Transcript Energy - wvhs.wlwv.k12.or.us

How much work does a 154 lb. student do when climbing a flight of stairs that are 6
meters in height and 30 meters in length? If the stairs are climbed in 4 seconds calculate
the power in Watts and horsepower. (1kg = 2.2 lbs.)
Mass in kg
154 lb x 1 kg = 70 kg
2.2 lb.
Weight of student Fg = mg
W = Fd
P=W
t
Fg = (70 kg) (9.8 m/s2) = 686 N
W = (686 N) (6 m) = 4,116 N
P = 4,116 N = 1,029 watts
4s
1029 watts x 1 HP
= 1.4 HP
746 watts
(1HP = 746 watts)
Conversion factors
How many inches are there in 2 kilometers?
2 km x 1 mile x
1.61 km
(1 mile = 1.61 km) (1 mile = 5280 ft.)
5280 feet x 12 inches = 126,720
1 mile
1 foot
1.61
= 78,708 feet
What is“energy”
Two forms of mechanical energy are?
Potential energy – gravitational potential energy
“energy of position”
Kinetic energy – “energy of motion”
Kinetic Energy – energy of motion
When chasing prey, a cheetah can
reach a top speed of 120 m/s for a
short period of time. What is the
kinetic energy of a 40 kg cheetah
when moving at this speed?
KE = ½ mv2
KE = ½ (40 Kg) (120 m/s)2
KE = 288,000 Joules
Kinetic Energy KE = ½ mv2
Why do stopping distances increase by four times when the speed
is doubled?
Calculate the kinetic energy of a car with a total mass of 900 kg
going 40 m/s? …… 80 m/s?
For car going 40 m/s
KE = ½ (900 kg) (40m/s)2 = 720,000 J
For car going 80 m/s
KE = ½ (900 kg) (80m/s)2 = 2,880,000 J
So stopping distances increase by four times when the speed is doubled because……..
– there is four times the kinetic energy to overcome by the braking force.
Work energy theorem – whenever work is done energy changes
work = ∆KE
so Fd = ∆KE
the force applied by the brakes times the stopping distance equals the change
in kinetic energy (720,000 J or 2,880,000 J)
The amount of potential energy of
an object is equal to the amount
of work done to get an object to
that position.
An 8 kg rock is carried to the
top of a 5m hill. What is the
potential energy of the rock?
PEg = mgh
F
Amount of work done to get
rock to the top of the hill?
PEg = (8 kg) (9.8 m/s2) (5m)
PEg = 392 Joules
392 J
Amount of kinetic energy of rock just before it hits the ground?
392 J
Conservation of Energy
The law of conservation of energy states that energy cannot be created or destroyed. Energy
can be transformed from one form into another but the total amount never changes.
PEg = mgh
For a ball moving down a ramp, in the absence of
friction, the gravitational potential energy at the top of
the ramp would be equal to the kinetic energy at the
bottom of the ramp.
KE = 1/2 mv2
6m
A 2kg ball rolls down a frictionless ramp. The ball is 6 vertical meters above a level section of
track. What is the velocity of the ball as it moves on the level section of the track?
If there is no friction then PEg (top) = KE (bottom)
So
mgh = 1/2 mv2
Rearrange to v2 = 2gh
Then simplify to gh = 1/2 v2
So v = √2gh
So v = √ (2) (9.8 m/s2) (6 m) = 10.8 m/s