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Published byElisha Larner Modified over 3 years ago

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Types of Levers First Class: fulcrum located between effort and resistance forces, used to multiply force. If a large force is used on shorter end it can multiply distance.

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**More first class levers…**

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Second Class Lever Resistance is located between the effort force and the fulcrum, always multiply the force

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**More second class levers…**

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Third class levers The effort force is located between the resistance force and the fulcrum. Effort arm is always shorter that the resistance arm Can’t multiply force and it’s MA is always less than one.

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**More third class levers…**

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Pulleys Works like a first class lever, uses a rope instead of a bar and the axle acts like the fulcrum. Two sides of pulley are the effort arm and the resistance arm. Fixed: can change the direction of force but doesn’t multiply the effort force Movable: multiplies the effort force, MA >1 effort distance must be greater to conserve energy.

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Using machines A combination of two or more simple machines is a compound machine. Ex) axe- wedge & lever Bicycle: Overall mechanical advantage is the ratio of the resistance force exerted by the tires on the road to the effort force exerted by the riders feet.

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Now…apply this Use at least four steps to design a device that performs a simple task. Draw it out. Use simple or compound machines.

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**Efficiency Machines can never reach 100% efficiency**

A measure of how much of the work put into a machine is changed to useful work put out by a machine. Efficiency= Wout X 100% = Fr X dr Win Fe Fe Machines can never reach 100% efficiency

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**Gravitational Potential Energy**

PE = mgh Mass, m = Height, h= Free-fall acceleration, g= 9.8m/s2 Ex) a 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35m above the base of the cliff? M=65 kg h= 35 m g= 9.8 m/s2 PE= (65 kg)(9.8 m/s2)(35m) = 2.2 x 104 J

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Practice: Calculate the gravitational potential energy in the following systems: A car with a mass of 1200 kg at the top of a 42 m high hill A 65 kg climber on top of Mt. Everest (8800 M high) A 0.52 kg bird flying at an altitude of 550 m

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Practice: Lake Mead, the reservoir above Hoover Dam, has a surface area of approximately 640 km2. the top 1 m of water in the lake weighs about 6.3 X 1012 N. The dam holds that top layer of water 220m above the river below. Calculate the gravitational potential energy of the top 1 m of water in Lake Mead. PE=(6.3 X 1012 N)(220 M)

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Practice A science student holds a 55 g egg out a window. Just before the student releases the egg, the egg has 8.0J of gravitational potential energy with respect to the ground. How far is the student’s arm from the ground in meters? h = PE/mg PE=8.0 J m = .055kg g= 9.8 m/s2 15m

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Practice A diver has 3400 J of gravitational potential energy after stepping up onto a diving platform that is 6.0 m above the water. What is the diver’s mass in kilograms? M= PE/ gh PE= 3400 J g = 9.8 m/s2 h = 6.0 m 3400J/ (9.8 m/s2 )(6.0 m) = m =58 kg

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Bonus: A spider has J of gravitational potential energy as it reaches the halfway point climbing up a 2.8 m wall. What is the potential energy of the spider at the top of the wall? (hint- you don’t need to find the mass!) 0.16J

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Bonus: A 0.50 g leaf falls from a branch 4.0 m off the ground to a bird’s nest 2.5 m off the ground. How much gravitational potential energy did the leaf lose? J

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Kinetic Energy KE = 1/2mv2 What is the kinetic energy of a 44 kg cheetah running at 31 m/s? mass, m= 44 kg speed, v= 31 m/s KE = ? KE = 21000J

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Practice: Calculate the kinetic energy in joules of a 1500 kg car moving at the following speeds: 29 m/s 18 m/s 12 m/s

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Practice A 35 kg child has 190 J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill? 3.3 m/s A bowling ball traveling 2.0 m/s has 16 J of kinetic energy. What is the mass of the bowling ball in kilograms? 8 kg 3.3m/s 8 kg

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