Lecture5- Newton s Laws Chapter 5

Transcription

1 1 / 45 Lecture5- Newton s Laws Chapter 5 Instructor: Prof. Noronha-Hostler Course Administrator: Prof. Roy Montalvo PHY-123 ANALYTICAL PHYSICS IA Phys- 123 Oct. 6 th, 2018

2 2 / 45 Nobel Prize in Physics Announced!

3 3 / 45 First Women Physics Nobel Laureate in 55 years!

4 4 / 45 Lasers as minature tools transforming laser light into miniature tools "optical tweezers" to manipulate microscopic objects including viruses and bacteria enabling manufacturers to drill tiny, precise holes allowing for the invention of Lasik eye surgery See more

5 5 / 45 Tutoring The Rutgers Learning Centers offers free peer tutoring, which might be appropriate for intro classes: Our department maintains a list of graduate students who are available for tutoring (for a fee): keeton/ugrad/2018- tutors.pdf Both are linked under "Tutoring" on the undergraduate program webpage:

6 6 / 45 Test No notes. Equations will be provided Simple calculators allowed (NO phones, scientific calculators etc). No. 2 pencils. Fill in the bubbles well. No talking, no helping. Ask a proctor. Don t panic and carry a calculator.

7 7 / 45 Objectives Revist Frame of Reference Understand Forces Newton s Laws Internal vs. External forces Equilibrium Examples of forces

8 Frame of reference 1D Previous slides used: Motion described in FIXED reference frame (coordinate system) However: Nothing prevents us from choosing another frame moving at constant velocity v = const 8 / 45

9 9 / 45 Frame of reference- position Coordinate in v = const frame x PA = x PB + x BA

10 10 / 45 Frame of reference- velocity Velocity in v = const frame d dt (x PA) = d dt (x PB) + d dt (x BA)

11 11 / 45 Frame of reference- velocity Velocity in v = const frame v PA = v PB + v BA

12 Frame of reference- acceleration Acceleration in v = const frame d dt (a PA) = d dt (a PB) + d dt (a BA) }{{} =0 12 / 45

13 13 / 45 Frame of reference- acceleration Acceleration in v = const frame a PA = a PB

14 14 / 45 2d Relative Motion Frame B is has a velocity of v BA = const relative to Frame A.

15 15 / 45 2d Relative Motion Position r PA = r PB + r BA Velocity v PA = v PB + v BA Acceleratoin a PA = a PB v BA = const, so d v BA /dt = 0

16 Dog running up the aisle in a airplane From the perspective of the bird, how fast is the dog running? A.) ω B.) v C.) v-w D.) u+v-w E.) u+v 16 / 45

17 Dog running up the aisle in a airplane From the perspective of the bird, how fast is the dog running? A.) ω B.) v C.) v-w D.)u + v w E.) u+v 17 / 45

18 18 / 45 What are forces?

19 19 / 45 What are forces? forces change an objects velocity i.e. accerlate Forces are vectors object Newton s laws

20 20 / 45 Forces as vectors 2+ Forces acting on a body Vector addition! The net force is the sum of all external forces acting on a body. F net = F 1 + F

21 Newton s first law (demo) Newton s first law an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. Friction is an external force that slows down moving objects! 21 / 45

22 22 / 45 Types

23 23 / 45 Intertial Reference Frame Newton s 1st law does not hold in all reference frames. In the reference frame of a car (noninertial), coffee could spill if the car itself accelerates.

24 Newton s second law (demo) Newton s second law the net force on a body is equal to the product of the body s mass and its acceleration. F = m a Indentify the body vs. external forces. Only external forces acting on the body (not on something else!). Separate Axes. These are vector quantities, so F net,x = ma x, F net,y = ma y, F net,z = ma z 24 / 45

25 25 / 45 Units Newton (N) Newton (N)= 1 kg m/s 2 the acceleration a force would give to 1 kg.

26 26 / 45 Can you pick up a bowling ball by this string? A.) No, it will break 100% of the time. B.) Yes, you can always pick up a bowling ball by this string without breaking. C.) Sometimes. D.) Only if the string were more massive than the bowling ball. E.) Only on the moon.

27 27 / 45 Can you pick up a bowling ball by this string? A.) No, it will break 100% of the time. B.) Yes, you can always pick up a bowling ball by this string without breaking. C.)Sometimes. D.) Only if the string were more massive than the bowling ball. E.) Only on the moon.

28 28 / 45 Forces in Equilibrium

29 29 / 45 Free body diagram Draw forces using arrows Include all the external forces Label Accurate picture of the body itself not important e.g. a cow can look like a sphere

30 30 / 45 Where will I move on the cart? A.) Forwards B.) Backwards C.) Towards the ceiling D.) Won t move E.) Towards the seats

31 31 / 45 Where will I move on the cart? A.) Forwards B.) Backwards C.) Towards the ceiling D.)Won tmove E.) Towards the seats

32 32 / 45 External vs. Internal forces System- 1 or more bodies. If there is a ridgid connection between them, treat as one composite body. Internal forces cannot accelerate the system Only forces outside of the system can accelerate it

33 33 / 45 Where will I move on the cart? A.) Forwards B.) Backwards C.) Towards the ceiling D.) Won t move E.) Towards the seats

34 34 / 45 Where will I move on the cart? A.)Forwards B.) Backwards C.) Towards the ceiling D.) Won t move E.) Towards the seats

35 35 / 45 Types Friction Tension Gravity Normal Force Weight Magnetism

36 Gravity Pull directed toward a second body. Free Fall F g = m( g) or F g = mg 36 / 45

37 37 / 45 Gravity at rest Gravity acts on bodies in rest Free Fall F g = F g ĵ = mgĵ

38 38 / 45 Weight The weight W of a body is equal to the magnitude F g of the gravitation force on the body F net,y = ma y W F g = m 0 = 0 So, if gravity changes... W = F g

39 39 / 45 Normal Force When a body presses against a surface, the sufarce deforms and pushes on the body with a normal force F N perpendicular to the surface Free Fall F N F g = ma y F N mg = ma y F N = m(g + a y ) (1)

40 40 / 45 Tension When a cord attached to a body is pulled tight with a force T away from the body. Often assume the chord to be massless

41 41 / 45 Friction Resistance of a force attempting to slide a body over a surface

42 Newton s third law (demo) Newton s second law when two bodies interact, the forces on the bodies from eachother are always equal in magnitude and opposite in direction. 42 / 45

43 Newton s third law (demo) Normal force F N = F g This holds even if the table was moving! 43 / 45

44 If an object is moving can you conclude there are forces acting on it? If an object is at rest, can you conclude there are no forces acting on it? Consider each of the following situations. In which one of the following cases, if any, are there no forces acting on the object? a) A bolt that came loose from a satellite orbits the earth at a constant speed. b) After a gust of wind has blown through a tree, an apple falls to the ground. c) A man rests by leaning against a tall building in downtown Dallas. d) Sometime after her parachute opened, the sky diver fell toward the ground at a constant velocity. e) Forces are acting on all of the objects in choices a, b, c, and d.

45 If an object is moving can you conclude there are forces acting on it? If an object is at rest, can you conclude there are no forces acting on it? Consider each of the following situations. In which one of the following cases, if any, are there no forces acting on the object? a) A bolt that came loose from a satellite orbits the earth at a constant speed. b) After a gust of wind has blown through a tree, an apple falls to the ground. c) A man rests by leaning against a tall building in downtown Dallas. d) Sometime after her parachute opened, the sky diver fell toward the ground at a constant velocity. e) Forces are acting on all of the objects in choices a, b, c, and d.

46 44 / 45 Examples

47 45 / 45 Next Week Applications of Newton s Laws