The Work-Energy Theorem Derivation: v f 2 =v i 2 +2ad Kinematics Equation mv f 2 =mv i 2...
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Transcript of The Work-Energy Theorem Derivation: v f 2 =v i 2 +2ad Kinematics Equation mv f 2 =mv i 2...
The Work-Energy The Work-Energy TheoremTheoremDerivation:Derivation:
vvff22=v=vii
22+2ad +2ad Kinematics Kinematics EquationEquationmvmvff
22=mv=mvii22+2mad+2mad Dynamics Dynamics
equationequation(1/2)[mv(1/2)[mvff
22=mv=mvii2+2mad]2+2mad]mad= ½ mvmad= ½ mvff
22- ½ mv- ½ mvii22
KE=kinetic energy = ½ mv2
Kinetic Energy: The energy associated with the motion of a mass The amount of capable work associated with the motion of a mass.Measured in Joules (J)KE is a scalar quantity.
Kinetic Energy Kinetic Energy ConsiderationsConsiderations
A small mass moving at a high speed can have the same kinetic energy (do the same work) as a large mass that travels slowly.
You have two equivalent masses, mass A and B. Suppose you double the mass of A and double the speed of mass B. Which mass now has more KE?
Why? A car traveling at a certain speed stops after travel a
distance, d, after the brakes are applied. Suppose that the speed of the car is double, by what factor does the distance to stop change provided that the braking for is the same as the first situation.
d4d
4mv
21
)m(2v21
FdFd
0v
)m(2v21mv
21Fd
mv21mv
21Fd
1
2i
2i
1
f
2i
2f1
2i
2f
Things for ThoughtThings for Thought A 2 g bumblebee moving at 2 m/s has a
kinetic energy of 4x10-3 J which means it is capable of doing 4x10-3 J of work
A comet with a mass of 7.9x1011 kg traveling at 25,000 m/s has a kinetic energy or capable of doing 2.5x1020J of work. The most powerful atomic bomb has an energy yield of 5x1015 J, thus this comet can do 5000 times more work.
Various Forms of the Work-Energy Various Forms of the Work-Energy TheoremTheorem
mad= ½ mvmad= ½ mvff22- ½ mv- ½ mvii
22
(Fnet)d=KE(Fnet)d=KEff-KE-KEii KEKEff=final kinetic energy, KE=final kinetic energy, KEii=initial kinetic =initial kinetic energyenergy
Wnet=KEWnet=KEff – KE – KEiiWnet=Wnet=ΔΔKEKE The net work accomplished on mass causes a change in The net work accomplished on mass causes a change in
its kinetic energy.its kinetic energy. The work-energy theorem is a cause and effect The work-energy theorem is a cause and effect
relationship. The net work on a mass causes a change relationship. The net work on a mass causes a change in its kinetic energy.in its kinetic energy.
Positive Net Work – increases the speed of a mass Positive Net Work – increases the speed of a mass causing a positive change in kinetic energy.causing a positive change in kinetic energy.
Net Negative Work – decreases the speed of a mass Net Negative Work – decreases the speed of a mass causing a negative change in kinetic energy.causing a negative change in kinetic energy.