# Forces KINE 3050 - Forces KINE 3050 3050 SI Units Force...

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Unformatted text preview: Forces KINE 3050 3050 SI Units Force / Weight Mass Time Time Length Newton Newton Kilogram Second Meter N kg s m Force Facts SI unit for force: Newton (N) 1 N of force accelerates of a 1 kg mass 1 ms-2 kg mass ms On earth g = 9.82 m/s2 1lb 4.448 N (kg*m*s-2) 1kg 9.8 N 1 Newton s Laws of Motion 1. Every object in a state of uniform motion Every tends to remain in that state of motion unless an external force is applied to it. (law (law of inertia) [resistance to change] 2. The relationship between an object's mass The m, its acceleration a, and the applied force its F is F = m*a. is m*a. 3. For every action there is an equal and For opposite opposite reaction. Force = mass x acceleration Force: Force: Push or Pull between objects that may may OR may not result in a movement. Force Facts A net force accelerates or deforms an net force object.  A rigid body does not deform Forces come in pairs action / reaction equal but opposite forces 2 Force Facts: Forces are represented by Vectors. A vector is an arrow More about vectors later Types of forces Internal Internal Forces: Forces that act within the object or system whose motion is being investigated. investigated. Created Created by muscles, tendons, bone, ligament, cartilage. Internal Internal Forces: Tensile A pulling force acting on pulling the ends of an internal internal structure Tensile force often cause of ACL injury 3 Internal Forces: Compressive Pushing Pushing force acting on the ends of an internal internal structure Compressive force Often Often cause of cartilage injury injury What What happens when the tensile or compressive force acting on a structure is greater than the structure can withstand? Internal Internal forces cannot change the the bodies center of mass Must Must interact with the outside world to change position of center center of mass 4 External Forces Act Act on an object as a result of its interaction with the environment surrounding surrounding it External External forces are mostly contact forces Force of gravity is a non-contact external non-contact force force Weight vs. Mass Weight vs. Mass Force of gravity acting on an object Weight Weight of an object is its mass times acceleration due to gravity:  Gravitational constant: -9.8 ms-2 (negative?)  Unit for mass is Kilogram (Kg) W = m*g  Unit for weight = Newton (N) 1kg weighs 9.8 N (on earth) 5 Center of gravity The central balance point The The point through which the resultant force acts acts Center of gravity Weight W = m*g W = 4500kg * 9.8 m/s/s = 44,100 N 44 6 Contact Forces Forces Forces between objects objects in contact Solid or fluid Forces Forces transmitted through through contact Resolved Resolved into components components Contact force components Reaction force line of line action is perpendicular to surface surface Friction force line of line action is parallel with surface surface  Responsible for Responsible horizontal horizontal movement RF FF Friction Friction facts: facts:  Opposes motion  Responsible for changes in HORIZONTAL Responsible motion motion  Dry or Fluid  Static or Dynamic 7 Friction Static Friction: between 2 surfaces that are not between moving. moving. -max friction that develops just before surfaces max surfaces slide slide Dynamic Friction: friction between sliding surfaces. Static Static > Dynamic (harder to begin movement than to to keep movement going) How do cleats impact friction? Factors affecting friction Contact Contact force and friction force are proportional; as one increases, so does as the the other Nature Nature of materials: greater friction developed between softer and rougher surfaces surfaces than harder and smoother Pressure P= F A What does this mean? The The larger the surface area; the smaller the force force per unit area. (inverse of friction) 8 Pressure The The pascal (Pa) is the SI unit of pressure. (Pa) 1 Pa: pressure generated by a force of 1 N Pa: acting on an area of 1 m2. ac It is a rather small unit as defined and is more more often used as a kilopascal [kPa]. [kPa]. Example What pressure if: F = 100 N A = 10 mm2 = 10*1/1,000,000 m2 = 0.00001 m2 0.00001 P = 10,000,000 = 10,000 kPa 10,000,000 Pressure example 9 Mechanical stress Depend Depend on the force and the area More More Force Stuff Vector: How we represent a force Magnitude Direction Net Net Force: Sum of all external forces acting on on a system Resultant Resultant Force: The result of vector addition addition of 2 or more forces Addition of Forces Must take into account: Magnitude AND Magnitude AND Direction 10 Summary SI Units Newton s 3 laws  Inertia  F=m*a  Reaction forces forces Force facts Internal vs. external forces Mass vs. Weight Center of gravity Friction and pressure Mechanical loads 11 ...
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