WORK, ENERGY, AND POWER SPH4C – Unit 3 Day 2 Energy

BELL WORK

| #1 A farmer applies a constant horizontal force of magnitude 21 N on a wagon and moves it a horizontal distance of 3.2 m. Calculate the work done by the farmer on the wagon.

| #2 A truck does 3.2 kJ of work pulling horizontally on a car to move it 1.8 m in the direction of the force. Calculate the magnitude of the force.

Wedd= 21N × 3.2 m

a 67 J

0

Ws 3.2 kJ = 3200J W = Fed

Ddt 1.8 m EWeetnoooononT.EE#k

ZERO WORK

| If the force and displacement are perpendicular to each other, the work done by the force is zero.

| If no force is applied work is zero.

| If there is no displacement work is zero.

90°

SOME USEFUL DEFINITIONS

| Energy y the capacity (ability) to do work

| Kinetic Energy (EK) y energy possessed by moving objects

| Work – Energy Principle y the net amount of mechanical work done on an object

equals the objects change in kinetic energy

MORE USEFUL DEFINITIONS | Potential Energy

y a form of energy an object possesses because of its position in relation to forces in the environment

| Gravitational Potential Energy

y energy possessed by an object due to its position relative to the surface of the earth

| Reference Level y a designated level to which objects may fall y Considered to have a gravitational potential energy value of

0 J | Mechanical Energy

y the sum of kinetic energy and gravitational potential energy

KINETIC ENERGY

| scalar quantity

| Equation:

| OR

2

2kmvE

212kE mv

Check Units

←-

R

bug D mimIN s sxs

9 =÷.

kg_m.mkg.gr#s2=1Nm

= 1J

WORK ENERGY PRINCIPLE

| The net amount of mechanical work done on an object equals the object’s change in kinetic energy

Net kW E '

Net kf kiW E E �

2 2

2 2f i

Net

mv mvW �

GRAVITATIONAL POTENTIAL ENERGY

gE mgh Check Units ThighFyE!k Teena .

N . m= 1J

MECHANICAL ENERGY

mechanical K gE E E �

+Gravitational

Kinetic PotentialEnergy Energi

EXAMPLE #1 GRAVITATIONAL POTENTIAL ENERGY

| A 0.45 kg book is resting on a desktop 0.64 m high. Calculate the books gravitational potential energy relative to the desktop and the floor.

Given MTO -45 kgo

- hoo .64m

$0 .64m ( above floor)Requited Eg ( above desk) h=0

Eg( above floor) ( above desk)Select : Egimgh Relative to floor

Eps : Relative todeskEg=¢4skg)(98s⇒¢.64m)

tEjs←8↳l.sn#dEg=2.8T

EXAMPLE #2 GRAVITATIONAL POTENTIAL ENERGY

| The elevation at the base of a ski hill is 350 m above sea level. A ski lift raises a skier (mass 72 kg) to the top of the hill. If the skier’s gravitational potential energy relative to the base of the hill is now 9.2 x 105 J, what is the elevation at the top of the hill?

Pg . 142*4

Elevations '3S0ntBo4m Steps

jI→¥¥wIHIY¥÷g↳¥on sedtsttnghhquooom

Givei_m=72kg Utterly

h.LI#Eg=9.2X10IEkjatwngh=l304m

EXAMPLE #3 KINETIC ENERGY

| During a shot put a 5.0 kg shot leaves an athletes hand at 13 m/s. Calculate the kinetic energy.

EXAMPLE #4 MECHANICAL ENERGY

| A roller coaster car (mass 350 kg) is moving at a speed of 2 m/s at the top of a 65 m hill. Calculate the mechanical energy of the roller coaster car.

QUESTIONS

| Pg. 137 #11-14 | Pg. 138 #1-7 | Pg. 142-143 #1,3,5-10 | Pg. 146 #1-4,6

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