may the FORCE be with you 

1. Chapter 4: Forces and the Laws of Motion may the FORCEbe with you 

2. A Review…. • Force • push or pull exerted on some object • - Unit of Force is a NEWTON (N) • 1 kg x m/s2 • - Force is a VECTOR • Has a magnitude and direction

3. Review • Acceleration is a change in velocity per unit of time interval. • It has both magnitude and direction. • Vectors have both magnitude and direction. • What causes motion? • Why do some objects accelerate at higher rates than others do? Forceis any mechanism that can cause a change in an object’s velocity.

4. 4.1 Changes in Motion • How does force affect the motion of an object? • Forces can cause • Changes in velocity (fig 4-1) • Stationary objects to move (throwing a ball) • Moving objects to stop (catching a ball) • A change of direction (hitting a ball) • These all involve a change in velocity an acceleration Force the cause of acceleration, or the change in an object’s velocity

5. 4.1 Weight vs. Mass • The weight of an object is a measure of the magnitude of the gravitationalforce exerted on an object. (mass is a measure of an object’s inertia) • It’s the result of the objects mass interacting with Earth’s gravitational field. • Terms we often use to describe weight are really units of force • Ex. A ¼ lb. stick of butter has a weight equivalent to a force of 1N. • 1 lb. =4.448N • 1N=0.225 lb

6. Contact and Field Forces • Forces can act through contact or at a distance • Contact forceis a force that arises from the physical contact of two objects (like a push or a pull) • Usually easy to identify • Ex: • frictional force • Tension force • air resistance • Applied force

7. Field Forces • Forces that do not involve physical contact are called field forces • The presence of an object affects the space around it so that a force is exerted on any other object placed within that space. • The region of influence is called a field • Ex: Gravity – earth exerts a force on an object even when it’s not in immediate contact with the object (also electrical and magnetic) • Ex: the attraction or repulsion between electrical charges

8. Forces • Applied Force Fapp a force that is applied to an object by a person or another object. (a person pushing a desk across the room) • Gravity Force F grav (also known as weight) the force with which the earth or other large objects attract another object towardsitself (not the same as mass) • Normal Force Fnorm the support force exerted upon an object that is in contact with another stable object. (like a book resting upon a surface, the surface is exerting an upward force on the book in order to support the weight of the book)

9. Forces • Friction Force Ffrict the force exerted by a surface as an object moves across it or makes an effort to move across it. (Both sliding and static friction) • Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules • Air Resistance Force Fairfrictional force that acts upon objects as they travel through the air (usually negligible except for objects traveling at high speeds or having high surface area)

10. Forces • Tension Force Ftens Force transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. • Spring Force Fspring the force exerted by a compressed or stretched spring upon any object that is attached to it. • Mass the amount of matter that is contained by the object. (how much stuff is there) • Weight the force of gravity acting upon an object (the pull of the earth on that stuff)

11. Force Diagrams • If you push a toy car, it does not move as fast as it does when you give a harder push. • The effect of force depends on its magnitude • The effect of force on an object’s motion also depends on the direction of the force • A force diagram is a diagram of the objects involved in a situation and the forcesexerted on the objects • force diagram shows all the forces acting in a situation (b) free-body diagramshows only the forces acting on a particular object

12. Force Diagrams • Show force vectors as arrows • Tail of the arrow is attached to the object on which the force is acting • Used as tools to analyze collisions • For now, disregard the size and shape of the object and assume that all forces act on a point at the center of an object

13. FREE BODY DIAGRAMS • A free-body diagram helps analyze a situation • Used to analyze only the forces affecting the motion of a single object • Work just like vector diagrams – find component and resultant forces • Ex. Engineers use to analyze test-car crashes to determine which forces affect the car and passengers

14. Tow truck example • There are many forces acting on this car • The tow truck exerts a force on the car in the direction of the cable • The road exerts forces on the car • The car is acted on by gravitational force • To draw a free-body diagram, you must first isolate and identify all the forces acting on the car • Figure 4-4

15. Free Body Diagrams isolate an object and the forces acting on it Steps to draw free body diagrams: • Draw a diagram to represent the isolate object (b). Draw in same position as the actual object. • Draw and label vector arrows of all external forces acting on the object (c). All forces are assumed to act on a single point at the center of an object. • Label the arrow with the size of the force • The force of the tow truck exerts 5800 N on the car. The arrow is pointing in the same direction as the force of the cable on the car.

16. Drawing Free Body Diagrams • The gravitational forceacting on the car is 14700 N directed toward the center of the earth (d). • The road exerts an upwardforce of 13690 N (e). • Because of the interaction between the road and the car’s tires, the road also exerts a backward force of friction equal to 775 N (f).

17. Drawing Free Body Diagrams • Make sure that only the forces acting on the carare included in the free body diagram. (leave off the forces the car is exerting on other objects) • F is the completed free-body diagram. A free-body diagram can be used to find the net external forceacting on an object, using the rules for vector analysis. Section Review 1-6

18. 3 Laws of Motion NEWTON’S LAWS OF MOTION

19. Newton’s 1st Law • Predicts the behavior of stationary objects and moving objects. (Insert chart from physicsclassroom.com) • An object at rest remains at rest and an object in motion continues in motion with a constant velocity unless acted on by an outsideforce

20. Law of Inertia • Also known as the LAW OF INERTIA • Inertia the tendency of an object not to accelerate • When the net external force on an object is zero, its acceleration (or change in velocity) is zero • If the forces acting upon an object are balanced, then the acceleration of that object will be 0 m/s2 • Objects tend to “keep on doing what they’re doing”.

21. Newton’s First Law • Acceleration is determined by netforce • Net external force is the total force resulting from a combination of external forces on an object; sometimes called the resultant force • It’s the vector sum of all the forces acting on an object • When all external forces are known, the net external force can be found by finding the resultant vector • An objects acceleration is determined by all the forces acting on it.

22. Newton’s first law concepts • Mass is a measurement of inertia • A basketball and a golf ball that are side by side on the ground remain at rest as long as no net external force acts on them. • If you strike each ball with a golf club, which one will accelerate more? • The basketball experiences a smaller acceleration because it has more inertia than the golf ball. • The inertia of an object is proportional to its mass. • The greater the mass of the body, the less the body accelerates under an applied force.

23. Newton’s First law concepts • Objects in motion tend to stay in motion. • The Zebra’s stripes stay in motion!

24. Equilibrium • Objects that are either at rest or moving with constant velocity are said to be in equilibrium. • All forces are balanced • Newton’s first law describes objects in equilibrium, whether they are at rest or moving with constant velocity. • For equilibrium to occur, the net external force must be zero.

25. Equilibrium • The bob on the fishing line is at rest. • We know Fnet is equal to zero • If a fish bites the bait, the fish exerts a force on the line, the bob accelerates downward. • To return the bobber to equilibrium , the person must apply force to the fishing line.

26. Equilibrium • The force that brings an accelerating object into equilibrium, must be equal and opposite to the force causing the object to accelerate. • An object is in equilibrium when the vector sum of the forces acting on it is equal to zero. • The easiest way to do this is to resolve forces into their x and ycomponents. • When the sum of all forces in the x direction is zero (Fy=0). Then the vector sum is equal to zero and the body is in equilibrium.

27. Newton’s Second Law • Newton’s Second Law • Predicts the behavior of objects for which all existing forces are NOTbalanced. • States: The acceleration of an object is dependent on two variables: Fnet and mass.

28. Newton’s Second Law • Acceleration is directly proportional to the Fnet acting on the object and inversely proportional to the object's mass. • As force is increased, acceleration is increased • As mass is increased, acceleration is decreased Acceleration = Net Force divided by mass a= Fnet/m Net Force = mass * acceleration  Fnet=m*a

29. Newton’s Second Law • Remember that 1 Newton (N) = kg * m/s2 a=F/m Example: Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2 kg book toward Roberto. If the net external force acting on the book is 2.6 N to the right, what is the book’s acceleration?

30. EXAMPLE 4B • Given: m=2.2 kg • Fnet=F=2.6N to the right • a=? • Use Newton’s second law and solve for a. a= F/m Practice 4B

31. Misconception Alert • Be aware of the common misconception that sustaining motion requires a constant force. • Read the scenarios and we will discuss the answers!

32. Newton’s 3rd Law • Forces result from an object’s interaction • Such as contact interactions (normal, friction, tensional and applied) • Action-at-a-distance interactions (gravitational, electrical, magnetic) • When you sit in a chair, you exert a downward forceon the chair and the chair exerts an upward force on your body • These two forces are called action and reaction pairs • “a pair of simultaneous equal but opposite forces resulting from the interaction of two objects”

33. Newton’s Third Law • Newton’s Third law deals with action and reaction pairs • For every action there is an equal and opposite reaction. • “If 2 objects interact, the magnitude of the forceexerted on object 1 by object 2 is equal in magnitude of the force simultaneously exerted on object 2 by object one, and these forces are opposite in direction.” • A single isolate force cannot exist!

34. Action-reaction pair • In every interaction, there is a pair of forces acting on the 2 interacting objects • The size of the forces on the first object equal the size of the force on the second object • The direction of the force on the first object is opposite to the direction of the force on the second object. • Forces always come in pairs! Equal and opposite action-reaction force pairs.

35. Action- Reaction Pair • Either force can be called the action or reaction • Misconception alert: don’t confuse ‘reaction’ with the everyday use of the word. Reaction here does not mean something that happens in response to something, but happens at the SAME TIME as the action force! • Most important to remember: each force acts on a different object.

36. Action-Reaction forces • Misconception alert: equal and opposite forces do not balance each other a inhibit a change in motion. • Consider a hammer striking a nail? • Why doesn’t it remain at rest?

37. Example • Think of the flying motion of birds. • The wings of a bird push air downwards. • Since forces result from mutual interactions, the air must also be pushing the bird upwards. • The size of the force on the air equals the size of the force on the bird • The direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).

38. A truck driving down the road runs into a bug flying the other way. a) Which object has the greater force exerted on it? b) Which object will change its motion the most? FTB FBT Both objects have the same small force exerted on them during the collision according to Newton’s third law. The bug squishes easily! The bug obviously changes its motion significantly more than the truck because of its much lower inertia (mass). Bug: - aT = - F / M Truck: aT = F / M

39. The moon moves around the earth. a) Which object has the greater force exerted on it? b) Which object will change its motion the most? FME FEM Both objects have the same large force exerted on them during the collision according to Newton’s third law. The Moon obviously changes its motion significantly more than the Earth because of its much lower inertia (mass). Moon: - aM = - F / M Earth: aE = F / M

40. Check for Understanding • Read the questions on “Check for Understanding”, answer them and we will discuss. • Do Section Review 1-5 pg 140

41. Laws of Motion Review • 1st Law  what happens when there is no net force (inertia) • On object at rest stays at rest and an object in motion stays in motion • 2nd law  what happens when a net force does act • Acceleration depends on net force and mass • 3rd law  forces occur in pairs, no isolated forces (action-reaction law) • For every action there is an equal and opposite reaction

42. 4-4 Everyday Forces • Which weighs more, a bowling ball or a tennis ball? • How do you know? • Imagine one ball in each hand and imagine the downward forces acting on your hands. • Because the bowling ball has more mass the force of gravity pulls more strongly on the bowling ball and it pushes on your hand with a stronger force.

43. Weight vs. Mass • Mass is an inherent property of an object. The mass of a bowling ball is the same on earth as it is the moon. • Weight is NOT an inherent property. It is dependent on the force of gravity- which changes with location. • An astronaut weights 800N (180 lb) on Earth, but only 130 N (30lb) on the moon. • Because gravity is smaller on the moon

44. Weight • Is weight constant on earth? Does your weight vary depending on where you are? • Actually, YES ! G decreases as distance from the center of Earth increases, so objects weigh less at higher altitudes than at sea level. • Also, because earth is not symmetrical, g also decreases a little as latitude decreases. • Weight is the magnitude of the force of gravity acting on an object

45. The Normal Force • Normal force (Fn )a force exerted by one object on another in a direction perpendicular to the surface of contact. • One meaning of the word “normal” is ‘perpendicular’. • It is a support force exerted upon an object that is in contact with another stable object • The Fn is ALWAYSperpendicular to the surface but NOT always opposite of Fg Perpendicular means at a right angle to

46. Refrigerator on a ramp • This is a fridge on a loading ramp • Notice the Fn is perpendicular to the ramp but NOT Fg • The magnitude of normal force can be calculated with: Fn = mg(cos )

47. The force of Friction • Friction is the force exerted by a surface as an object moves or makes an effort to move across the surface • Two types of friction • Static friction (Fs) results when the surfaces of two objects are at REST relative to one another and a force exists on one of the objects to set it into motion • Kinetic Friction (aka Sliding friction) (Fk) results when an object slides across a surface

48. Friction (a) Because the jug of is in equilibrium, any horizontal force will cause it to accelerate (b) when a small force is applied, the jug remains in equilibrium because the force of friction is equal/opposite to the appliedforce. (c) When a larger force is applied, the jug begins to move as soon as the applied force exceeds the opposing static friction force

49. Friction • Friction opposes the applied force • Fapplied increases, static friction also increases • Fs = -Fapplied • Fsmax is when the force of static friction reaches its max value – the applied force is as great as it can be without causing the jug to move • Kinetic friction is less than static friction (Fsmax) • The net external force (F) is equal to the difference between Fapp and Fk(F-Fk)

50. Friction • The force of friction is proportional to the normal force (due to mass of the object) • Friction depends on the surfaces in contact • Objects that appear smooth really aren’t at the microscopic level