Transcript Work Review

ATB - Work Review
How much Work is done in lifting a 5 N book 3
meters?
How much Work is done in carrying that same
book up a flight of stairs that are 2 meters
long and 3 meters high?
Physics Chapter 7
ENERGY
Mechanical Energy
• Potential
– Energy that is stored
because of its
position relative to
something else.
• Kinetic
– Energy of motion
Potential Energy
Energy that is stored because of its position relative to
something else.
• Examples 
– A drawn bow
– A stretched or compressed spring
– Slingshot
– Chemical energy
– Water in an elevated reservoir
– Ram of a pile driver
Gravitational
Potential
Energy
Potential Energy
• Gravitational Potential Energy
– PE due to an elevated position
– Amount of PE is equal to the Work done in lifting it
• W = Fd
• The force (F) needed to lift it is the force equal and
opposite to the objects weight  Fw = mg
• The distance (d) is the height, h above the Earth
– PE = mgh
– Only changes in PE are meaningful. That is,
how much PE is converted into some other form.
Potential Energy
• It does not matter how the object gets to
the height, h. The PE of the ball depends
only on the height, not the path taken
there. Whether it is lifted, pushed up a
ramp, or carried up stairs, the PE is equal.
Example #1
A 2000 kg car is picked up by a crane to a height
of 10 m.
A)
What is the PE of the car?
B)
How much work was done by the crane?
Kinetic Energy
Energy of Motion
• Examples 
– Pendulum
– Roller Coaster Car
– Any object that is in motion, i.e. has a velocity
Kinetic Energy
• Depends on…
– Velocity
– Mass
– Kinetic Energy = ½ mass x velocity2
– Variable:
KE
– Unit:
Joule
– Equation: KE = ½ mv2
Example #2
A 6.0kg cat runs after a mouse at 10.0m/s.
What is the cat’s kinetic energy?