The Work-Energy Theorem
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Transcript The Work-Energy Theorem
Net Work
Net work (Wnet) is the sum of the work
done on an object by all forces acting
upon the object.
The Work-Energy Theorem
• Consider a force applied to an object
(ΣF ≠ 0).
• Newton’s second law tells us that this
net force will produce an acceleration.
• Since the object is accelerating, its
displacement will change, hence the
net force does work.
The Work-Energy Theorem
FWs mas
F ma
(v v ) 2
W
W
mmv mvi
2
2
2
f
f
1
2
2
1i
2
F s W
v v 2as
(v v )
as
2
2
f
2
f
2
i
2
i
Kinetic Energy
A form of mechanical energy
Energy due to motion
K = ½ m v2
– K: Kinetic Energy in Joules.
– m: mass in kg
– v: speed in m/s
The Work-Energy Theorem
F ma
W mas
W mv mv
1
2
2
f
1
2
2
i
KE
Wnet
The Work-Energy Theorem
Wnet = KE
– When net work due to all forces acting upon
an object is positive, the kinetic energy of the
object will increase.
– When net work due to all forces acting upon
an object is negative, the kinetic energy of the
object will decrease.
– When there is no net work acting upon an
object, the kinetic energy of the object will be
unchanged.
Power
Power is the rate of which work is
done.
No matter how fast we get up the
stairs, our work is the same.
When we run upstairs, power demands
on our body are high.
When we walk upstairs, power
demands on our body are lower.
Power
The rate at which work is
done.
Pave = W / t
P = dW/dt
P = F • v
Units of Power
Watt = J/s
ft lb / s
horsepower
• 550 ft lb / s
• 746 Watts
Power Problem
Develop an expression for the
power output of an airplane cruising
at constant speed v in level flight.
Assume that the aerodynamic drag
force is given by FD = bv2. By what
factor must the power be increased
to increase airspeed by 25%?