Download Unit 6 Work and Energy Review 2012 KEY

Unit 6 Work & Energy Review ANSWER KEY
1) Energy is the ability to ____ .
A) produce no change
C) do work
B) burn something
D) none of the above
2) When a force is exerted on an object, no work is done if the object ____ .
A) is heavy
B) is not heavy
C) moves parallel to the force
D) moves perpendicular to the force
3) How much work is done if you raise a 5.0 N weight 3 m above the ground?
A) 0.0 J
B) 2.5 J
C) 7.0 J
D) 15.0 J
4) What is the potential energy of a 5.0 N weight 3 m above the ground?
A) 0.0 J
B) 2.5 J
C) 7.0 J
D) 15.0 J
5) One definition of power is ____ .
A) force exerted per unit
B) work done per unit of time
C) amount of energy expended distance
D) force applied multiplied by time
6) An object that has energy due to its shape or position possesses____.
A) potential energy B) chemical energy
C) kinetic energy
D) momentum
7) Which of the following is needed to find the kinetic energy of an object?
A) its shape
B) its size
C) its velocity
D) its chemical composition
8) As the height above the Earth increases, the potential energy of the object ____.
A) increases
B) decreases
C) remains the same
D) cannot tell
9) The kinetic energy of a book weighing 20.0 N book perched on the edge of a table 0.5 m
high is ____.
A) 10.0 J
B) 0 J
C) 40.0 J
D) 20.0 J
10) The potential energy of a book weighing 20.0 N book perched on the edge of a table 0.5 m
high is ____.
A) 10.0 J
B) 0 J
C) 40.0 J
D) 20.0 J
11) The amount of work done against gravity by lifting a 20.0 N book from the ground to the
edge of a table 0.5 m high is ____.
A) 10.0 J
B) 0 J
C) 40.0 J
D) 20.0 J
12) A change in the kinetic energy of an object is equal to the ____ .
A) net work done on the object
B) work minus potential energy
C) change in mass times velocity
D) 0.5 (W + PE)
13) Find the kinetic energy of the crate (below) that has a weight of 50 N.
F = ma
(50 N) = m (9.8 m/s2)
m = 5.1 kg
6.0 m/s
50 N
KE = (1/2)mv2 = (1/2)(5.1 kg)(6m/s)2
KE = 91.8 J
14) Which of the following are forms of mechanical energy?
I) Elastic potential energy
II) Gravitational potential energy
III) Kinetic energy
IV) Thermal energy
A)
II, III and IV
B) I, II, and III
C) I, II, IV
D) I, II, III and IV
15) What is happening when some kinetic energy is converted to thermal energy?
A) Mechanical energy is being conserved.
B) Mechanical energy is not being conserved.
C) Mechanical energy is being converted to non-mechanical energy.
D) Both B and C
16) Suppose for a second that a VW beetle and Ford F150 have the same kinetic energy. The vehicle with more
speed is:
A) VW beetle (less mass)
B) Ford F150
C) Both are the same speed
17) Which of the following would have the same units?
A) force : energy
B) power : work
C) force : power
D) work : energy
18) A 30 kg student at rest is sitting on top of a frictionless slide that is 4.0 m high. At the top of the slide, the
student has
A) all potential energy, no kinetic energy
B) mostly potential energy, some kinetic energy
C) mostly kinetic energy, some potential energy
D) all kinetic energy, no potential energy
19) A 30 kg student at rest is sitting on top of a frictionless slide that is 4.0 m high. When the student reaches the
bottom of the slide, what has happened to the energy?
A) Some energy was loss due to friction.
B) All the kinetic energy was converted to potential energy
C) All the potential energy was convert to kinetic energy
D) All the potential energy was converted to mechanical energy
20) A 30 kg student at rest is sitting on top of a frictionless slide that is 4.0 m high. When the student reaches a
height of 1.0 m, what is the kinetic energy of the child?
Diagram: (Given + Unknowns)
Equation: ME = PE + KE
ME = PE + KE
m = 30 kg
h = 4.0 m g = 9.8 m.s2
m = 30 kg
h = 1.0 m g = 9.8 m.s2
KE at 1 m in ???? J
Top: PEi = mgh
Middle: PE = mgh
Bottom PEf = 0 J
KEi = 0 J
KE = 1/2mv
Substitute: at 4 m
ME = PE
+ KE
ME = mgh + 0 J
ME = (30kg)(9.8 m/s2)(4 m)
ME = 1176 J
Mechanical Energy is conserved since no thermal energy is
produced.
at 1 m
2
KEf = 1/2mv2
ME = PE
+ KE
ME = mgh + KE
1176 J = (30kg)(9.8 m/s2)(1 m) + KE
1176 J = 294 J + KE
882 J = KE
Conservation of Energy Table for Question 20 (Notice Mechanical energy is conserved.)
Height
Potential Energy
Kinetic Energy
Mechanical Energy
4m
1176 J
0J
1176 J
1m
294 J
882J
1176 J
0m
0J
1176 J
1176
21) How much work is done by a 75 W light bulb that is left on for 90 seconds?
A) 6750.0 J
B) .833 J
C) 8.16 J
D) 80.0 J
P = Work
75 = Work
Work = 6750 J
t
90
22) Use the following diagram for the following next six questions.
You throw a 0.50 kg basketball off the Eiffel Tower in Paris, France.
Lab
I) Where does the ball have the most kinetic energy? ____D___
II) Where does the ball have the least kinetic energy? ___B___
III) Where does the ball have the most potential energy? __B__
IV) Where does the ball have the least potential energy? __D__
V) Where does the ball have no vertical speed ?
_____B___
VI) Where is the ball moving the fastest? _____D_____
23) How much energy does your 1600 W blow drier transfer if you dry your hair in 300 sec?
Diagram: (Given + Unknowns)
Equation: P = W/t
or E = Pt
Substitute: (1600 ) = Work
(300 )
P = 1600 W
t = 300 s
W = ???? J
Work = (1600 ) (300 ) = 480,000 J
Solve: Work = 480,000 J
24) How much power is developed by an electric motor that moves a 80 kg load through a distance of 20 m in 10 s?
Diagram: (Given + Unknowns)
m = 80 kg
d = 20 m
t = 10 s
80 kg
P = ??? W
Equation: F = ma then W = fd then P = W/t
d = 20 m
Substitute:
F = ma = (80 )(9.8) = 784 N
Work = fd = (784)(20 ) = 15,680 J
P = Work / t = (15,680 ) / (10 ) = 1,568 W
F = ?? N
a = 9.8 m/s2
Solve: P = 1,568 W
25) It takes 800 J to lift a box 4 m in the air in 8 seconds. What is the weight of the box?
Diagram: (Given +
Unknowns)
W = 800 J
d=4m
t=8s
Equation:
Substitute:
W = Fd
800 = F (4 )
F = 200 N
Fg = ??? N
Time isn’t needed
Solve: F = 200 N
26) A spring is stretched 0.75 m. How much elastic potential energy is stored in the spring?
(spring constant of 12 N /m)
Diagram: (Given + Unknowns)
Equation: PEelastic = (1/2)kx2
d = x = 0.75 m
spring constant = k = 12 N/m
Substitute: PEelastic = (1/2)(12 N/m)(0.75 m)2
PEelastic = 3.375 J
PEelastic = ????? J
Solve: PEelastic = 3.375 J
32) It takes 500 J of work to lift a puggle 1.7 meters off the floor. What is the magnitude of the force needed to lift
the puggle?
Diagram: (Given + Unknowns)
Equation: W = F(d)
Substitute:
W = 500 J
d = 1.7 m
(500 J) = F(1.7 m)
F = ???? N
Solve: F = 294 N
28) A puggle is sitting hanging out in her penthouse suite in that most lavish doggie hotel in Allen, TX. She is looking
outside the window when she accidently drops her favorite doggie toy (a.200 kg stuffed mallard). It takes 3.19
seconds for the mallard to hit the ground. What is the final kinetic energy of the mallard before just before it hits the
ground.
Diagram: (Given + Unknowns)
Equation:
a = (vf – vi ) / t
then KE = 1/2mv2
KEf = ??? J
m = .200 kg
vf = ??? m/s
Substitute:
-9.8 = vf – 0
3.19
Vertical
a = -9.8 m/s2
vf = ??? m/s
vi = 0 m/s
t = 3.19 s
h = Δd = ??? m
-31.2 m/s = vf
direction.)
multiply -9.8 and 3.19
(remember the negative only tells me
KEf = (1/2)mv2
KE = (1/2)(.200)31.22 = 97. 7 J
You could have also found the height first and then did PE = mgh
Then PEi + KEi = PEf + KEf (where KEi and PEf equals zero.)
Solve: KE = 97.7 J
29) Name one form of non-mechanical energy. Heat / thermal energy
30) A soccer player having less mass than a runner also accelerates uniformly from rest to a speed of 8 m/s. Compare
the kinetic energies of the less massive soccer player to the kinetic energy of more massive runner when they are both
traveling the same speed?
The soccer player with lass mass would have a lower kinetic energy.
The runner with more mass would have a higher a kinetic energy.