PHYSICS

Unit 3 Kinetic and Potential EnergyConservation of Mechanical Energy

PORTFOLIO 2 CONSERVATION OF MECHANICAL ENERGYDUE 9/26/19

If/Then Thesis Statement

Required for full credit for

this lab

This info in included in your Intro

section

This info in included in

your Materials and Methods

section

THIS INFO IN INCLUDED IN YOUR DATA AND ANALYSISSECTION

CONCLUSION SECTION

FINAL CHECK LIST

DO NOT NEED TO COMPLETE HYPOTHETICAL

AS USUAL – VIRTUAL LAB = IDEAL CONDITIONS

•Real spring will not oscillate forever

•Friction acts on the spring and will cause heat

•Mechanical energy is also not conserved

• Therefore kinetic energy that is lost is due to heat loss from friction

•This is why the potential and kinetic energy are not the same in real life

WORK

• Work = force x displacement

• Work can be positive or negative

• Unit = Joules (J) or kilojoule (kJ)

• displacement needs to be in meters

• force needs to be in N (kg ◦ m/s2)

• Student holding chair has completed no work!

• There is a trick – if no displacement then NO PHYSICAL WORK is done

• Also work is done only when component of force is parallel to displacement

• Even though it is hard “work” holding huge gear bag even when you move = no work

• only work is moving gear bag onto back and again when lowering to ground

• Climber has displacement up while gravity force downward = negative work

•A good samaritan stops to help a person with their broken

car. A 400N force is applied to the move the 1325kg car

the 25.0 m to get off the road. How much work was

completed?

• W = F x d

•= 400N x 25.0m

•= 1.00 x 104 J

•A child pushes his toy truck with a 68.0N force at a 42.0°

angle downward from the horizontal for .0025km

• When we have an angle

•W = F cos x d

•= .0025km = ?m

• x = 2.5m

•= (68 cos 42°) 2.5

•=126 J

•A neighbor pushes a lawnmower 4 times as far as you do

but exerts only ½ of the force, who does more work and

by how much?

•Wneighbor = F x 4d

•Wyou = ½F x d

•Easiest way to figure it out is to choose a value and

calculate

•Wn = 2 x 4(5) = 40J Wy = ½2 x 5= 5J

•Neighbor will do 8x more work

•Kinetic Energy – energy of an object due to the objects motion

•KE=

•Unit = J

•

•

•What is the kinetic energy of a .0012 kg golf ball hit at 62 m/s ?

•KE=

•=

•2.3 J

•Work-Kinetic Energy Theorem

•allows us to think of kinetic energy as energy stored in the motion

of an object

• even when object changes speed

•Wnet =

•Dash, who is 21kg runs with a speed of 1.8 m/s till he crosses the

finish line. The coefficient of friction of the new age track is .22.

How far is his race?

• W = F cos x d

• We sub in using the Fk for force with kinetic friction

• Wnet =

• We also know

• Wnet = 𝑖2

• Set each equation equal to each other and then solve for displacement

• = 1.8 m/s .22 m= 21kg g = 9.81 m/s2

• =

•= Running on horizontal so

•

•d =

•=

= .075.08 meters

• Potential Energy – energy due to position

•Can be thought of as “stored” energy

• PEg = mgh units = Joules

• In ideal conditions energy is conserved so all potential energy

becomes kinetic energy

•What is the kinetic energy of a bowling ball dropped from the

third story of a building just before it reaches the ground? Each

story of the building is 3.8m and the ball has a mass of 5.0kg.

• PEg = mgh

• h = 3.8x3

•=11.4m

• PEg = 5.0 (9.81)(11.4)

•= 559.17

•= 560 J

• Elastic Potential Energy = PEelastic

•Depends on the distance an object is compressed or stretched

• PEelastic = ½ kx2

• k = spring constant x = distance compressed or stretched

• Total PE in a system PEtot takes into account PEg due to position

and elastic potential energy if there is a compressed or stretched

component

• PEtot = PEg + PEelastic

•Deriving equations

•As we build our equation bank we will need to substitute in and

solve for specific variables more frequently!

• This course is cumulative so you don’t get to forget as we move

forward

•Keep resources organized

•Use organized equation sheets

•Know what all components mean and labeled clearly

•On U3 Quiz 1 you will be required to derive an equation using

your knowledge of these 3 given equations:

• Power = work/change in time P=

•W= and v =

•Don’t overthink and psych yourself out!