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

Mechanical Energy is… The total amount of (macroscopic) kinetic and (gravitational) potential energy Changed when an external* force does work on a “system” Conserved when no external forces do work PEi + KEi + Wext= PEf + Kef * ALL our mechanical forces are EXTERNAL except the force of gravity)

(fairly good) Examples of Mechanical energy being conserved…(Wext= 0) PEi + KEi + Wext= PEf + KEf Swinging pendulums Roller coasters Water slides Sleds on icy hills Coasting down a small hill on a bike while tucking and wearing those silly looking lycra shirts and shorts https://www.physicsclassroom.com/mmedia/energy/pe.cfm

Examples of Mechanical energy NOT being conserved…(Wext≠ 0) PEi + KEi + Wext= PEf + KEf A paratrooper falling with a parachute open Sliding down a sliding board A car slowing to a stop or speeding up by the brakes or the engine Peddling a bike up a hill

Ex 1, 2 How fast can a 30kg child go by the bottom of a frictionless sliding board that is 3.0m tall and 6.0m long? If the child is only going 4.0m/s at the bottom of a real 3.0m tall, 6.0m long slide, how strong was the friction force on the child?

Ex 3 If an athlete can exert a force of 300N horizontally to a 90kg blocking sled that is initially at rest, but a friction force of 250N acts on the sled also, how fast can the athlete get the sled moving in 5.0m?

Ex 4 You wish to make riders feel a normal force as strong as the gravitational force on them when they are upside-down in the top of a circular roller coaster loop. How many times taller is the start hill than the top of the loop? Assume no friction or drag.

Helpful steps to solving problems with PEi + KEi + Wext= PEf + KEf Make a sketch. Label all the things you know on the sketch. Make the height zero at the lowest point, UNLESS the problem has already noted you are computing things relative to a certain vertical position. Remember the individual formulas for PE, KE, W

Power Lab Objective: Determine one’s own power output while climbing a flight of stairs, at a constant speed. Report: Word problem that includes all measurements. Solution with succinct explanations Answer written in one sentence (include the value in watts AND horsepower)