Force and Newton’s Laws of Motion

1. Force and Newton’s Laws of Motion

2. Forces Unit Problem Set • Problem Set #2: (Giancoli, chapter 4, Pages 98 - 102) • # 13, 15, 17, 19, 25, 27, 31, 37, 43, 53, 55

3. Galileo’s Inclined Planes 1. A ball rolling down an inclined plane accelerates. 2. A ball rolling up an inclined plane decelerates. 3. In the absence of friction, does a ball rolling on a level surface roll forever?

4. Galileo’s Inclined Planes What if the second inclined plane is very, very far away? A ball rolled down one incline plane will roll up a second until it reaches its initial height

5. Galileo’s Inclined Planes Ball loses speed quickly on a steep slope. Speed is lost more slowly on lesser slope. If there is no slope, does the ball ever lose its speed?

6. Newton’s First Law of Motionaka The Law of Inertia An object continues in a state of rest, or moves at a constant speed in a straight line, unless a net force acts on it.

7. In other words … a stationary object will stay put, and a moving object will keep moving at the same speed in the same direction.

8. The property of an object to resist changes in motion. Inertia: • The mass of an object is directly proportional to its inertia • High mass, high inertia • Low mass, low inertia

9. Inertia of a train A heavy freight train and a much light passenger train are moving down some track as the same speed. Which requires more distance to stop? Suppose the fright train is three times more massive than the passenger train. How much more inertia does it have? Three times as much.

12. Inertia The property of things to resist changes in motion Why does it work with heavy glass objects? Would this experiment work well using foam plates and plastic cutlery?

13. Inertia A massive pile of books, then a wooden block, is placed on a person’s head. A nail is hammered into the block. Why doesn’t this hurt? The stack of book has a lot of mass, so it also has a lot of inertia. When struck by the hammer, it resists moving … into the person’s head.

14. Scientists at NASA take advantage of the vacuum of space when planning deep-space missions With no air resistance, even a very small engine will make the spacecraft travel forever.

15. How much force is needed to keep the rocket ship moving in deep space? None. Without any opposing forces (gravity or air resistance), rockets in motion stay in motion.

16. Inertia of Moving Things The Earth moves at 30 km/s, and so do walls that are attached to the Earth. Stand next to a wall, and then jump so that your feet break contact with the ground. Does the 30 km/s wall smash into you? Why or Why not? No, You also are moving at 30 km/s before your jump, during your jump, and after your jump.

17. Inertia of Moving Things When riding in a plane or a car, does a coin flipped into the air land back in your hand? Why or why not? A coin held in your hand has the same speed that you do. The coin also has inertia, so it tends to maintain that speed, even when you are no longer holding it.

18. Inertia of Moving Things The bird sees a plump worm below its perch on the tree branch. If the Earth move through space at 30 km/s, is it possible for the bird to drop directly down from its branch and catch the worm? Yes, the bird, the worm, the tree and the earth are all moving at 30 km/s.

19. Using the Law of Inertia, explain why: • You stamp your feet to remove snow from your shoes. • You shake a rug to remove dust from it. • You can tighten the head of a hammer by pounding the handle on a hard surface.

20. Force, Mass, and Weight Force: A push or a pull The unit for force is the Newton. Definition of Newton Weight is a force that is produced by all objects that have mass. Measured in Newtons Directed downwards, toward center of the Earth Proportional to mass

21. Mass vs. Weight Mass is the amount of matter in an object. Weight is the force of gravity acting on an object with mass.

22. Mass Which has more mass: a kilograms of bricks or a kilogram of feathers? They have the same mass – 1 kilogram.

23. Forces, Mass, and Weight: Units Mass: Weight: Force: Kilograms Newtons Newtons A mass of 1 kilogram, acted on by gravity, has a weight of about 10 Newtons. 2 Pounds equals about 9 Newtons. Weight = Mass * Gravity Weight = 1 kg * 10 m/s2 W = m*g Weight = 10 kg•m/s2 Weight = 10 N

24. Mass vs. Weight What is the weight, in Newtons, of a plate with a mass of 20.4 kg? W = mg W = (20.4 kg) (9.81 m/s2) W = 200. N An athlete supports a barbell with a weight of 1580 N overhead. What is the mass of the barbell? W g W = mg  m = 1580 9.81 m = = 161 kg … 355 lbs

25. Mass/Inertia vs. Weight Weight vs. Mass

26. Mass vs. Weight Weight changes depending on the gravitational pull of the planet an object is found on. How much would 1 kg weigh on each body? Earth g = 9.81 W = 9.81 W = m*g Mercury g = 3.61 W = 3.61 Jupiter g = 26.0 W = 26.0 Moon g = 1.62 W = 1.62

27. Mass vs. Weight Weightless objects still have mass. In outer space, far from any objects that have a gravitational force, objects are weightless.

28. Weight on Other Planets The acceleration due to gravity on Mars is 3.8 m/s2. Would a 100 kg rover weigh more on Earth or Mars? W = mg W = (100)(3.8) W = 380 N W = mg W = (100)(9.81) W = 981 N

29. Weight on Other Planets What would the mass of the same rover be on Earth? On Mars? The rover has a mass of 100 kg, no matter where in the universe it is found.

30. The Normal Force A book rests motionless on a tabletop... The Earth exerts a gravitational force on the book – its weight. Because the book is not accelerating, a force with a magnitude equal to the book’s weight but opposite in direction must be present. A perpendicular contact force exerted by a surface on an object resting on that surface Normal Force:

31. The Normal Force A massive piece of construction equipment has a mass of 15,000 kg. What is the normal force exerted on the earthmover? W = mg W = 15,000*9.81 W = 147150 N The normal force is equal to 147,150 N as well.

32. Mass, Weight, and Acceleration A falling body (hardy har har) with a mass of 60 kg and a weight of 588.6 N accelerates toward the surface of some planet. What is the body’s acceleration? W = mg 588.6 = (60)g 588.6 60.0 g = = 9.81

33. Net Force The sum of the forces that act on an object Net Force: Forces produce a change in motion. In what direction is the net force? To the right.

34. Whether or not an object moves depends on the net force. If the forces sum to zero, or cancel, there is no movement. Unopposed forces produce an acceleration.

35. Net Force and Acceleration A skydiver falls directly downwards at a constant velocity. What is the skydiver’s acceleration? Zero – she is falling at a constant velocity What is the net force on the skydiver? Zero – since she is not accelerating, there must be no net force

36. Calculate the Net Force

37. Net Force A balloon with a mass of 0.01 kg (ignoring the mass of the air inside) is released. The air exiting the balloon exerts a force of 10 N on the balloon. If the balloon travels directly upwards, what is the net force on the balloon? Net Force = 10 - W Net Force = 10 – 0.01*9.81 Net Force = 9.90 N

38. Two weights are hung from strings draped over pulleys. The string are attached to either end of a spring scale, as shown below. If the weights hang motionless in the air, what is the reading on the scale?

39. Nellie Newton pushes the box with a force of 75 N. If a friction force of 75 N opposes the motion, does the box move? No, the net force is zero.

40. If the airplane’s engine can produce a thrust of 1250 N, but the air drag of 250 N opposes the motion, what is the net force on the plane? 1250 N – 250 N = 1000 N Is the plane accelerating? Yes, there is a net force acting on it.

41. System: An object of interest A force produced by contact between the system and an external object Contact Force: A force that is exerted on the system without making physical contact Field Force: Magnetism

42. Free-Body Diagrams Pictorial model used to solve motion problems. Free-body diagram: • A dot is used to represent an object • Arrows are used to represent forces • Arrow points indicate the direction of the force • Arrow tails are placed on the dot

43. Draw the free-body diagram for the lantern. Ft Ft • Define the system by circling it. • Replace object with a dot. • Identify contact forces • Identify field forces. Fg

44. An apple tows an orange around in a wagon. Given that the orange is the system, draw the free-body diagram. FNormal FTension FFriction Fgrav

45. Net Force and Acceleration A kitty cat falls at a terminal (constant) velocity. What is the acceleration of the kitty? Zero. What is the net force on the kitty? Zero.

46. The Equilibrium Rule According to Newton’s First Law, if an object is not acclerating, there must be no net force. Tension Tension 9 N - 9 N 0 N Weight Weight Net Force Because the tension force and the weight cancel, the bag remains at rest. It is in mechanical equilibrium. There are two forces acting on the bag of sugar.

47. The Equilibrium Rule The state of an object when there is no change in motion. Equilibrium Rule: • If at rest, it continues at rest. • If in motion, continues in motion. ∑F = 0 “The sum of the forces is zero.”

48. The Equilibrium Rule 2. The sum of the forces upwards and downwards is zero. 1. The sum of the forces in the x-direction is zero. 3. All the forces on the object are balanced. 4. There is no net force.

49. The Equilibrium Rule Burl and Hewitt are on a scaffold suspended by two ropes. The weight of the scaffold points down; it is balanced by the tension in the ropes, which points up. Which is greater: the weight or the tension force? The forces are equal Burl is heavier than Hewitt. Is the tension is his rope greater? Yes.

50. The Equilibrium Rule If Hewitt walks over to Burl’s side, what happens to the tension in Burl’s rope? What happens to the tension in Hewitt’s rope? The tension in Hewitt’s rope decreases. What happens to the tension in Burl’s rope? The tension in Burl’s rope increases.