Transcript The Work - SHS Science

In terms of motion, what does
energy allow you to do?
Units?
How much work is done in each…?
A man squats 200 kg masses a
distance of 0.5 m.
A waiter carries a 10 kg tray 10 m
across the room.
A 65 kg woman skies down from
the top of Schweitzer (el. 1951m)
to the lodge (el. 1220m).
A child pulls on a rope with 10 N,
dragging a block 10 m across the
floor which the rope makes a 30o
angle to.
WORK ≡ Force · Distance
Force and Distance have to be in the SAME DIRECTION
How much WORK is done by the fork lift?
ZERO!
Force
W = f1000
· dN
Distance
6 meters
WORK ≡ Force · Distance
Force and Distance have to be in the SAME DIRECTION
How much WORK is done by the fork lift?
Work = force · distance
Force
Distance
1000 N
1.5 meters
How much WORK does the runner do?
600 N
5m
Work = F * d
3m
How much WORK do these fitness
buffs do as they enter the gym?
In order to do “Work” what must be expended?
Work expended = Energy required/given
What kinds of energy are there?
http://www.vikingrange.com/recipes/recipe_images/banana
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http://www.freewebs.com/server1988/bomb%209.jpg
http://www.bradfitzpatrick.com/weblog/wpimages/my_art/fullcolor/daring_01.jpg
http://resources.yesican-science.ca/energy_flow/images/kinetic_energy1.png
Who did more WORK?
Which required more energy?
How can they be equal?... Isn’t something different?
Time is different… so…
http://www-db.stanford.edu/~widom/photos/temple.jpg
UNITS
WORK Force·Distance
POWER ≡
=
Time
Time
Joules
= Second
= Watts
A 40 Watt light bulb Means…
40 Joules of WORK done every second.
The energy in a BIG MAC
could run the 40 W bulb for…
An average horse can lift 550 pounds
a distance of 1 foot in 1 second.
And do it all day long.
1 Horsepower ≡ 550 ft·lbs/s
James Watt
550
lbs.
1 ft/s
FIELD TRIP
Calculate YOUR
Horse Power
Clifton Bluhm
How much time will it take the 2000 lb car to
climb the 80 ft hill if the engine is 115 Hp?
115 Hp
550 ft·lb
s
1 Hps
= 63,250
ft·lb
s
s
Force·Distanc
POWER =
e
2,000Time
· 80
63,250 =
Time
Time = 2.53 s
80 ft
ENERGY ≡ The ability to do . . .
WORK
An object that has
100 Joules of ENERGY
has the ABILITY to do
100 Joules of WORK
** Energy is conserved! **
Energy
Work
POWER ≡
≡
Time
Time
POWER is how quickly
WORK is done.
POWER is how quickly
ENERGY is expended.
The Work – Energy connection (in motion)
10,000 N of force applied for 10 m… how much work?
How much energy does the bus now have?
Wdone =ΔE
Energy appears to be based on velocity…
Let’s substitute in velocity somehow…
KE =
2
½mv
• A 1000 kg car is traveling at 25 m/s… how
much work must be done to stop it?
• If the Ff has a max value of 10,000 N…
what is the shortest distance that car can
stop in?
The Work – Energy connection (potential)
The box has potential… the potential to become KE.
** How calculate its Potential??? **
40 m
How much KE at the bottom??? – what is “v”???
PEg = mgh
• A 50 kg object hits the ground at 30 m/s…
how high was it dropped from?
KE = ½ m·v2
WORK = F ·
1000 kg car
d
Assume the friction force is 10,000 N.
Velocity
(m/s)
Kinetic
Energy
Work Needed
to Stop
Stop Dist.
Needed
http://www.wolflink.net/~jstorey/porsche/968/js968aa.jpg
KE & PE applied…
“But officer… I was only going 10 over?”