opposite reaction.”

18. Equal but Opposite • She pushes on wall. • Wall pushes on her. • Equal and opposite.

20. Dragster Seat Action Reaction

21. Newton’s Law of Gravitation • Every object in the universe attracts every other object with a gravitational force: F = Gm1m2 r 2 • G is a universal constant, 6.67x10−11 N-m2/kg m1 r m2

22. Newton’s Law of Gravitation • A body of mass, m, close to the surface of the Earth is attracted as if the entire attracting mass of the Earth (assumed to be spherical) is concentrated at the center of the Earth. m R ME

23. Law of Gravitation • Appears to be a universal law. • Extends to an infinite distance. • Near the Earth’s surface, the force is called “gravitational force,” or “weight W.” W = mg m

24. Weight W = mg Weight equals mass times g It’s a measure of gravitational force

25. U. S. Units • One pound in English units is a unit of force. • The force of 2.2 lbs is equal to force of gravity on 1.0 kg of mass. 121 lbs 540 N 55 kg Mass Force, Earth Surface

26. Weight on Sun and Moon • On Sun • Gravitation = 28 g’s • 150 lbs → 4,200 lbs • On Moon • Gravitation = 1/6 g • 150 lbs → 25 lbs • Web Astronaut with Gear On Earth 430 lbs On Moon 72 lbs

28. Weight Is there a gravitational force in free fall in a skydive? On Sunday morning Oct. 13, 2012, Austrian daredevil Felix Baumgartner broke the world record for highest-ever skydive, leaping from a balloon nearly 24 miles (128k feet) above Earth’s surface.

29. Is there a gravitational force in orbit?

30. Basic Units Review

31. Centripetal Force • From Newton’s Second Law, F = ma • The force that is required to cause circular motion is centripetal force. • Centripetal force equals mass times centripetal acceleration. FC = mac = mv2/r • Directed towards the center of curvature. Fc v

32. Swing Yo-Yo • m = 45 grams • T = 1.1 s • R = 0.80 m

33. Find the Speed • T = period, time to go around once, T. • v = distance/time = 2πr/T • A yo-yo does a “round-the-world” in 1.1 s. The yo-yo is 0.80 meters long. The speed is − v = d/t = 2πR/T = 2π(0.8)/1.1 = 4.57 m/s

34. Centripetal Force • ac = v2/R = 26.1 m/s2 • FC = mac • m = 45 grams = 0.045 kg • FC = (0.045)(26.1) = 1.2 N ... this is the tension in the string

35. Swing Yo-Yo • Which way will the yo-yo fly if the string breaks when the yo-yo is at the top of its “orbit?” • Why?

36. Amusement Rides In Rotor, ride rotates, riders are pinned to the wall. What holds the riders to the wall? Rotor

37. Centrifugal Force • “Center-fleeing” force. • Newton’s equal-but-opposite reaction to centripetal force. Centrifugal: Rider against wall A Short Ride Centripetal: Wall presses on rider towards the center of rotation.

38. Another Ride

39. Gravitron • 24 rpm. • Riders experience 4 g’s. Gravitron

40. Nuclear Enrichment • Natural uranium approximately 99.3% 238U and 0.7% 235U. • 235U is lighter and fissionable. • How to make the uranium with higher percentage of 235U? • Use “centrifuges” to separate heavier U238 from lighter U235.

41. Centrifuges • The centrifuges spin very fast ~ 100,000 revolutions per minute (rpm). • More massive U-238 goes to outside, less massive U=235 forced to center of rotation, and removed.

42. In the News • BBC News: Iran unveils 'faster' uranium centrifuges. • Iran’s been hiding the enrichment program since 2003. “Only for electricity.” • “The IAEA report said 8,610 centrifuges had been installed in known enrichment facilities in Iran, of which 3,772 were operating.” 4/9/10

44. Force on Space Station Int’l Space Station altitude h = 400 km. R, distance from center of Earth: RE + h = 6,360 + 400 = 6,760 km = 6.76x106 m MISS = 2.33x105 kg ME = 6.0x1024 kg From Newton’s law of Gravitation FG = GMEMISS/R2 = 2.04x106 N ... we’ll use this later

45. Speed of Space Station Centripetal force comes from gravity. FC = FG From the centripetal force equation … FG = mv2/R = 2.04x106 N (from earlier slide) 2.04x106 = (2.33X105)v2/ (6.76x106) v = 7,700 m/s (~17,000 mi/hr)

46. Period of an Orbit v = 7,700 m/s Circumference of orbit d d = 2πR = 2 (3.14)(6.76x106) = 4.25x107 m v = d/t 7,700 = 4.25x107/ t t = 5,500 seconds = 91 minutes Track

47. Linear Momentum

48. Linear Momentum • Abbreviated with symbol “p” • A simple product of mass times velocity. p = mv • Momentum is also a vector—it’s sign (+ or – ) in straight line motion is important.

49. Momentum • A 0.45-caliber bullet (m = 0.162 kg) leaves the muzzle of a gun at 860 m/s. What is its momentum? Given Formula • m = 0.162 kg p = mv • v = 860 m/s • p = (0.162) (860) = 139 kg-m/s

50. System • Total momentum of a system is calculated by adding momentums, taking direction into account. • Conservation of Momentum: • With no unbalanced forces on the system, there is no change in total momentum. • Momentum of the system is “conserved.”