Transcript Energy-Work-Power - juan

WHAT IS WORK?
• A force causing displacement
• Time is not a factor----can be fast or slow
• Force must be applied in the same direction as
the displacement of the object or at some
angle theta (Θ)
• Work = force x displacement x cos Θ
• If force and displacement are in the same
direction then Θ is 0 (W = Fd)
• Measured in N • m or Joules
POTENTIAL ENERGY
• Energy that is stored and waiting to be used
later
• Energy of an object due to its condition or
position
POTENTIAL ENERGY
Potential energy due compression or
expansion of an elastic object.
POTENTIAL ENERGY
Potential energy stored within the
chemical bonds of an object
POTENTIAL ENERGY
KINETIC ENERGY
• Energy resulting from motion of an
object.
• Work must be done to cause an object
to be in motion.
• Work = change in kinetic energy
W = ΔKE
• KE = 1/2 mv2 (measured in Joules)
EXAMPLE
A 105-g hockey puck is sliding across the ice. A
player exerts a constant force of 4.50-N over
a distance of 0.150 m.
• How much work does the player do?
• What is the change in the puck’s energy?
• What is the change in velocity of the puck?
W = (4.50 N)(0.150 m) = 0.675 J
ΔKE = W = 0.675 J
0.675 J = (0.5)(0.105 kg)v2 = 3.6 m/s
EXAMPLE
When a golf club strikes a 46-g
golf ball, the ball picks up 43 J
of KE. A constant force of 2300
N is applied. Over what distance
is the club in contact with the
ball?
W = Fd  d = W/F
d = 43 J/ 2300 N = 0.019 m
EXAMPLE
A sailor pulls a boat a distance of 30.0 m
along a dock using a rope that makes a
25° angle with the horizontal. How
much work does the sailor do on the
boat if exerts a force of 255 N on the
rope?
W = Fdcos Θ
W = (255 N)(30.0 m)(cos 25°) = 6933.3 J
EXAMPLE
Will N. Andable and Ben Pumpiniron are in
the weight room. Will lifts a 100-lb.
barbell over his head 10 times in 1
minute while Ben lifts a 100-lb. barbell
over his head 10 times in 10 seconds?
Which student does the most work?
Answer:
EXAMPLE
Calculate the work done by a 2.0-N force
(directed at a 30° angle to the vertical)
to move a 500 gram box a horizontal
distance of 400 cm across a rough floor
at a constant speed of 0.5 m/s.
TOTAL MECHANICAL ENERGY
POWER
• Power is the rate at which work is done.
• Measures how quickly energy is being
transferred
• Power = work/time or P = W/t
• Measured in Watts (W) which equals J/s
• 1 horsepower (hp) = 750 W
• Substituting Fd for W  P = Fd/t
• Substituting v for d/t  P = Fv
EXAMPLE
A net force of 2800 N accelerates a 1250kg vehicle for 8.0 s. The vehicle travels
80.0 m during this time. What power
output does this represent?
P = W/t = Fd/t
P = (2800 N)(80.0 m)/8.0 s = 28,000 W
Can also be expressed in kiloWatts (28
kW)
EXAMPLE
Remember Will N. Andable and Ben Pumpiniron?
Will lifted a 100-lb. barbell over his head 10
times in 1 minute while Ben lifted a 100-lb.
barbell over his head 10 times in 10 seconds?
Which student delivers the most power?
EXAMPLE
If Nellie Newton is doing chin-ups during
her physical fitness test and lifts her 42kg body a distance of 0.25 m in 2
seconds, what power is delivered by
Nellie’s biceps?
Fg = mg = (42 kg)(9.8 m/s2) = 411.6 N
P = Fd/t = (411.6 N)(0.25 m)/2 s = 51.5 W
EXAMPLE
A squirrel (mass of 1 kg) does push-ups by
applying a force to elevate its body by 5 cm.
Determine the number of push-ups a squirrel
must complete in order to do 1 Joule of work. If
he does them in 4 s, determine his power
output.
W = Fd = mgd = (1 kg)(9.8 m/s2)(0.05 m) = 0.49 J
The squirrel must do ____ push-ups.
P = W/t = 1.0 J/ 4 s = 0.25 W
SIMPLE MACHINES
Eases the load by changing either the
magnitude or direction of a force to
match the force or capability of the
operator.
Mechanical advantage (MA) is a ratio of
the resistance force (Fr) exerted by the
machine to the effort force (Fe) exerted
by the operator.
MA = Fr/Fe
TYPES OF SIMPLE MACHINES
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Pulley
Inclined Plane
Lever
Wheel & axle
Screw
Wedge
SIMPLE MACHINES
• Ideal Mechanical Advantage (IMA) is the
ratio of the displacement of the effort
force (de) to the displacement of the load
(dr)
• IMA = de/dr
• In theory, all machines would be 100%
efficient….but in reality they are not
• Efficiency (e) = MA/IMA x 100 or work
output divided by the work input [Wo/Wi x
100]