Work, Energy&

Power

Quick Review

We've discussed FORCES Magnitude – How hard is the “push” Direction – Which way does it act upon the

object Applying a FORCE causes an object to

accelerate (F=ma)

F

(Units) lbs = lb-sec2/ft x ft/sec2

m

F = m x a

lb-sec2/ft = slug

Quick Review

We've discussed TORQUE A FORCE that serves to “spin” an object

around a given point Torque = Force x Distance

F

T = F x DD

T

(Units) ft-lbs = lbs x ft

Quick Review

We discussed gaining “mechanical advantage” Linear forces - Lever Mechanisms Rotary force (torque) - Gears, sheaves/belts,

sprockets/chain

Take the Next Logical Step!

Work Work is the application of a force over a

distance Lifting a weight from the ground and putting it

on a shelf is a good example of work

Wt

Wt

D(Units) ft-lb = lbs x ft

W = F x D

Energy

Capacity for doing Work Two types -

Potential Energy (stored energy) Battery Stretched rubber band Elevated weight

Kinetic Energy (energy of motion) Car speeding down the road

Many times both are present

Energy

Kinetic Energy For an object of mass m, moving with

velocity of magnitude V, this energy can be calculated from the formula

E = ½ m x V2

(Units) ft-lbs = lb-sec2/ft x ft2/sec2

POWER Power is the work done in a unit of time Power is a measure of how quickly work can be

done POWER (P) is the rate of energy generation (or

absorption) over time: P = E/t The unit of power is the Watt

746 Watts = 1 Horsepower

Work & Power What can we say about the two examples

shown below? What can you say about how much work is

done for each? How about power requirements? (watts)

Lift in 4 Seconds

Lift in 2 Seconds

Wt

Wt

Wt

Wt

10 ft

Work & Power Work = F x D

Force and Distance is independent of time Work done is identical

Power = E/t Energy (E) = ½ m x V2

Time (t) = halved So E goes by V2 and t is halved means Power

required is doubled

Lift in 4 Seconds

Lift in 2 Seconds

Wt

Wt

Wt

Wt

10 ft

3 Ways We Deliver Power Mechanical Stored Energy

Bungy, rubber band, spring / trigger required Use lever principles to obtain “mechanical

advantage” Pneumatics

Stored compressed air acts on cylinder Use lever as above for “mechanical advantage”

Motors Variety of 12 VDC motors allowed Use sprockets, sheaves and gears to gain

advantage

MOTOR POWER

1HP = 746 watts HP = Torque x Speed

Constant So let's look at 2 different motors...

Typical Motor Curve

Typical Motor Curve

Work-Energy-PowerSummary

Work – application of force over a distance W = F x D

Energy - capacity for doing Work E = ½ M x V2

Power – How quickly work can be done P = E/t t = time

Horsepower = T x N

Constant

?

1. An example of Kinetic Energy would be: a) a moving car b) a stretched rubber band that was just

released c) a charge particle in an electric field d) all of the above

An example of Potential Energy would be: a) a moving car b) a battery c) a book resting on a table d) both b and c

An example of a system having both kinetic and potential energy would be:

a) a book resting on a table b) a piece of sugar c) an object in free fall d) a stretched rubber band

Which of the following statements is not correct a) energy is the capacity to do work b) Work can be express as Force x Distance c) power is the amount of work done in a unit

of time d) the unit of power is the ft-lb