Chapter 12- Forces Physical Science. 12.1 Forces Force, Units Measuring Force vectors Combining forces Balanced Forces – Equilibrium Unbalanced forces

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Chapter 12- Forces Physical Science Slide 2 12.1 Forces Force, Units Measuring Force vectors Combining forces Balanced Forces Equilibrium Unbalanced forces acceleration Friction. Static, sliding, rolling, fluid Gravity acceleration Terminal velocity Air Resistance Projectile Motion Slide 3 Force Force (F) - A push or a pull on an object An influence or interaction Net Force (Fnet) Total Force on an object Forces in opp directions will subtract, forces in same direction will add Slide 4 Combining Forces Net Force Multiple forces may be acting on an object at the same time, the Net Force is the combination (resultant) of all these forces. Slide 5 Slide 6 Slide 7 Balanced Forces Forces in each direction cancel each other Net force of ZERO If forces are balanced, object is in equilibrium Object is either AT REST, or AT A CONSTANT VELOCITY Slide 8 Unbalanced Forces Net force is NOT Zero Unbalanced Forces (net force) causes acceleration Slide 9 Slide 10 Friction Static Friction Resistance to the start of motion Sliding Friction Resistance to one object sliding over another object Rolling Friction Resistance to one object rolling over another object About 100-1000 times less than Static/Sliding Static > Sliding > Rolling Fluid Friction Air Resistance Moving through water Slide 11 Gravity Force that acts between any two masses Only significant when one of the objects is very large (Earth, Sun, etc.) Pulls objects together (attractive force) Force of Gravity is also called weight Acts downward toward center of Earth Used balanced by some other force Support force from ground, chair, etc. asdf Slide 12 Falling Objects Gravity accelerates objects downward Air resistance acts opposite to gravity and slows down the acceleration Air resistance gets greater with velocity Eventually if falling so long that air resistance balances gravity you will reach terminal velocity Terminal Velocity constant velocity of a falling object when the force of air resistance equals force of gravity Slide 13 Terminal Velocity Slide 14 Projectile Motion Any object that is only under the influence of gravity and air resistance is a projectile If given some initial forward velocity, projectiles will follow a curved path Combination of initial forward velocity and downward vertical force of gravity causes it to follow curved path. Slide 15 12.2 Newtons 1 st & 2 nd Laws of Motion Is a force required for motion? Aristotle (300 BC) Proposed that force is required to keep an object moving at a constant velocity Galileo & Newton (1500s-1600s) Moving objects are that are not experiencing friction or any other force will continue to move indefinitely Would keep moving forever, if there is no friction Objects have a natural tendency to stay in motion or at rest Slide 16 Newtons 1 st Law of Motion Objects at rest will stay at rest, objects in motion will stay in motion unless acted upon by an outside force Objects have a natural tendency to resist changes in their motion Inertia property of matter that resists changes in its motion Mass is a measure of inertia.. More mass, more inertia Slide 17 Examples of Inertia Seatbelts Rollercoaster Car dashboard Tennis ball/string Earths rotation Helicopter Slide 18 Newtons 2 nd Law of Motion Slide 19 Weight & Mass Weight Force of gravity acting on an object Mass How much matter is in an object A measure of how much inertia an object has Weight = Mass x acceleration due to gravity W = mg Weight in Newtons Mass In Kilograms (kg) Acc of gravity in m/s 2 Slide 20 Gravitational force differs on other planets Mercur y VenusEarthMarsJupiterSaturnUranusNeptun e Gravity at equator (Earth=1) 0.38g0.9g1g0.38g2.64g0.93g0.89g1.12g Slide 21 .16.38 10 2.64 Slide 22 Slide 23 Why do astronauts float around inside the international space station? They are falling with the space station around Earth Continuously falling without ever running into anything. Gravity is still pulling them Slide 24 Why do satellites orbit Earth? Satellites are forced to circle the Earth by the pull of Earths gravity. To orbit a satellite must be travelling at a very fast speed! Around 17,000 mph or faster Satellites are falling around the Earth Objects in orbit feel weightless Not actually weightless Slide 25 12.3 Newtons 3 rd Law Newtons 3 rd Law For every action force there is an equal and opposite reaction force Equal in magnitude, opposite in direction Forces only exist in pairs (Action/Reaction) Cant have an action without a reaction Slide 26 * Action forces produce the interaction, and the reaction force must be in existence at the same time, have the same size, and be opposite in direction * You apply a 10 N eastward force to wall, wall applies a 10 N westward force to you Slide 27 * An astronaut in spacenot moving with just a spacesuit on and hes not tethered to the ship. If he is only a few meters away from his ship and there are no other people to help him how could you get back to his spaceship? Oh No!! Slide 28 Whats really pushing you? To accelerate you need an outside force. You cannot apply a net force to yourself! Something outside of you needs to push you in order to move/accelerate The astronaut in the previous slide need his tool to push him. Slide 29 Should always be able to identify each action/reaction x Slide 30 s Slide 31 Slide 32 * Car?? * Rocket?? * Person?? Slide 33 Are these forces equal? Bug/windshield Ball/Bat Small car/truck Slide 34 Slide 35 Momentum (p) Inertia in motion How much an object in motion wants to stay in motion How much motion an object has Momentum = mass x velocity P = mv Vector quantity Slide 36 Example Questions A 100 kg cart is moving with a velocity of 5 m/s, what is its momentum? 500 kg*m/s A 2 kg bowling ball is rolling with a speed of 5 m/s, what is its momentum? 10 kg*m/s Slide 37 f Slide 38 f Slide 39 Law of Conservation of Momentum The total amount of momentum in a system must stay constant unless an outside net force acts Internal forces cannot change momentum Collisions If no outside forces. Momentum is only exchanged between objects never lost Recoil from gun Slide 40 g Slide 41 12.4 Universal Forces There are four fundamental forces that exist in the universe that can act across a distance (push/pull things without touching) Strong Nuclear Force Acts on neutrons and protons in the nucleus of an atom to hold them together Electromagnetic Force Holds electrons to the positively charged nucleus Weak Nuclear Force Responsible for radioactive decay Gravity Attractive force that acts between any two masses Every object in the universe attracts every other object Slide 42 Slide 43


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