Chapter 15
Energy Assignments
15.1 Math Skills 1-3 p 448; AQs 1-9 p452
15.2 Math Skills 1-3 p 458; AQs 1-6 & 9-10
15.2 Learning Targets for Energy
Conversion and Conservation
Describe a situation in which energy
is transformed from one form to
another
Explain how energy is conserved
Give an example in which
gravitational potential energy and
kinetic energy are changed from one
to the other
Solve equations that equate initial
energy to final energy
Describe the relationship between
energy and mass
MP 1.
A 10 kg rock is dropped & hits the ground at 60
m/s. Calculate PE before it was dropped.
Ignore friction!
PE beginning = KE end
PE beg. = KE end = ½ m*v2
PE beg.= KE end = 0.5*10 kg*(60 m/s)2
PE beg. = 18,000 J
MP 2.
Diver w/mass = 70.0 kg stands atop 3.0 m high
platform. Calculate PE on platform and KE as
he enters water (ignore friction)
PE beginning = KE end = mgh
KE end = 70.0*9.9 m/s/s*3.0 m
KE end = 2100 J (? speed entering water)
KE = ½ m*v2
2100 J = 0.5*70.0 kg*v2
2100 = 35*v2 (divide both sides by 35)
60 = v2 (take the square root of both sides)
V = 7.7 m/s
MP 3.
A pendulum w/ 1.0 kg weight is set in motion
from 0.04 m above lowest point of path. What
is KE at the lowest point? Ignore friction.
PE beginning (KE=0) = KE low point (PE=0)
= mgh
mgh = 1.0 kg*9.8 m/s/s*0.04m
PE beginning = mgh = 0.4 J
PE beginning = KE end = 0.4 J
1. Describe how energy can be
converted from one form to another
in a wind-up toy
Elastic PE stored in the wound up
spring is converted to KE as the
spring unwinds and the toy moves.
2. State the law of energy
conservation.
Energy cannot be created or
destroyed (only transformed)
3. What energy change is taking place
as an object falls in free fall?
PE is being converted into KE as it
falls.
4. Explain Eintstein’s conclusion
about the relationship between
energy and mass.
Energy and mass are equivalent, and
can be converted into one another
5. What type of energy change results
when friction slows down an object?
Friction causes the conversion of
kinetic energy into thermal energy
6. Describe the energy of a playground
swing at its highest position.
The swing has maximum PE and no
KE at its highest point.
9. A 0.15 kg ball is thrown into the air
and rises to a height of 20.0 m. How
much KE did the ball initially have?
KE beginning = PE end
KE beg. = PE end = mgh
KE beg.= PE end = 0.15 kg*9.8 m/s/s*20 m
KE beg. = 29 J
10. A 125 g steel ball with a KE of 0.25 J rolls
along a horizontal track. How high up an inclined
track will the ball roll if friction can be ignored?
KE beginning = PE end
KE beg. = PE end = mgh (solve for h)
KE / mg = h
h = 0.25 J / 125 kg*9.8 m/s/s
h = 0.20 m