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…And Energy
Work – Measured in Joules
Energy – Measured in Joules
Power – Measured in Watts
Work and Energy are
both measured in Joules.
Are they the same thing?
What do you think???
Work (W)
• Work is the Transfer of Energy
• Force exerted over a distance
• W = Fd
– F = Magnitude of Force (Newtons)
– d = Magnitude of Displacement (distance - m)
Remember displacement?
W=F (Newton) x displacement
(Meters)
So 1 Joule = 1 Newton x 1 Meter
Energy of a body is
its Capacity to do
Work
Energy
There’s 3 Types!
Kinetic Energy (KE or K)
• The Energy of Motion
• KE = ½ mv²
– m = mass of the object
– v = velocity of the object
Gravitational Potential Energy
• The stored energy an object has due to
with its position above earth’s surface
• PE = mgh
– m = mass of the object
– g = acceleration of gravity
– h = height of the object relative to earth (or
some other zero level)
Elastic Potential Energy (PEs)
• Energy a Spring has by virtue of being
stretched or compressed
• PEs = ½ kx²
– k = Spring Constant – Measures the strength
of the spring in N/m
– x = Distance the spring is stretched or
compressed
Are Work and Energy related?
Remember:
Work is the transfer of Energy
Energy is the ability to do work
Work and Energy
• Work exerted on an object is equal to the
difference between an object’s initial and
final Energies.
• Or…
W=Ef-Ei
WKE = KEf – KEi = mv²f - ½ mv²i
Wgravity = PEf – PEi = mghf – mghi
WPEs = PEsf – PEsi = ½ kx²f - ½ kx²i
Mechanical Energy – the ability
to do work
Mechanical energy is the energy that an
object has due to its motion (kinetic
energy) or its position (potential energy.
• (ME or E) of an object is the sum of the
kinetic and potential energies of that object
Etotal= KE + PE
KEi + PEi = KEf + PEf
…In this case, KE and PE can
be transformed into one another
as the object moves.
But, the energy is never lost or
gained, it is only transformed…
Ei = E f
KEi + PEi = KEf + PEf
Power
• The Rate at which Work is done
• P = Work/time
or
• P = Force x velocity
• remember
Work = Force x Displacement
• Force =ma
• Measured in Watts (W) = J/s
Time to Practice!
Find the work exerted on a
suitcase if the suitcase is being
pulled with a force of 16.0 N and
the displacement of the suitcase
is 63.0 cm.
W = Fd
W = (16.0N)(0.63m)
W = 5.64 J
An archer pulls a bowstring
back a distance of 0.470m and
then releases the arrow. The
bow and string act like a spring
whose spring constant is 425
N/m. What is the elastic
potential energy of the system?
PE = ½kx²
PE = ½(425N/m)(0.470m)²
PE = 46.9 J
A motorcyclist is trying to leap
across a canyon by driving
horizontally off the cliff at a speed
of 38.0 m/s. Ignoring air
resistance, find the speed at which
the motorcycle strikes the ground
on the other side. The motorcycle
starts at a height of 70.0m and will
end at a height of 35.0m.
KEf + Pef = KEi + PEi
½ mv² + mgh = ½mvi² + mghi
(mass cancels out)
½v² + (9.8m/s²)(35.0m) = ½(38.0m/s)² + (9.8m/s²)(70.0m)
V = 46.2 m/s