Self-Check Quiz 12-4 – SOLUTIONS
This quiz is designed to help you determine if you are “keeping up” with things
and/or understanding the NOTES/TEXTBOOK. It will NOT be collected
C
1) The Principle of Conservation of Energy states that: A) The total mechanical energy of a system of
objects remains the same
B) The sum of the kinetic energy and the gravitational potential energy of a
system of objects remains the same
C) The total energy in all its forms of a system of objects remains
the same
D) When an object loses gravitational potential energy it gains kinetic energy
E) The total
gravitational potential energy of a system of objects remains the same
D
2) If there were no “losses” of mechanical energy due to friction: A) a ball dropped from a height h meters
above the ground would bounce back to a height h meters above the ground. B) all of the gravitational
potential energy of the ball would be converted into kinetic energy when the ball reached the ground
C)
the ball would keep bouncing “forever.” D) A, B, and C E) A and B only
TRUE
3) TRUE or FALSE: When scientists say that some energy is “lost” during a process, what they really
mean is that some energy is transformed into a form that cannot be used to generate useful work.
E
4) In an “ideal pendulum”, the pendulum would swing back and forth forever, continually changing
gravitational potential energy into kinetic energy and back to gravitational potential energy. In a real
pendulum, the swinging eventually stops due to: A) air resistance between the pendulum and the air
B) frictional forces at the point where the pendulum pivots
C) the force of gravity between the
pendulum and the Earth D) A, B, and C E) A and B only
4.43 m/s
5) Consider the “ideal pendulum” shown below where the zero of gravitational potential energy is where
the pendulum has “swung completely down.” The mass of the pendulum “bob” is 1 kg and mass of the
string holding the pendulum “bob” can be neglected. At its highest point, the pendulum is ½ meter above
the zero level of gravitational potential energy. What is the (maximum) speed of the pendulum “bob”
when the “bob” it at the level of the zero of gravitational potential energy? The acceleration due to gravity
is 9.8 m/s2. SHOW WORK
1 kg
½ meter
zero of gravitational potential energy
initial gravitational potential energy = mgh = (1 kg)(9.8 m/s2)(½ m) = 4.9 J
final kinetic energy = initial gravitational potential energy
½mv2
= (½)(½)(v2) =
¼v2
= 4.9
v2 = 4(4.9) = 19.6
v = 19.6
continued on other side
Physical Science Self-Check Quiz 12-4 – SOLUTIONS
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v = 4.43 m/s
TRUE
6) TRUE or FALSE: Although it may seem that energy has “entered” or “left” a system of objects, that
energy “came from” or “went to” some other system of objects. When the entire Universe is
considered, energy is neither/never created or destroyed.
E
7) Which of the following is a good approximation of a closed system with respect to energy transfer? A)
a well-constructed almost-frictionless pendulum
B) a skydiver floating to Earth using his/her parachute
C) a billiard ball striking another billiard ball on an almost-frictionless billiard table D) A, B, and C E)
A and C only
FALSE
8) TRUE or FALSE: With respect to energy transfer, the Earth IS a good example of a closed system.
TRUE
9) TRUE or FALSE: With respect to energy transfer, a person is NOT a good example of a closed
system.
B
10) The reason why NO machine can EVER be 100% efficient is: A) people are not smart enough to
figure out how to build such a machine
B) a 100% efficient machine would require that ALL frictional
forces be eliminated and this is not possible C) machines may be useful but they are only as “smart” as
the people who build them D) A, B, and C E) A and C only
C
11) Andrea and James calculate that they must do 1,800 J of work to push a piano up a ramp. However,
when they actually push the piano up the ramp, they find that to overcome friction they must do 2,400 J
of work. The efficiency of the ramp is: A) 1,800% B) 2,400% C) 75% D) 133⅓% E) 600%
A
12) To lift a car high enough to change a tire, 1,200 joules of work must be done. If a jack is only 25%
efficient, how much work must be done by a person using the jack to lift the car? A) 4,800 joules
B)
300 joules C) 48 joules D) 0.0208 joules E) 1,200 joules
D
13) A windmill has an efficiency of 37.5%. If a gust of wind does 125 joules of work on the blades of the
windmill, how much useful output work can the windmill do? A) 125 joules
B) 87.5 joules
C) 162.5
joules D) 46.875 joules E) 37.5 joules
TRUE
14) TRUE or FALSE: It is impossible to build a perpetual motion machine because of energy “losses”
due to friction.
E
15) What is the efficiency of a bicycle pump if a person does 200 joules of work on the pump but only 40
joules of work are done by the pump in pushing air into the bicycle tires? A) 200% B) 100% C) 30%
D) 2.5% E) 20%
780 joules 16) A river does 6,500 joules of work on a water wheel every second. The efficiency of the water wheel
a
is 12%. a) How much work can the axel of the wheel do every second? b) What is the power output of
780 watts the water wheel? SHOW WORK
b
a) useful output work = input work × efficiency = (6,500 J)(0.12) = 780 joules
b) since the work is done in 1 second,
the power output is 780 watts
600 joules 17) Samson is using a pulley system to lift a sail that weighs 150 N to a height of 4.0 meters above the
a
deck of a sailboat. a) What is the minimum amount of work that must be done to raise the sail? b) If
1,200 joules the pulley system is only 50% efficient, what is the actual amount of work that Samson must do to raise
b
the sail? SHOW WORK
a) minimum amount of work = mgh = wh = (150 N)(4.0 m) = 600 joules
b) actual amount of work = twice minimum amount (only 50% efficient) = 1,200 joules
Physical Science Self-Check Quiz 12-4 – SOLUTIONS
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