Lecture Note

FORCE, MOTION AND ENERGY 4.1 | SCIENCE | QUARTER 04 2023 Asian Philosophers were interested indefining and describing the motion. Aristotle stated to keep an object movinga force should be continuously applied onit, but this statement is missingsomething. FORCE ● Its either push or pull ● Push - it is directed away from thatwhich exerted it. ● Pull - exerted effort towards you ● Expressed in a unit called Newton(N) to honor the British physicistIsaac Newton ● Inanimate objects aso exert forcescalled “pull of gravity” or the “forceof gravity” NORMAL FORCE - is a pushing force which always acts perpendicular to the surface itexerts. TENSION - is a kind of pulling force exerted on a non-rigid object such as a rope, stringor chain MEASURING FORCES SCALES - used to measure forces ● You may not encounter manyscales that measure weight innewton ● Units of mass (kilograms orgrams) ● Older, non-standard unit - pound(lb) ● INTERNATIONAL SYSTEM OF UNITS(SI) - the standard unit of force is the N 1N = 1kg x cm/s^2 ● CENTIMETRE-GRAM-SECOND SYSTEM OF UNITS (CGS) - Force is measured in a unitcalled dyne. It is neverabbreviated. 1 dyne = 1g x m/s^2 REPRESENTING FORCES THROUGH DIAGRAM ● Use one-way arrow torepresent a force on apaper ● The length of the arrowitself is an indication of thestrength of the force ● Should always have at oneend an arrowhead thatpoints in the direction ofthe force. ● Vector Quantity has amagnitude and direction. CONTACT AND NONCONTACT FORCES Contact ● Contact is needed for the force toact on objects ● Include normal force, tensioncompression, and friction Noncontact Forces ● Also called action-at-a-distanceforces Reviewer by: Khasandra Chloe S. Serrano 1

FORCE, MOTION AND ENERGY 4.1 | SCIENCE | QUARTER 04 2023 ● Include gravitational forces ● Between charged particles called Electrostatic Forces . ● Magnets also exert noncontactforces between them. THE LAWS OF MOTION Sir Isaac Newton ● Summarize the reasons whyobjects move, stop or keep moving ● The three Laws of Motion ● Law of Inertia ● Law of Acceleration ● Law of Interaction Dynamics (Dynamics of Motion) ● Ascertains the reasons whyobjects move the way they do, andwhy some objects move fast whilesome move slowly LAW OF INERTIA - Also known as the law ofequilibrium - Centered on the concepts ofinertia and equilibrium Inertia ● Tendency of an object to keepdoing what it is already doing(Galileo) If something pushes to start moving or toslow it down or speed it up, the object putsup “resistance”Resistance is applied because of Inertia. Important things to remember aboutInertia: 1. Inertia is the tendency of an objectto resist any change in its state ofmotion. 2. Inertia is not a force. It is the property of an object. 3. Inertia is measured by an object’s mass. EQUILIBRIUM ● Exist when all the forces acting onan object balance each other out. ● Sum of all forces acting on theobject is zero. FINDING THE NET FORCE ● The net force is also known asresultant force, and needs to bezero. ● Added using vector addition ● Positive signs for forces that are tothe right and negative signs forforces which are to the left. Example: 1. Fnet = 5N + 10n = 15N (not equilibrium) 2. Fnet = 5N + (-5N) = 0 (equilibrium) EQUILIBRANT - A nonzero net force acting on anobject is balanced by a singleforce. - It causes the object to be inequilibrium Example: Reviewer by: Khasandra Chloe S. Serrano 2

FORCE, MOTION AND ENERGY 4.1 | SCIENCE | QUARTER 04 2023 1. 5N, -5NFnet = 5N + (-5N) = 0 2. 5N, 7.5N Fnet = 7.5N + (-5N) = 2.5 N to theleft2.5 + 5 = 7.5 LAW OF ACCELERATION ● When the net force acting on an object is not zero the objectaccelerates. This change caneither be a change in speed, achange in direction, or a change inboth speed and direction. ACCELERATION - Measure how fast the velocity ofan object changes. ● The magnitude of the accelerationis directly proportional to the netforce and inversely proportional tothe object mass. Note: First part, Net force causes acceleration and defines the direction ofthe acceleration. Relates the quantity ofthe net force to that of the acceleration. ★ Acceleration increases when thenet force increases. Yet,mass playsinto the equation. Example: Fnet = ma Given:Fnet = 10N m = 5kgm2 = 10kga1 = ?a2 = ? Solution:Fnet = maFnet = ma m m Derived formula. To find the accelerationwe must have to divide the mass andacceleration to mass and so in Net force.Then we must cancel out the mass on theright side. Formula: a = Fnet m Fnet = 10nm = 5 kg 10N 5 kg Lets cancel out the kg and we must use theformula of SI, 1N = kg x m/s^2 10 kg x m/s^2 5 kg We must cancel out kg and 10 divided by 5is 2, so our answer is.. = 2 m/s^2 Reviewer by: Khasandra Chloe S. Serrano 3

FORCE, MOTION AND ENERGY 4.1 | SCIENCE | QUARTER 04 2023 MASS AND WEIGHT Show how much Inertiaan object has. The forces that earthexerts object or near it Not a force A force Remains the sameregardless of loc. Varies in magnitude fromplace to place. Standard SI unitkingdom (kg ) Newtons or dynes (dyn) Weight can be calculated from the mass using an equation similar to F = ma W = mg When: ● Weight of the object is W ● Mass of the object is m ● g is the acceleration due to gravityat the specific location on Earth,g = 9.8 m/s^2 = 980 cm/s^2directed downward. Example: W = mgw = ?gm = 1.62 m/s^2ge = 9.8 m/s^2 Let’s proceed to Earth gravitational. Wemust use multiplication. w = (100 kg) (9.8 m/s^2) = 980 kg x m/s^2= 980 N Same formula for the second one. LAW OF INTERACTION ● Have you ever awakened to seethat your pillowcase has left markson your face? This is a reminderthat when you touch something, ittouches you back. The weight (aforce) of your head is directedtoward the pillow; the pillow has toapply an upward force on you atthe same time. ● This manifests Newton’s third lawof motion, also known as Law ofInteraction. The Law of Interactionalways involves two objects, suchas your face and the pillowcase.The Law states: When a body A exerts force on a body B, Bexerts an equal but opposite force on A. FRICTION ● Friction is a contact force thatserves to slow down a movingobject or prevent motion altogether. ● Friction is always parallel to thesurface in contact and if the objectis moving, it is the oppositedirection of motion. ● Friction may occur between solidobjects, It may also exist betweena solid and a fluid. The two kinds of friction are: Static Friction - Prevents two objects from slidingagainst each other. Reviewer by: Khasandra Chloe S. Serrano 4

FORCE, MOTION AND ENERGY 4.1 | SCIENCE | QUARTER 04 2023 Kinetic Friction - Exist when objects slide againsteach other, like in the case ofmoving machine parts. Beneficial Effects of Friction: 1. Use materials that are naturally rough.Sandpaper comes in various “grits” orroughness. Tires come in various treads,making some tires better than others forwet conditions. 2. Increase the pressure between thesurfaces. When cleaningoff tough stains from a surface, it is best topress down harder as you scrub. Theadded pressure increases the frictionbetween the surfaces. In places whereheavy snowfall occurs people drive aroundwith a load of snow on top of their vehicle,increasing the vehicle’s weight andtherefore the friction. 3. Use the right combination of materials.Friction does not just depend on one of thetwo surfaces, but on the pair of surfaces.Rubber on concrete produces more frictionthan rubber on glass, for example. UNDESIRABLE EFFECTS OF FRICTION 1. Friction causes the soles of our shoes towear out. 2. In industry, machine parts grind againstone another, generating heat and resultingin tremendous losses of energy. Machineparts have to be replaced due to wear asa result of this grinding. WAYS TO REDUCE FRICTION 1. Use a lubricant. A lubricant is anysubstance that spreads over theminute grooves and cracks onsolid surfaces, making the surfaces more even. 2. Reduce the contact between surfaces. This is the opposite ofpressuring surfaces harder toincrease friction. This is how someinter island ferries increase theirspeed over water. 3. Use bearings. Ball bearings are used to reduce or eliminate slidingfriction between the wheel and theaxle. As the wheel turns, so do allthe bearings. There is no sliding,only rolling. With some addedgrease, rolling friction is furtherreduced. CIRCULAR MOTION Circular motion is more common than onemight think at first. We see it in the potter’swheels, Ferris wheel, and merry-go-round. Uniform Circular Motion (UCM) ● The simplest type of circularmotion. The characteristics of this type of motionare the ff: 1. The path of the objects is an arc ora circle. In other words, the objectmay move through a portion of acircle. 2. The speed (magnitude of the velocity) is constant during themotion. 3. The direction of motion changes Reviewer by: Khasandra Chloe S. Serrano 5

FORCE, MOTION AND ENERGY 4.1 | SCIENCE | QUARTER 04 2023 moment by moment. 4. The acceleration of the object is constant and is alwaysperpendicular to the velocity. It isdirected toward the center alongthe radius. The net force that pulls the object towardthe center of the circle is referred to as the centripetal force of Fc. Example: A car negotiating a turn needsthe friction between the tires and the roadto provide centripetal force. Uniform circular motion governed byNewton’s second law in which the net forceis replaced by the centripetal force. In thiscase, The acceleration is referred to as the centripetal acceleration because it also points inward. It is related to centripetalforce by equation: Fc = mac The centripetal acceleration is related tothe speed of the object around the circleby the formula: ac = v^2 r Thus, we can also calculate the centripetalforce using: Fc = mv^2 r Example: a) Calculate the centripetal force exerted on a 900.0 kg car that negotiates a 500.0 m radius curveat 25.00 m/s. We know that: Fc = mv^2 r Thus, Fc = mv^2 r = (900.0 kg) (25.00 m/s)^2 (500.0 m) = 1125 N Moving In Many Circles Period - the time within which an orbiting body completes its movement in a circle. For example, the period of the Earth’sThe revolution around the Sun is one year.Period T may be given in seconds (s) orany other unit of time. Frequency - the reciprocal of period - denoted as f- It is given in the unit “per second” or /s.- also called Hertz (Hz) named after theGerman scientist Heinrich Hertz. In symbol, period and frequency are related by the equation: f = 1 T For an object with a given frequency orperiod moving in a circle of radius r, we Reviewer by: Khasandra Chloe S. Serrano 6

FORCE, MOTION AND ENERGY 4.1 | SCIENCE | QUARTER 04 2023 can determine its speed v (also known asthe orbital speed ) around the circle. v = 2πa T Reviewer by: Khasandra Chloe S. Serrano 7

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Force, Motion And Energy

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