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Transcript
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 (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 + W = PEf + KEf Remember… W=… PE=… KE=… W= (F)(Dr) cosq PE = mgh KE=1/2 2 mv PEi + KEi + W = PEf + KEf A Dodge Viper (1490kg) starts at rest and coasts down a 400m long incline that is tilted 5° relative to horizontal. The road then is level. a) Ignoring friction and drag, what speed will the car reach by the bottom of the hill? b) If the coefficient of static friction for the tires on the road is 0.95, how far will it take the car to stop? PEi + KEi + W = PEf + KEf A 30kg child on a 5 kg sled starts from rest at the top of a 100m long, 17m tall hill. a) Compute the speed the child would reach by the bottom of the hill if it was frictionless. b) Real hills, even icy ones have friction. When the child reaches the bottom, she is going 10.0m/s. Compute the work done by friction, and size of the friction force. 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.
