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Unit 2. Force. Force – push or pull. a force always acts in a certain direction ex. if you push something, the force is in the direction of the push. Forces in Nature. Electric force ex. lightning Magnetic force ex. magnetic poles Gravity. Weight is a force. Newton.

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## Unit 2

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**Unit 2**Force**Force – push or pull**• a force always acts in a certain direction • ex. if you push something, the force is in the direction of the push**Forces in Nature**• Electric force • ex. lightning • Magnetic force • ex. magnetic poles • Gravity**Newton**• metric unit of force**What is gravity?**• Sir Isaac Newton • 1642 – 1727 • The Principia**Gravity**• force of attraction between all objects in the universe**Gravity**• the larger an object’s mass, the greater the gravitational force**Gravity**• the greater the distance between two objects, the smaller the gravitational force • the force of gravity decreases by the amount equal to one divided by the distance (d) squared • 1/d2**What is air resistance?**• air resistance • force that opposes the movement of an object in air**Air Resistance**• a larger surface area usually results in greater air resistance • lighter objects feel more air resistance than heavier objects**Terminal Velocity**• speed at which air resistance and gravity acting on an object are equal • as an object falls, it’s speed increases at a steady rate until it reaches it’s terminal velocity**Vacuum – empty space**• in a vacuum, all objects fall at the same speed**What is friction?**• Friction • force that opposes the motion of an object • the force of friction works in the opposite direction of the force of motion**Types of Friction**• Sliding Friction • the source of friction is the contact between two surfaces, at least one of which is in motion**Types of Friction**• Air Resistance • friction results from air pushing on an object as it is moving**Types of Friction**• Rolling Friction • friction between two surfaces that are not in constant contact • ex. wheels**Friction**• friction makes motion possible • friction also makes it hard to move objects • reducing friction makes it easier to move objects**How can friction be reduced?**• by changing sliding friction into rolling friction • by using lubricants • Lubricants – substances that reduce friction**What is pressure?**• Pressure • force per unit area**Pressure**• pressure is the amount of force acting on a surface • pressure equals force divided by area • p = f/a**Pressure**• pressure can be changed by changing the amount of force acting on an area • pressure can be changed by changing the area on which a force acts**Fluid Pressure**• pressure in gases and liquids**Air Pressure**• air pressure is caused by the motion of particles in the air • you do not feel the weight of the air because the pressure inside your body is equal to the air pressure around you**Air Pressure**• air pressure changes with altitude • the higher you are, the lower the air pressure**Air Pressure**• Barometer • instrument used to measure air pressure**Water Pressure**• the force of water pressure on an object is the same in all directions • water pressure changes with depth • the deeper you are, the greater the water pressure**Water Pressure**• Manometer • instrument used to measure pressure in a liquid**What is Bernoulli’s principle?**• Bernoulli’s Principle • principle that states that as the speed of a fluid increases, its pressure decreases**Bernoulli’s Principle**• airplane wings are designed to use Bernoulli’s principle • air traveling over the wings moves faster than air underneath • this reduces the pressure on top of the wing, allowing it to be pushed upward**Bernoulli’s Principle**• three forces combine to help an airplane fly • the upward force on a plane’s wing is called lift • a forward force, or thrust, helps the plane take off and maintain air speed • the air resistance on a plane is called drag

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