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# Physics chapter 10 and 11

## by shaffelder on Jan 19, 2012

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Ch 10,11 Notes

Ch 10,11 Notes

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## Physics chapter 10 and 11Presentation Transcript

• Chapter 10 Work, Power & Simple Machines
• Work
• Work – product of force exerted on an object and the distance the object moves in the direction of the force.
• W = F (d) Units: N*m = 1 Joule
• W = F (d) cos θ
• Power
• Power – the rate of doing work; rate at which energy is transferred
• P = W / t Units: J/s = 1 Watt
• Machines
• Machines –
• ease the load by changing the magnitude or direction of a force, but it does not change the work done.
• 2 Parts of the Machine
• Effort Force (F e ) - force exerted by you
• Resistance Force (F r ) - Force exerted by the machine
• W in = F e (d e )
• W out = F r (d r )
• Mechanical Advantage (MA) is the ratio of resistance force to effort force
• MA = F r / F e
• assumes transfer of all energy (No friction)
• IMA = d e / d r
• Efficiency of a Machine
• Efficiency of a machine is the ratio of output work to input work.
• Efficiency = (W out / W in ) x 100
• Efficiency = (MA / IMA) x 100
• 6 Simple Machines
• Lever
• Pulley
• Wheel and Axle
• Simple machines in the inclined plane family
• Inclined Plane
• Wedge
• Screw
• 1 st Class Lever
• The pivot is between the effort and the resistance (load)
• Ex: Catapult, See-saw
• 2 nd Class Lever
• Pivot is on one end and resistance force is in the middle
• Ex: Wheelbarrow
• 3 rd Class Lever
• Pivot is on one end and effort force is in the middle
• Ex: Broom, Hockey Stick, Shovel
• Chapter 11 Potential and Kinetic Energy
• Energy
• Energy – the ability to change an object or its surroundings
• There are several types of energy, but we will focus on these two.
• Potential Energy – energy stored in an object because of its position or state
• PE = mgh
• Kinetic Energy – energy due to the motion of an object
• KE = 1/2mv 2
• Work Energy Theorem
• The net work done on an object is equal to its change in kinetic energy.
• Work = ∆KE = KE f – KE i
• If the net work is positive (Force is in the same direction as motion) then KE increases; if the net work is negative (Force in the opposite direction as motion) then KE decreases.
• F d = KE f – KE i
• The Law of Conservation of Energy
• Within a closed, isolated system energy can change form, but the total amount of energy is constant.
• KE i + PE i = KE f + PE f
• Mechanical Energy = KE + PE
• Energy of a Pendulum
• Where is the potential energy the greatest?
• 1 and 5
• Where is the kinetic energy the greatest?
• 3
• If the pendulum has 10 J of kinetic energy at point 3, how much mechanical energy does it have at point 4?
• 10 J
• Why is the first hill the highest on a roller coaster?
• It has to store the most potential energy so it can be converted to kinetic energy to finish the ride.
• The energy conversions in the pole vault
• Kinetic on the runway
• Elastic when the pole bends
• Potential and Kinetic as he moves off the ground
• All potential at bar clearance
• Kinetic as he hits the mat
• Thermal after the landing
• Elastic and Inelastic Collisions
• Perfectly Elastic Collision (Bounces) – A collision where the kinetic energy is conserved
• Inelastic Collision (Sticks) – A collision where some of the kinetic energy is changed into other forms (usually heat)