Transcript Work Horsepower

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)
m
F
F=
(Units)
m
x
a
lbs = lb-sec2/ft x ft/sec2
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
T
F
D
T=FxD
(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
W=FxD
(Units)
ft-lb = lbs x ft
D
Wt
Energy

Capacity for doing Work

Two types 
Potential Energy (stored energy)


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
Kinetic Energy (energy of motion)


Battery
Stretched rubber band
Elevated weight
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=½mx
(Units)
2
V
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)
Wt
Lift in 4
Seconds
Wt
Wt
10 ft
Lift in 2
Seconds
Wt
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)

So E goes by V2 and t is halved means Power
required is doubled
= halved
Wt
Wt
Lift in 4
Seconds
Wt
10 ft
Lift in 2
Seconds
Wt
3 Ways We Deliver Power


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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-Power
Summary
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Work – application of force over a
distance
W=FxD
Energy - capacity for doing Work
E = ½ M x V2
Power – How quickly work can be done
P = E/t
t = time
Horsepower
= T xN
Constant
?
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1. An example of Kinetic Energy would be:
a) a moving car
b) a stretched rubber band that was just
released
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c) a charge particle in an electric field
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d) all of the above
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An example of Potential Energy would be:
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a) a moving car
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b) a battery
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c) a book resting on a table
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d) both b and c
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An example of a system having both kinetic and
potential energy would be:
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a) a book resting on a table
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b) a piece of sugar
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c) an object in free fall
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d) a stretched rubber band
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Which of the following statements is not correct
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a) energy is the capacity to do work
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b) Work can be express as Force x Distance

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c) power is the amount of work done in a unit
of time
d) the unit of power is the ft-lb