FORCES

1. FORCES • Inertia, Mass, Weight • Newton’s Law • Types of Forces • Free-body diagram • Friction

2. Focused Learning Target • By  analyzing  data , SWBAT support the claim that Newton’s second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration. • Performance Expectations : • I can analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship among the net force on a macroscopic object , its mass, and its acceleration • Learning Targets : • I can use Newton’s second law to accurately predict changes in motion of objects .

3. What changes Motion ? Tug of War NGSS Paper

4. FORCE • Push/pull • Causes an object to accelerate or change its velocity. • Net force: sum of all forces • Two types of forces: a) Contact forces b) Field forces • Gravitational …objects • Electromagnetic…Charges • Nuclear…subatomic particle • Weak…radioactive decay • Vector

5. Inertia, Mass, and Weight • Inertia • Resistance to change in its state of motion • Depends on mass • Example • Pushing heavy things • When moving…difficult to stop • Seatbelts • Galileo and inertia • Incline plane: initial height = final height

6. Inertia, Mass, and Weight • Mass • Amount of matter it has • Measures inertia • Units: kg • Remains constant • Weight • Changes with gravity • Measures force • Units: Newton

7. Experiment : Mass VS Weight

8. What Changes Motion ? • Provide a scientific explanation of the phenomenon you just observed, to an intelligent person not familiar with the science. Feel to draw, list graph, tabulate, write equation /formula or build flow chart as needed to explain the core scientific concepts in a clear and concise way.

9. Newton’s First Law • Law of inertia • Two parts • Object at rest, remains at rest • Object in motion, remains in motion • Unless an unbalance force acts upon it • No force needed to keep an object in motion • Resists acceleration

10. Newton’s First Law • Contradicted: • Aristotle: all objects have natural tendency come to rest • Newton: all objects come to rest due to friction; w/o friction object continues to move • Balanced and unbalanced forces

11. Newton’s First Law http://www.physicsclassroom.com/Class/newtlaws/U2L1a.html

12. Mass Mass is the measure of inertia of an object. In the SI system, mass is measured in kilograms. Mass is not weight: Mass is a property of an object. Weight is the force exerted on that object by gravity. If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.

13. Newton’s First Law

14. What changes Motion What determines the object’s acceleration ?

15. Newton’s Second Law • A net force or unbalanced forces causes an object to accelerate in the direction of the force. • Acceleration is directly proportional to the force

16. Newton’s Second Law of Motion Force is a vector, so is true along each coordinate axis. The unit of force in the SI system is the newton (N). Note that the pound is a unit of force, not of mass, and can therefore be equated to newtons but not to kilograms.

17. Newton’s Second Law • Acceleration is inversely proportional to the mass • Fnet = ma • Units: Newton (N) = kg·m / s2 • Vector sum of all forces • FORCES CAUSE ACCELERATION!!

18. Newton’s Second Law Equilibrium means “Zero” acceleration

19. Newton’s Second Law • Free Fall • All objects will fall with the same rate of acceleration, regardless of their mass • Air Resistance • Decreases the acceleration • Depends on cross-sectional area and speed • Terminal Velocity • Weight equals force due to air resistance; a = 0 m/s/s

20. Newton’s Second Law

21. NEWTON’S SECOND LAW

22. NEWTON’S SECOND LAW

23. Problems: Pg 92 • 1) When a shot-putter exerts a net force of 140 N on a shot, the shot has an acceleration of 19 m/s/s. What is the mass of the shot? • 7.4 kg • 2) Together a motorbike and rider have a mass of 275 kg. The motorbike is slowed down with an acceleration of –4.50 m/s/s. What is the net force on the motorbike? Describe the direction of this force and the meaning of the negative sign. • - 1.24 x 103 N; direction of force is in opposite direction of the velocity.

24. Newton’s Second Law of Motion What is Newton’s 2nd Law of Motion? Express this law in an equation. What is the unit of Force ? What is the equivalent of Newton ?

25. F = ma F is Force in newtons (N) m is mass in kilograms (kg) a is acceleration in m/s2 FormulaForce = m a Units  Newtons = kg m /s2

26. Manipulation of Force Formula F = m X aa = m = W = m X g g = m =

27. Classwork 30pts: Changes in mass, net force and acceleration Explain what happens when Net force,mass and acceleration changes .Fill in the table with data and compare each row .

28. Problems: Pg 92 • 3) A car, mass 1225 kg, traveling at 105 km/h, slows to a stop in 53 m. What is the size and direction of the force that acted on the car? What provided the force? • - 9.8 x 103 N; road surface pushing against the car tires • 4) Imagine a spider with mass 7.0 x 10-5 kg moving downward on its thread. The thread exerts a force that results in a net upward force on the spider of 1.2 x 10-4 N. a) What is the acceleration of the spider? b) Explain the sign of the velocity and describe in words how the thread changes the velocity of the spider. • 1.7 m/s/s

29. Experiment :Forces Phet simulation Lab

30. Newton’s Third Law • Two different objects • Action-reaction pair • For every action force there is a equal and opposite reaction force • F object A on object B = - F object B on object A

31. Newton’s Third Law

32. Newton’s Third Law

33. Newton’s Third Law

34. 4-4 Newton’s Second Law of Motion What average net force is required to bring a 1500 kg car to rest from a speed of 100 km/h within a distance of 55 m?

35. I do Ex 4.1 p 109 a) what is the a? b) what is the a if friction is 140N c) how far did it travel from rest after 4 sec if the acceleration remains the same . Ex 4.2 p 110

36. Princeton ReviewWe Do and You do Ex3.1 Ex3.2 Ex3.3 Ex 3.4 Ex3.5 Ex3.6

37. Ex 4.3 Text p 110-112 Ex 4.4 Text p 112-113

38. 4-5 Newton’s Third Law of Motion Any time a force is exerted on an object, that force is caused by another object. Newton’s third law: Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.

39. Notes 4 : Newton’s 3rd Law of Motion: Law of Action and Reaction I. What are the 2 things reacting in the balloon race? 2. What force caused the balloon to move forward ? 3. Draw the diagram with the balloon moving forward and the direction of air exit .

40. 4-5 Newton’s Third Law of Motion A key to the correct application of the third law is that the action and reaction forces are exerted on different objects. Make sure you don’t use them as if they were acting on the same object.

41. Newton’s 3rd Law of Motion Balloon Race Class work : Quick Write “ What Provide a scientific explanation of the phenomenon you just observed, to an intelligent person not familiar with the science. Feel to draw, list graph, tabulate, write equation /formula or build flow chart as needed to explain the core scientific concepts in a clear and concise way.

42. 4-5 Newton’s Third Law of Motion Rocket propulsion can also be explained using Newton’s third law: hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket. Note that the rocket does not need anything, such as air, to “push” against. If it did rockets would not operate in space.

43. 1. What is Newton’s 3rd Law of Motion? 2. What are the 2 things reacting in the movement of the swimmer 3. Draw and explain the movement of the rocket.

44. 4-5 Newton’s Third Law of Motion What exerts the force on a car? What makes a car go forward?

45. 4-5 Newton’s Third Law of Motion Helpful notation: the first subscript is the object that the force is being exerted on; the second is the source. This need not be done indefinitely, but is a good idea until you get used to dealing with these forces. (4-2)

46. 4-5 Newton’s Third Law of Motion We tend to associate forces with active objects such as humans, engines, or a moving object like a hammer. However, inanimate objects at rest can exert a force due to elasticity. A force influences the motion of an object only when it is applied on that object. A force exerted by an object does not influence that same object. It only influences the other object on which it is exerted.

47. What is a Normal Force ? Spring measurer Weights

48. 4-6 Weight – the Force of Gravity; and the Normal Force Weight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight is: What do the bold letters mean? On Earth a mass of 1.0 kg weighs about 2.2 lb

49. 4-6 Weight – the Force of Gravity; and the Normal Force An object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there? The force exerted perpendicular to a surface is called the normal force. It is exactly as large as needed to balance the force from the object (if the required force gets too big, something breaks!)

50. 4-6 Weight – the Force of Gravity; and the Normal Force Weight and Normal Force are NOT action-reaction pairs. What is the reaction force to Weight and the Normal Force?