ENERGYPart I

Learning Goals for Section 1The 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) Work measured in Nm

1 Nm = 1 Joule

Power = Work / time Power measured in J/s

1 J/s = 1 Watt1 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

energy of motiondepends 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.

Potential Energy

stored energystored energy in an object means the object has potential to cause change

Gravitational Potential Energy

stored energy due to positiondepends 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.