3 General OverviewThree physical laws that describe relationships between forces acting on a body and that body’s motion.Formulated by Sir Isaac Newton, English scientist and mathematician, in 1687.Have stood the “test of time” Only two exceptions:Very fast objects: explained by Einstein’s Special Theory of RelativityVery small objects: explained by Theory of Quantum Mechanics
4 The Three Laws WAY TOO MANY UNFAMILIAR TERMS!!! First Law of Motion An object will maintain its current state of rest or uniform velocity unless acted upon by an unbalanced forceSecond Law of MotionThe acceleration of an object is directly proportional to the net force exerted upon it and inversely proportional to its massThird Law of MotionFor every action there is an equal and opposite reactionWAY TOO MANY UNFAMILIAR TERMS!!!
5 Press the Easy Button! First Law of Motion: inertia Second Law of Motion: Fnet = maThird Law of Motion: action-reactionYOU MIGHT STILL NEED A VOCAB LIST!!!
6 The Vocab Breakdown Force: a push or pull on an object Rest: not movingVelocity: speed in a given directionAcceleration: change in velocityUniform: constant, not changing
7 The Vocab Breakdown Net: overall sum, considering different directions Mass: amount of matter in an objectDirectly Proportional: as the independent variable increases, the dependent variable increasesInversely Proportional: as the independent variable increases, the dependent variable decreases
8 So… Newton used the force too?!? NO!!! NOT THAT KIND OF FORCE!
9 Types of ForcesApplied Force (Fapp): force which is applied to an object by another object or by a personTension (Ftens): force which is transmitted through a string, rope, or wire when it is pulled tight by forces acting at each end.Normal Force (Fnorm): support force exerted upon an object which is in contact with another stable object. Exerted perpendicular to the surface.
10 Types of ForcesFriction (Ffric): force exerted by a surface as an object moves across it or makes an effort to move across it. Opposes the motion of the object.Air Resistance (Fair): special type of frictional force which acts upon objects as they travel through the air. Opposes the motion of the object.Gravitational Force (Fgrav): force with which the earth, moon, or other massive body attracts an object towards itself. By definition, this is the weight of the object.
11 Newton’s First Law of Motion An object will maintain its current state of rest or uniform velocity unless acted upon by an unbalanced force.What it means: In the absence of an unbalanced force…an object at rest will remain at restan object in motion will remain in motion at the same speed and in the same direction
13 1st Law Examples: Auto Collisions What is the occupant’s state of motion before the collision?Constant forward velocityWhat is the occupant’s state of motion after the collision without a seatbelt?What does a seatbelt provide?An unbalanced force on the occupantIt decelerates the driver to rest
14 1st Law Examples: Whiplash Suppose you are sitting at a stoplight when you are rear-ended by another car.To an onlooker on the sidewalk, what initially happens to your head?It stays at rest as the car and your body move forward.It was at rest and will remain at rest.An unbalanced force does not act on it until…the headrest pushes your head forward.
15 1st Law Examples: The Tablecloth Parlor Trick TRY THIS AT HOME... with PAPER plates!!!Why does it work?If the tablecloth is pulled quickly enough, the frictional force is minimal, so the table setting remains practically at rest as the tablecloth is pulled out from underneath.
16 Inertia The First Law is often called the Law of Inertia Inertia is the tendency of an object to maintain its state of rest or constant velocityInertia is really a measure of MASSMore mass Greater tendency to stay at rest or in motionLess mass Less tendency to stay at rest or in motionWhich is easier to move from rest: a pebble or a boulder? Which is easier to stop when moving?
17 So inertia is just weight?!? Not quite, young grasshopper!Mass and weight are not the same thing!Mass: amount of matter in an objectMeasured in kilograms (kg) in metric systemMeasured in slugs in English systemNever changes!Weight: force of gravity on an objectMeasured in Newtons (N) in metric systemMeasured in pounds (lbs) in English systemChanges based on location (altitude & planet)
18 Calculating WeightWeight can be calculated very easily from the mass of the object. weight = mass x acceleration due to gravity On Earth, acceleration due to gravity is 9.8 m/s2 weight = mass x 9.8 m/s2
19 Calculating WeightSolved Example: What is the weight of a 50 kg high school student? weight = mass x 9.8 m/s2 equation weight = 50 kg x 9.8 m/s2 plug & chug weight = 490 kgm/s2 weight = 490 N answer Note: 1 N = 1 kgm/s2
20 Calculating Weight Try this one on your own! Show all work! What is the weight of 4 kg sack of potatoes?Solution: weight = mass x 9.8 m/s2 equation weight = 4 kg x 9.8 m/s2 plug & chug weight = 39.2 kgm/s2 weight = 39.2 N answer
21 Newton’s Second Law of Motion The acceleration of an object is directly proportional to the net force exerted upon it and inversely proportional to its mass.as net force increases, acceleration increases (as long as mass is constant) Example:Just you pushes a heavy piano. Piano doesn’t speed up quickly.Both you and a partner push the piano. Piano speeds up quicker.
22 Newton’s Second Law of Motion The acceleration of an object is directly proportional to the net force exerted upon it and inversely proportional to its mass.as mass increases, acceleration decreases (as long as force is constant) Example:You pull an empty wagon with all your force. Wagon speeds up quickly.You pull a wagon loaded with bricks with all your force. Wagon speeds up slowly.
23 Newton’s Second Law of Motion The 2nd Law is expressed as an equation: Fnet = m·a where: Fnet is the net force acting on the object is measured in newtons (N)m is the mass of the object is measured in kilograms (kg) a is the acceleration of the object is measured in meters per second squared (m/s2)
24 What’s a Net Force? Sum of forces acting in different directions Lefts vs. Rights Ups vs. DownsHow do we know if there’s a net force? The object’s MOTION will tell us!!!At rest: No net forceMoving at constant velocity: No net forceAccelerating: Net force actsThe easiest way to analyze forces and determine the net force is by drawing!!!
25 Drawing ForcesWe need to diagram both the magnitude and direction of these forcesMagnitude: size, amount, how muchDirection: which way is the push or pullA force ALWAYS needs both magnitude and directionINCORRECT: a force of 5 NCORRECT: a force of 5 N to the right
26 Drawing Forces To draw forces, we use arrows These arrows are called “force vectors”Length of vector: magnitude of forceDirection of vector: direction of forceF5 N right10 N left3 N down
27 Free Body DiagramsTo diagram ALL the forces acting on an object, we use a FREE BODY DIAGRAMBox represents the objectArrows represent forcesArrows drawn outward from box in direction of forceArrows are labeled with name of forceFapp
28 Net force is always in the direction of the acceleration! Free Body DiagramsTo draw correct FBDs, you need to think about the forces acting on the object.This is difficult at first.Practice makes perfect!Key to success: determine if there’s a net forceRemember:At rest: No net forceMoving at constant velocity: No net forceAccelerating: Net force actsNet force is always in the direction of the acceleration!
29 Free Body DiagramsExample: What does the FBD for a physics book resting on the desk look like?Solution: Is there a net force? No, the book is at rest. Consider all forces and decide which ones act.Gravity? YES! Gravity always acts & pulls down.Applied? NO! Nobody is pushing the book.Tension? NO! There’s no string, rope, etc.Friction? NO! It isn’t sliding or trying to slide.Normal? YES! The desk pushes up on the book.So… GRAVITY pulls down & NORMAL FORCE pushes up. Together it looks like…
30 BUT, PRACTICE MAKES PERFECT… FnormBUT, PRACTICE MAKES PERFECT…Fgrav
31 Determining Net Force from FBDs Add forces in same directionSubtract forces in opposite directionsThe VECTOR SUM is the NET FORCE6 N5 N1 N2 N(All Forces)(Net Force)
32 Determining Net Force from FBDs Treat horizontal and vertical forces separately.(All Forces)(Net Force)3 N1 N4 N1 N5 N2 NTrigonometry is needed to determine a single net force from these two!!!
33 So, what about Fnet = m·a?Working with this equation is a synthesis of 2nd Law knowledge:First, identify forces acting and draw FBDIf forces are balanced, Fnet = 0If forces are unbalanced, express Fnet as either:A single forceVector sum (combination) of of two or more forcesSecond, identify given info from problemThird, Plug & Chug (you may need to rearrange!)Finally, express your answer in the correct units!!!
34 2nd Law Example Problem #1 Using a rope, you pull your kid brother on a sled across the ice with a force of 75 N. If your brother and the sled have a combined mass of 30 kg, what is the resulting acceleration of the sled? (Ignore friction)
35 2nd Law Example Problem #1 Identify forces:Vertically: Gravity & Normal balance outHorizontally: Tension only!!!Net Force? Yes!!! Fnet = FtenIdentify info:Ften = 75 N m = 30 kg a = ?Plug & Chug: Fnet = m·a 75 N = (30 kg)·a (need to rearrange!) a = 75 N / 30 kgAnswer: a = 2.5 m/s2FnormFtenFgrav
36 2nd Law Example Problem #2 You pull your kid brother in wagon with a force of 90 N. The resulting acceleration is only 2 m/s2. If your brother and the wagon have a combined mass of 30 kg, what is the force of friction acting on the wagon?
37 2nd Law Example Problem #2 Identify forces:Vertically: Gravity & Normal balance outHorizontally: Applied & FrictionNet Force? Fnet = Fapp - FfricIdentify info:Fapp = 90 N m = 30 kg a = 2 m/s2 Ffric = ?Plug & Chug: Fnet = m·a 90 N – Ffric = (30 kg)·(2 m/s2) 90 N – Ffric = 60 N Ffric = 90 N – 60 N Ffric = 30 NAnswer: Ffric = 30 NFnormFtenFgrav
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