aim: what are potential and kinetic energy? do now: a girl lifts a 10 kg object 1.5 m straight up....
TRANSCRIPT
Aim: What are Potential and Kinetic Energy?
Do Now:
A girl lifts a 10 kg object 1.5 m straight up. How much work has she done?
W = Fd
W = Fgd
W = (10 kg)(9.8 m/s2)(1.5 m)
W = 147 J
Work – Energy relationship
The work that you put into a system is equal to the energy that system gains
Potential EnergyPotential EnergyStored energyStored energyGravitational Potential Gravitational Potential
EnergyEnergy – the energy stored – the energy stored due to objects vertical due to objects vertical positionposition
Measured from a specific Measured from a specific reference point (ground: h = reference point (ground: h = 0 m)0 m)
PE = mghPE = mgh
Back to the Do Now Problem
How much potential energy does the object have?
PE = mgh
PE = (10 kg)(9.8 m/s2)(1.5 m)
PE = 147 J
Work – Energy Relationship
If 147 J of work go into lifting an object, then the object obtains 147 J of potential energy
Change in Potential energy (loss or gain)
ΔPE = mgΔh
The 10 kg object from the do now falls 1 m. How much potential energy has it lost?
ΔPE = mgΔh
ΔPE = (10 kg)(9.8 m/s2)(1 m)
ΔPE = 98 J
ExampleExampleWhat is the kinetic energy of What is the kinetic energy of
a 1200 kg car moving at 100 a 1200 kg car moving at 100 m/s?m/s?
KE = ½mvKE = ½mv22
KE = ½(1200 kg)(100 KE = ½(1200 kg)(100 m/s)m/s)22
KE = 6 x 10KE = 6 x 106 6 JJ
What is the velocity of a 75 kg person running with 1,250 J of kinetic energy?
KE = ½mv2
1,250 J = (½)(75 kg)v2
1,250 J = (37.5 kg)v2
v2 = 33.3 m2/s2
v = 5.8 m/s