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NEWTON’S LAWS FORCE • Force: The push or pull on an object • Net Force: The overall force on an object • Unbalanced force: When there is a net force that causes acceleration • Balanced forces: Equal forces acting in opposite directions that will not change an objects motion Examples of Force A flag being blown by the force of the wind. A jet engine propelling an airplane forward. MODELING FORCE UNBALANCED FORCES BALANCED FORCES MODELING FORCE UNBALANCED FORCES IN THE SAME DIRECTION PICTURING BALANCED FORCES PICTURING UNBALANCED FORCES Friction • The force that one surface exerts on another when the two objects rub against each other • Friction acts in a direction opposite the object’s direction of motion. • Without friction an object would continue to move at a constant speed forever Factors and Friction • The strength of friction depends on two factors –The type of surfaces involved –How hard the surfaces push together • Rough surfaces produce greater friction than smother surfaces Sliding Friction • Sliding friction - when two solid surfaces slide over each other. • A snowboarder slides over the snow covered slopes using sliding friction everyday. Rolling Friction • Rolling friction - when an object rolls over a surface. • Skate boarders take advantage of this type of friction all the time. Reducing the amount of friction between the surface and the wheels allow skaters to go really fast. Fluid Friction • Fluid friction - when an object moves through a fluid. • This is how a surfer glides over the water or a shark glides through the water. This type is called fluid friction. NEWTON’S FIRST LAW OF MOTION • An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by some outside force Inertia • Inertia is the resistance to change in motion • EXAMPLE: If a car stops suddenly inertia causes you to continue moving forward. In order for change in motion to occur a force is required, such as a seat belt or the windshield Mass • The amount of matter in an object • EXAMPLE: A jar of pennies has more mass than a jar of feathers • The amount of inertia on an object depends on the objects mass • The greater the mass, the greater its inertia NEWTON’S SECOND LAW OF MOTION • The net force of an object is equal to the product of its acceleration and mass • FORCE = MASS x ACCELERATION • UNITS = kg • m/s² =NEWTON (N) PRACTICE THE SECOND LAW • A 53kg water-skier is being pulled by a speedboat. The force causes her to accelerate at 2m/s². Calculate the force. • Force = mass x acceleration • Force = 52kg x 2m/s² • Force = 104 N LETS PRACTICE AGAIN • What force is needed to accelerate an object with a mass of 25000 grams at a rate of 10 m/s2? • Convert grams to kilograms – 25000 g = 25 kg • Force = 25 kg x 10 m/s2 • Force = 250 N LET’S TRY ANOTHER ONE • What is the force on a 1,000 kg elevator accelerating at 2 m/s²? • Force = 1000kg x 2 m/s² • Force = 2000 N SOLVING FOR ACCELERATION • You can use force to find acceleration • Acceleration = force/mass • You can increase acceleration by change force • Increase force = increase acceleration • Increase mass = decrease acceleration LETS FIND ACCELERATION • A 825N force is exerted on a 55kg cart. Find the carts acceleration. • Acceleration = force/mass • Acceleration = 825N / 55kg • Acceleration = 15 m/s² NEWTON’S THIRD LAW OF MOTION • For every action there is an equal but opposite reaction Example of the 3rd Law Momentum • Momentum is the product of its mass and velocity • The more momentum an object has the harder it is to stop • Momentum = mass x velocity • Unit = kg • m/s Let’s Practice • Which has more momentum: a 3kg sledgehammer swung at 1.5 m/s or a 4 kg sledgehammer swung at 0.9 m/s • Momentum = mass x veloctity Momentum1 = 3 kg x 1.5 m/s • Momentum1 = 4.5 kg•m/s Momentum2 = 4 kg x 0.9 m/s • Momentum2 = 3.6 kg•m/s Conservation of Momentum • Lw of Conservation of Momentum states that the total momentum of the objects that interact does not change. • The total momentum remains the same unless acted upon by an outside force Momentum- 2 moving objects • When a car bumps into another car that is going slower, it transfers its momentum into the other car. • The car that got bumped is now going faster, while the car that hit it is going slower Momentum- 1 object moving • If a moving objects hits a nonmoving object all the momentum is transferred. The nonmoving object begins to move and the moving object stops. Gravity • Gravity-The force that pulls objects towards to Earth. • Free fall: When gravity is the only force acting on an object –Objects in free fall accelerate as they move towards the Earth. Terminal Velocity • Terminal Velocity is the greatest velocity an object can reach will falling Gravity • Near the Earth’s surface the rate of acceleration due to gravity is 9.8 m/s² –This does not take into account friction • All objects will accelerate down at the same rate regardless of mass –A brick and a feather will hit the ground at the same time if it were inside an airless chamber (vacuum) Projectile versus free fall • An object thrown and an object dropped will both free fall at a rate of 9.8 m/s² • The will both hit the ground at the same time • Example: Shot a bullet and drop a bullet at the same time it would land at the same time. Air Resistance • Air resistance: Objects falling through the air experience this fluid friction • Air resistance pushes the object up as it falls causing it to slow down Air Resistance and Mass • Air resistance is not the same for all objects • -A feather will fall slower then a rock because of air resistance • Air resistance increase as the velocity of the object increases. • Eventually air resistance will equal the force of gravity and an object could stop falling Weight • Weight is the force of gravity on a person or object at the surface of a planet • Weight = mass X acceleration due to gravity • Weight = mass x 9.8m/s² Do the Math - weight • What is the weight of a person with a mass of 75kg • Weight = mass x acceleration due to gravity • Weight = 75kg x 9.8 m/s² • Weight = 735N Law of Universal Gravition • The law of universal gravitation states that the force of gravity acts between all objects in the universe • Any two objects in the universe without exception attract each other • We cannot tell most of the time because the force is small

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