ENERGY, WORK, AND POWERUnit 3

TYPES OF ENERGY Energy is the capacity for an object to

do work For example, when a car moves, the

engine performs work to get the car going.

There are many different types of energy, including: electrical, kinetic, gravitational potential, and elastic potential to name a few.

A more complete list can be found on p. 124

ENERGY TRANSFORMATION An energy transformation occurs

whenever energy changes from one form into another.

Examples of this would be a ball being held above the ground (gravitational potential) and then being released to fall to the ground (kinetic).

WORK

WORK This is the energy transferred to an

object The object must move a distance as a

result of the force applied Does it matter what direction the object

moves??

HOW TO CALCULATE WORK Work requires a force Work requires a distance This leads us to say: WαF and WαΔd This gives us: W = F Δd The units are Newton Meters (Nm) or,

more commonly, Joules (J)

EXAMPLES A 600 N force is applied by a person to

a dresser that moves 2 m. Find the work done if the force and the displacement are Parallel At right angles Oppositely directed

A horse pulls a barge along a canal with a rope in which the tension is 1000N. The rope is at an angle of 10° with the towpath and the direction of the barge How much work is done by the horse in

pulling the barge 100m? What is the net force on the barge?

REMEMBER!!!! For there to be work,

Force and Distance must be

in the same plane.

POSITIVE AND NEGATIVE WORK Any force applied in the same plane

causes work to be done If the force makes the object increase

in speed, then it is positive work If the force makes the object slow its

speed, then it is negative work. These forces are called Dissipative Forces

All friction is negative work.

GRAVITY When we lift something up, we do

work, why is this? When we look at this type of work, we

still must look at the force we are working with Fg = mg

This lead to the following W = Fgd W = mgd

EXAMPLE A bag of groceries of mass 8.1 kg is

raised vertically without acceleration from the floor to a counter top, over a distance of 92 cm. Determine The force needed to raise the bag without

acceleration. The work done on the bag against the

force of gravity

MECHANICAL ENERGY

MECHANICAL ENERGY There are 2 types of mechanical energy

Gravitational Potential Energy Kinetic Energy

Gravitational Potential Energy This is energy that can be used to do work

at a lower level Kinetic Energy

This is the energy of motion

DETERMINING POTENTIAL ENERGY To hit a nail with a hammer, what must you do? By lifting the hammer, Δh, you also need to

apply a force. The height is measured from a starting point or

equilibrium position. The force is found by lifting the mass against

gravity Ep = FΔh Ep = mg(-)

EXAMPLE Assume that a 59 kg

pole vaulter must raise their center of mass from 1.1 m off the ground to 4.6 m off the ground. What is the jumper’s gravitational potential energy at the top of the bar relative to where the jumper started to jump?

Ep = mgΔh Ep = (59)(9.81)(4.6-1.1) Ep = 2.0 x 103 J

APPLICATIONS OF MECHANICAL ENERGY Grain Auger Pile Drivers Hydro Dams We use this in Red Lake everyday

DETERMINING KINETIC ENERGY If you are interested in how the formula

is generated, see p. 134 Kinetic energy is the energy of motion,

so what do we need? Ek = ½ mv2

EXAMPLE Determine the

amount of kinetic energy of a 48 g dart travelling at a speed of 3.4 m/s.

Ek = ½ mv2

Ek = ½ (.048)(3.4)2

Ek = 0.28 J

LAW OF CONSERVATION OF ENERGY

ENERGY CONSERVATION We know that there are many types of

energy transformations When energy changes forms, energy is

conserved What does this mean? Energy is never lost, it just changes

form

EXAMPLE

An object which weighs 10 N is dropped from rest from a height of 4m above the ground. When it has free-fallen 1 meter, its total mechanical energy with respect to the ground is_______

An archer needs to exert 275 N of force to pull her bow string back 0.500m. If the mass of the arrow is 3.00 , what is the final speed of the arrow?

A skier glides down a frictionless hill of 100 meters, the ascends another hill, of height 90 meters, as shown in the figure below. What is the speed of the skier when it reaches the top of the second hill?

100m90 m

EFFICIENCY A comparison of the amount of energy

put into a system compared to the amount of energy output in a system

%E = Eout/Ein

ENERGY INPUT AND OUTPUT Energy input is the amount of energy

that is being supplied by the person doing the work

Energy output is the amount of energy that would be created if it could be vertically released

POWER Power is the rate of doing work or

transferring energy This is a scalar quantity P = W/ΔT P = ΔE/ΔT