What is it? What makes it change?

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What is it? What makes it change?

Presentation transcript:

What is it? What makes it change? Mechanical Energy What is it? What makes it change?

Mechanical Energy is… The total amount of kinetic and (gravitational) potential energy Changed when an external force does work on a “system” Conserved when no external forces do work PEi + KEi = PEf + KEf

(fairly good) Examples of Mechanical energy being conserved… PEi + KEi = PEf + KEf Swinging pendulums Roller coasters Water slides Sleds on hills

Reality: The Law of the Conservation of Energy PEi + KEi + Wext = PEf + KEf

Remember… W=… PE=… KE=…

W= (F)(Dr) cosq PE = mgh KE=1/2 mv2

PEi + KEi + Wext = PEf + KEf A Dodge Viper (1490kg) starts at rest and coasts down a 400m long incline that is tilted 5° relative to horizontal. Ignoring friction and drag, what speed will the car reach by the bottom of the hill? If the car actually reaches the bottom with a speed of 18.0m/s, how much work was done by external forces (drag, friction) c) How much work was done by the normal force?

PEi + KEi + W = PEf + KEf A 3000kg roller coaster starts from rest at the top of a 100m long, 20.0m tall hill. The first drop reaches ground level, then the coaster goes over the crest of a 15.0m tall hill. Compute the speed the coaster would reach by the bottom of the first hill (if friction and drag are ignored). Compute the radius of the crest if the riders are to feel weightless there.

PEi + KEi + W = PEf + KEf An 8m tall water slide features a 45° frictionless slope followed by a long level portion with 20cm deep water in it to gradually bring the riders to a stop. Compute the speed of riders at the bottom of the ride. Compute the work done by resistive forces to bring a 50kg child to a stop. How would your answers change to a&b if the rider was 100kg?

PEi + KEi + W = PEf + KEf http://www.physicsclassroom.com/mmedia/energy/pe.cfm For additional support, see the PCR W&E Lesson2: http://www.physicsclassroom.com/class/energy

What is the mathematical relationship between final speed (m/s) and amount of gravitational potential energy (J) “lost”? (Note: the initial speed is zero.) Use the Law of the conservation of energy to predict the shape of the graph you will see when you perform this experiment. Devise a way to perform this experiment using the set-up shown on the front board. Compare your results to your hypothesis. Account for any differences.