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Transcript
FORCES FORCES PUSH OR A PULL DESCRIBED NOT ONLY BY HOW STRONG THEY ARE, BUT ALSO BY THE DIRECTION IN WHICH THEY ACT UNBALANCED FORCES 2 Forces act in the same direction----add together + The = width of the arrow = strength of force UNBALANCED FORCES 2 Forces act in opposite directions----add together + = Like adding “+” and “-” UNBALANCED FORCES 2 Forces acting in opposite directions---add together + = **equal forces in opposite directions cancel each other out UNBALANCED FORCES Net force: overall force of an object after all the forces are added together UNBALANCED FORCES There is a net force acting on an object. Can caused an object to Start moving Stop moving Change direction Will change an object’s motion Cause an object to accelerate BALANCED FORCES Equal forces acting on an object in opposite directions Will NOT change the object’s motion NO Net change in acceleration force = 0 NEWTON’S FIRST LAW OF MOTION Inertia: the tendency of an object to resist change in its motion Example: air hockey puck NEWTON’S FIRST LAW OF MOTION An object at rest will remain at rest and an object that is moving at a constant velocity will continue moving at a constant velocity unless acted upon by an unbalanced force NEWTON’S FIRST LAW OF MOTION Also known as the Law of Inertia Explains many common events NEWTON’S FIRST LAW OF MOTION The amount of inertia an object has depends on its mass. The greater the mass of the object----the greater the inertia the object has Mass can be defined as a measure of the inertia of an object NEWTON’S 2ND LAW OF MOTION Explains how force, mass, and acceleration are related Net force on an object is equal to the product of its acceleration and its mass FORCE = MASS X ACCELERATION NEWTON’S 2ND LAW OF MOTION Mass measured in kg Acceleration measured in m/s2 Force measured in (kg x m/s2) or N (Newtons) N = 1kg x 1m/s2 N = the force needed to move 1 kg of mass at 1 meter per second per second NEWTON’S 2ND LAW OF MOTION Acceleration = Force Mass NEWTON’S 2ND LAW OF MOTION A 52 kg water-skier is being pulled by a speedboat. The force causes her to accelerate at 2 m/s2. Calculate the net force that causes this acceleration. Force = 52 kg x 2 m/s2 Force = 104 N NEWTON’S 2ND LAW OF MOTION What is the net force on a 1,000kg elevator accelerating at 2 m/s2? A net force of 825 N is needed to accelerate a 55kg cart. What is the acceleration? NEWTON’S 2ND LAW OF MOTION Acceleration and force change in the same way force causes acceleration Acceleration and mass change in opposite ways mass causes acceleration Friction & Gravity Friction: the force that one surface exerts on another when the two rub against each other Acts in a direction opposite the object’s direction of motion Friction Strength depends on: Type of surfaces involved How hard the surfaces push together Rough = friction Smooth = friction Type of Friction Sliding friction: solid surfaces slide over each other Rolling friction: object rolls over surface Fluid friction: object moves through gas/liquid Gravity A force that pulls objects toward each other Free fall: when the only force acting on it is gravity Projectile: object thrown Gravity A projectile in a free fall falls at the same rate 9.8 meters per second for every second, velocity increases by 9.8 meters per second Gravity All objects are supposed to fall at the same rate Don’t because of air resistance Upward force Type fluid friction Gravity Increase surface area, increase air resistance Air resistance increases with velocity---falling objects speed up so air resistance increases. Gravity Eventually air resistance equals the force of gravity Forces balanced—no acceleration Continues to fall, velocity no longer increases Terminal velocity: greatest velocity an object reaches Gravity Weight: measure of force of gravity on an object Mass: measure of amount of matter in an object Weight = mass x 9.8 m/s2 Gravity Law of Universal Gravitation: the force of gravity acts between all object in the universe Depends on mass and distance Farther apart the objects, weaker the force Action and Reaction Forces are NOT one-sided Newton’s 3rd Law: If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction o the first object Action and Reaction Do action/reaction forces cancel each other out? NO; forces can only be added together if they are acting on the same object. Momentum Momentum = Mass X Velocity Quantity of motion kg X m/s is unit for momentum Momentum is described by its direction as well as its quantity Momentum Which has more momentum: a 3kg sledgehammer swung at 1.5 m/s or a 4kg sledgehammer swung at .9 m/s? M = mass x velocity