Kinematics: how an object moves… what is its velocity, displacement, etc.

2. Kinematics: how an object moves… what is its velocity, displacement, etc. Dynamics …why an object moves the way it does

3. Aristotle If something is moving, then something MUST be pushing on it. Things don’t move all by themselves!

4. Oh, baloney! Once an object is moving in a straight line, it’s going to keep right on moving, unless it runs into something. That is, of course, as long as there is NO friction! Friction is what slows things down. Galileo

5. Galileo… Born in Pisa 1564 Remember... Galileo found that, ignoring the effect of the air, all freely falling objects had the SAME acceleration, regardless of their mass or the height from which they were dropped. The Law of Falling Bodies

6. Galileo described HOW things moved, but not WHY they moved that way. Galileo wrote, “the present does not seem to be the proper time to investigate the cause of the acceleration of natural motion….” Galileo died in 1642. When Isaac Newton, born Christmas day, 1642, began his studies of motion in the second half of the seventeenth century, that statement was no longer appropriate.

7. Because Galileo had been so effective in describing motion, Newton could turn his attention to dynamics. Dynamics is the study of why an object moves the way it does- why it starts to move instead of remaining at rest, why it speeds up or moves on a curved path, and why it comes to a stop. Newton’s most famous book was “Principia”, published in 1687. This book first listed what came to be known as Newton’s Three Laws of Motion

8. Forces are vectors. ( a push or pull is always in some direction!) The unit for Force is the Newton, N If someone was pushing on you with one Newton of Force, it wouldn’t hurt much. One Newton of Force is about the same as the weight of a quarter pound hamburger patty.

9. There are four fundamental forces, often called “interactions”, between the elementary particles (protons, electrons, etc.): • The strong force • The weak force • Gravitation • Electromagnetism All of these interactions are non-contact. In this course, we will really only deal with the gravitational force.

10. The strongest of these forces is…. The Strong Nuclear Force (it holds the nucleus of all atoms together, but only acts over a very limited distance range) The weakest of these forces is…. Gravity (but it acts over an infinite distance range)

11. Inertia Inertia: an object’s resistance to a change in its motion Mass: a way to measure inertia unit: kg

12. Newton’s First Law of Motion:“The Law of Inertia” An object at rest remains at rest unless a net external force acts on it. An object in motion continues that motion unless a net external force acts on it.

13. Newton’s First Law of Motion:“The Law of Inertia” An object at rest remains at rest unless a net external force acts on it. An object in motion continues that motion unless a net external force acts on it. Newton’s law of inertia confirmed what Galileo concluded: Once an object is moving, it requires no additional force to keep it moving. It will continue to move in a straight line unless a NET force acts upon it. I knew it first!!

14. Newton’s First LawAn object in motion continues that motion unless a net external force acts on it. If an object is moving, but not accelerating, the NET force acting on it is ZERO. (constant velocity no acceleration zero net force)

15. Second Law If there is a net external force acting on an object, it will accelerate... Newton’s Second Law is expressed as an equation: (I call this “the granddaddy of all physics equations”!)

17. Same Force on different mass….

18. Third Law For every force, there is an equal but opposite force. Action / Reaction forces

19. For every force, there is an equal but opposite force. Action / Reaction forces

20. For every force, there is an equal but opposite force. Action / Reaction forces

21. Action Force: Balloon pushes air downward Reaction Force: Air pushes balloon upward

22. The gun pushes on the bullet. The bullet pushes back on the gun!

24. The Action / Reaction forces do NOT act on the same object. I kick the wall (Action) The wall kicks me back (Reaction)

25. While driving down the road, an unfortunate bug strikes the windshield of a bus. • The bug hit the bus and the bus hit the bug. • Which of the two forces is greater: the force on the bug or the force on the bus? For every force, there is an EQUAL by opposite force!

26. Newton’s Three Laws • An object at rest remains at rest and an object in motion continues that motion unless acted upon by a net external force. • Fnet = mass x acceleration • For every force, there is an equal but opposite force.

27. Fnet = ma A child pulls on a 5.0 kg wagon with a net force of 20.0 N. What is the wagon’s acceleration? m = 5.0 kg Fnet = 20.0 N a = Fnet / m a = 20.0 N / 5.0 kg = a = 4 m/s2

28. Fnet = ma A dog pulling a man on a sled over ice can accelerate the sled at 2 m/s2. If the sled and man have a combined mass of 150.0 kg, what was the net force on the sled? m = 150.0 kg a = 2 m/s2 Fnet = ma Fnet = 150.0 kg x 2 m/s2 = 300 N

30. Fnet = ma When released, the lift provides a net force of 6 N on a 2 kg balloon. If it started at rest, how fast will it be moving in 4.0 s? Fnet = 6 N m = 2 kg vo = 0 t = 4 s vf = ? vf = vo + at, so we need “a”. a = Fnet / m a = 6 N / 2 kg = 3 m/s2 vf = 0 + 3 m/s2 x 4 s vf = 12 m/s

31. Fnet = ma A 1500 kg. car starts from rest and is moving at 10 m/s after 5.0 s. What was the net force on the car? m = 1500 kg Vo = 0 Vf = 10 m/s t = 5.0 s Fnet = ? Fnet = ma, so we need to find the acceleration. vf = vo + at a = ( vf – vo) / t a = (10 m/s – 0) / 5 s = 2 m/s2 Fnet = ma Fnet= 1500 kg x 2 m/s2 Fnet = 3000 N

32. Weight Weight, Wt. is the gravitational force acting on an object Your weight is determined by both your mass and the strength of the gravitational field (the acceleration due to gravity, “g”) Weight = mass x acceleration due to gravity Wt. = mg Since weight is a force, it is measured in Newtons, N Remember, “g” on Earth is 9.8 m/s2

33. Weight = mg What is the weight of a 42 kg child on Earth? Wt. = mg m = 42 kg Wt. = 42 kg x 9.8 m/s2 Wt. = 411.6 N

34. Weight = mg What is the mass of a 15000 N car on Earth? Wt. = mg m = Wt. / g m = 15000 N / 9.8 m/s2 m = 1530.6 kg

35. Weight = mg What is the gravitational field strength on a planet if a 90 kg rock weighs 1404 N? mass = 90 kg Weight = 1404 N g = ?

36. Remember, one Newton is not a very big force (about the same as a ¼ pound). So, your weight in Newtons is MUCH bigger than your weight in pounds! In fact, you would have to multiply your weight in pounds by 4.45 to get your weight in Newtons. How much do you weigh in Newton’s?

37. Even if you weigh 550 Newtons, You still wouldn’t be much of a Sumo Wrestler! (that’s only around 120 lbs)

38. “Net” Force Fnet = ma

39. Fnet = ma More than one force can act on an object at the same time. For example, two people could push on a book at the same time. One person could push toward the left and the other could push toward the right. In this case the two forces would act against each other. Since Newton’s Law requires NET force, what is the NET force? 2 N towards the left If the book had a mass of ½ kg, what would be its acceleration? a = Fnet / m a = 2 N / .5 kg a = 4 m/s2 (left)

40. Fnet = ma What if the opposing forces were equal? What is the net Force? What is the acceleration? If the forces are “balanced” forces, the net Force is zero and there will be NO acceleration!

41. Fnet = ma What if the forces were in the same direction? What is the net force? If the book had a mass of 2 kg, what is its acceleration? a = Fnet / m a = 14 N / 2 kg = 7 m/s2

42. 3 kg 6N 18N 15N Free Body Diagrams Free-body diagrams are pictures used to show the relative magnitude and direction of all forces acting upon an object in a given situation. These diagrams are often used in physics. The length of the arrow in a free-body diagram sometimes is used to represent the magnitude (size) of the force. The direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labeled to indicate the exact type of force. The object itself is either drawn as a box or squeezed down to a dot. The force arrows are always drawn pointing away from the center of the box.

43. Examples of Free-Body Diagrams • What are the unknown forces for the given net force?

44. Forces on an Airplane:if the forces are not “balanced”, there will be an acceleration! When Lift is larger than Weight, the plane will…. When Weight is larger than Lift, the plane will… When Thrust is larger than Drag, the plan will…. When Drag is larger than Thrust, the plane will…. When Thrust = Drag, the plane will… When Lift = Weight, the plane will….

45. + We usually label forces as negative or positive. Forces upward are positive. Forces downward are negative. Forces to the right are positive. Forces to the left are negative - + -

46. 3 kg 6N 18N Fnet = ma What is the net Force? - 6 N + 18 N = + 12 N What is the acceleration? a = Fnet / m 12 N / 3 kg = 4 m/s2

47. 3 kg 6N 18N 15N What is the net Force? - 6 N – 15 N + 18 N = - 3 N What is the acceleration? a = Fnet / m a = - 3 N / 3 kg = - 1 m/s2 Fnet = ma

48. Fnet = ma A parachute provides a lift force of 400 N on a parachutist that weighs 500 N. What is the net force on him? Use g = 10 m/s2 - 100 N What is his acceleration? a = Fnet / m What is his mass? a = -100 N / 50 kg a = - 2 m/s2 Parachute Weight

49. Tension Tension, T, is the force that cables, ropes, and strings pull with.

50. Fnet = ma A child pulls up on a string that is holding 2 fish of total mass 5 kg. If he is providing a tension of 60 N, what is the net force on the fish? Fnet = Tension – Weight Fnet = 60 N – 50 N Fnet = 10 N What is the acceleration of the fish? a = Fnet / m a = 10 N / 5 kg a = 2 m/s2