Download Physics 20 Energy – Conservation of Energy

Physics 20 Energy – Conservation of Energy ‐ Work Energy Theorem Work – Energy Theorem:
When work is done on an object, that object’s energy has either been__________________
or ___________________.
Example 1: A 3500 kg jet is
accelerated by a net force of 48 000 N.
How fast is the jet traveling after
80.0 m
Use the Work - Energy theorem when work is done on a system to either increase
(+W) or decrease (-W) the total energy in the system.
The work energy theorem states that the work done by the net force on an object is equal to
the change in the object’s energy
Example 2: Assuming no loss in mass,
what speed will a 3.50 kg rocket be
traveling at if a constant force of 5000 N is
applied as it rises 40.8 m?
Example 3: A 10.0 N book is lifted at a
constant rate over a distance of 1.00 m.
How much work is done in lifting the book?
Law of Conservation of Mechanical Energy
Kinetic energy and potential energy (gravitational/elastic) are both types of __________________energy. The total mechanical energy
is the sum of the kinetic and potential energy at any moment. Using the Law of ________________ __ ______________ statement
when no work has been done on the system, the _________ __________ ______________does not change and energy is just
changing forms.
The conservation of energy statement is that the __________ _____________ of all the energies at a given position (in time or space)
is the same as the ____________ ____________ of all the energies at any other position.
Example 1: If the 2000 kg 'hammer' is 5.00 m above the pile, what
velocity will it have when it strikes the pile?
1 S Molesky @ Notre Dame Physics 20 Energy – Conservation of Energy ‐ Work Energy Theorem Example 2: A box is at rest at position A in the diagram.
Assuming a frictionless system, what will the velocity of the box
be at position B?
Example 3: The trebuchet: how does the trebuchet achieve enough
energy to toss a piano?
Closure: Try these problems
1) A low friction, 200 kg roller-coaster cart traveling at
15.0 m/s at the top of a 400 m long ramp increases to a speed of
22.0 m/s by the time it reaches the bottom of the ramp. What is
the net force on the cart while on the ramp?
2) From what height must a 1500 kg pile driver fall, to drive a pile
0.50 m into the ground against an average opposing force of 3.5 x
105 N? (Ignore the weight of the pile driver along the 0.50 m path)
3) A 2.00 t car rolls down a hill 20.0 m high and 600 m long
(diagonally). a) If there is no friction, what would be the speed
of the car at the bottom of the hill?
b) If the actual speed of the car at the bottom of the hill is
12.0 m/s, what amount of energy would have changed to heat?
c) What is the frictional force on the car as it rolls down the
hill?
2 S Molesky @ Notre Dame