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FORCES A FORCE IS A PUSH OR A PULL To describe a force completely, you must know its… 1. 2. STRENGTH DIRECTION The sum of all of the forces acting on an object is the NET FORCE. 80N 50N 100N 12N 20N 18N 12N 70N NET FORCE = 70N 18N NET FORCE = 18N 20N GRAVITY is a force. GRAVITY causes objects elephants to fall toward earth. GRAVITY is a force that causes all objects in the universe to be attracted to each other. You exert a gravitational force on the stars as they exert a gravitational force on you! WEIGHT is a force Weight is caused by the force of gravity pulling on a object because of its mass. FRICTION IS A FORCE FRICTION is a force that OPPOSES MOTION between objects in contact with each other. The STRENGTH of a friction force is determined by… 1. How hard the objects press together ...press hard – MORE friction …press lightly – LESS friction 2. The type of surface Rough surface - MORE friction Smooth surface - LESS friction The 4 Main types of friction are STATIC FRICTION SLIDING FRICTION …exists before things move …exists when things slide past each other …exists when an object rolls ROLLING FRICTION …exists between FLUID FRICTION an object and a fluid AIR RESISTANCE is a type of FLUID FRICTION Free fall exists when gravity is the only force acting on an object – we don’t get free fall on earth’s atmosphere because of air resistance TERMINAL VELOCITY is the speed an object achieves when gravity is balanced out by air resistance INERTIA is the tendency of an object to resist changes in motion An object in motion will stay in motion… An object at rest will stay at rest… …Unless acted upon by an UNBALANCED force Ask her: Oh wait, you can’t ask her… …she wasn’t wearing a seatbelt …if the unbalanced force acting on you isn’t the seatbelt, it could be the dashboard or the windshield! NEWTON’s LAWS of MOTION Newton’s 1st Law of Motion AKA The Law Of Inertia An object at rest will remain at rest, and an object in motion will maintain its motion, unless acted upon by an unbalanced force. Remember: INERTIA is the tendency of an object to maintain its motion. Inertia is what makes us lean “out of” the turn in a car When the car turns to the left we feel like we are moving to the right, but really – we are just continuing in a straight line! Newton’s 2nd Law of Motion Force, Mass and Acceleration are related. FORCE = MASS x ACCELERATION Acceleration = Force / Mass Mass = Force / Acceleration NEWTON’s 3rd LAW of MOTION …if one object exerts a force on another object, then the second objects exerts an equal and opposite force on the first object. In simplified terms: For every action force, there is an equal and opposite reaction force. Action Force – the first force Reaction Force – the force that “reacts” to the action force Action / Reaction Forces DO NOT cancel each other out because they act on different objects! In order for you to walk… …you push backward on the floor …and the floor pushes forward on you MOMENTUM is related to an object’s mass and velocity MOMENTUM = MASS x VELOCITY Law of Conservation of Momentum The total momentum of any group of objects remains the same (is conserved) unless outside forces act on the objects. It can get more complicated – but we don’t have to get into that now! That’s why they teach physics in high school and college!