ENERGY Part I
Learning Goals for Section 1 The Nature of Energy Distinguish between kinetic and potential energy. Calculate kinetic energy. Describe different forms of potential energy. Calculate gravitational potential energy.
What is Energy? the ability to do work the ability to cause a change
Work, Power, Efficiency Work = (Force) (distance) Power = Work / time Work measured in Nm 1 Nm = 1 Joule Power = Work / time Power measured in J/s 1 J/s = 1 Watt 1 HP = 764 Watts Efficiency = (Wout / Win) x 100 Efficiency is a percentage (%)
Work Power Efficiency Problems Amy uses 20N of force to push a lawn mower 10 meters. How much work does she do? Frank does 4.8 E4 J of work in climbing a set of stairs. If he climbs the stairs for 2 minutes, what is his power output? A block and tackle does 1.25 E5 J of useful work, but friction limits the block and tackle to an efficiency of 45%. What is the amount of input on work block and tackle?
Kinetic Energy Kinetic Energy Equation energy of motion depends on mass and speed depends on speed more than it depends on mass – LOOK AT THE EQUATION!!! Kinetic Energy Equation kinetic energy (joules) = ½ mass (kg) × [speed (m/s)]2 KE = ½ mv2
Kinetic Energy Equations KE = ½ mv2 (Joules) m = 2 KE (kilograms) v2 v = √2KE (m/s) m
KE Problems What is the KE of an 8 kg mass at 5 m/s? What is the mass of an object that has 100 J of KE when moving at 5 m/s? Determine the velocity of a 6 kg mass has 75 J of KE. A rocket with a mass of 1.5 E4 kg accelerates at 220 m/s2 for 29s from an initial speed of 5200 m/s. Determine the kinetic energy of the rocket before and after the acceleration.
stored energy Potential Energy stored energy in an object means the object has potential to cause change
Gravitational Potential Energy stored energy due to position depends on both mass and height Gravitational Potential Energy Equation GPE (J) = mass (kg) × acceleration due to gravity (m/s2) × height (m) GPE = mgh remember… mass x acceleration due to gravity = weight acceleration due to gravity is 9.8 m/s2
GPE Equations GPE = mgh (Joules) m = GPE (kg) gh h = = GPE (m) mg g = GPE (m/s2) mh
GPE Problems What is the GPE of a 3 kg ball that is 2 m above the floor? How high do you have to lift a 5 kg box to give it 98 J of GPE? What is the weight of an stone that has 1200 J of GPE when setting on a 275 m cliff
Elastic Potential Energy energy stored by something that can stretch or compress – rubber band, spring PEE = ½ kd2 K is the spring constant for the material Measured in N/m d is the compression/expansion distance of the material Measured in meters
PEE Equations PEE = ½ kd2 (Joules) k = 2PEE (N/m) d2 d = √2PEE (m) k
PEEProblems The elastic force constant of a spring in a toy is 550 N/m. If the spring is compressed 1.2 cm, compute the potential energy stored in the spring. The spring constant (k) of a car’s front coil is 1800 N/cm. When the front tire rolls over a rock, the spring is compressed 15 cm from its equilibrium position. How much PEE is stored in the spring?
PEEProblems continued A 100.0 g arrow is pulled back 30.0 cm against a bow, which has an elastic force constant of 125 N/m. If the entire potential energy stored in the bow is converted to the kinetic energy of the arrow, compute the speed of the arrow.