Transcript mechanical energy

QUIZ
Who wants to be a
“future” millionaire??
Learning physics is not too bad
although you are already sickened.
Question 1
• A mass of 400 grams is accelerated
at 30 cm/s2. The force, in N, to
accelerate this mass is…
The answer to Q1
• 0.12 newtons
F = ma = 0.4 kg  0.3m/s2
Question 2
• A walker walks 30 m from the origin
toward the WEST to point A. She
then walks from point A, 20 m more
toward the EAST to point B. What is
the walker’s total displacement from
the origin?
Answer to Q2
10 m toward the WEST (or -10 m)
Distance and displacement are
different!!
Question 3
• A force of 56 N is directed 35
degrees NORTH of EAST. What is
the EAST component of the force?
Answer to Q3
45.9 N
Do you remember when to use cos or
sin for this type of problem?
Question 4
• A momentum change is found as 5.8 kg
m/s with the time interval, 0.22 s.
What is the force acting on it during
the time?
Answer to Q4
26 N
The change in momentum is equal to
the impulse, Fs.
Question 5
• What is the Kepler’s third law?
(a) The gravity is always the same with the
equal period of the orbit.
(b) The square of the orbit period of a
planet is proportional to the cube of
the semi-major axis of its orbit.
(c) A line segment joining a planet and the
Sun sweeps out equal areas during equal
intervals of time.
Answer to Q5
(b)
The formula is (T1/T2)2=(r1/r2)3
Question 6
• What is the meaning of the specific
heat?
Answer to Q6
Specific heats depend on materials.
The more specific heat it has, the
harder to raise (or lower) the
temperature with a certain heat.
Question 7
• What are conservative and
nonconservative forces?
Answer to Q7
A conservative force is derived from
the correspondent potential energy,
such as gravitational potential energy.
A nonconservative force is not derived
from the potential energy and its
work depends on the path taken, such
as frictional force, normal force, etc.
Question 8
• A ball is projected from the origin
with a velocity of 20.0 m/s at an
angle of 60 degrees above the
horizontal. What is the maximum
height of the ball?
Answer to Q8
15.3 m
First, obtain the y-component of the
velocity. Then, use v2=v02-2gy, which
is your favorite. 
Question 9
• A 4,000 kg satellite is traveling in a
circular orbit 200 km above the surface of
the earth. A 3.0 gram marble is dropped
inside the satellite. What is the force of
gravity on the marble as viewed by the
observers on the earth? (Me = 5.98 x 1024
kg, Re = 6.37 x 106 m, G = 6.67 x 10-11 Nm2
/kg2)
Answer to Q9
0.0277 N
Use F=GmM/r2 including the radius of
the Earth. However, do not use the
mass of the satellite.
Question 10
•
As for gravitational acceleration, 9.8
m/s2, which one of the following is true?
a) Gravitational accel. is greater when an
object is thrown up.
b) Gravitational accel. is smaller when an
object falls downward.
c) Gravitational accel. is always the same
wherever the object is placed on the
Earth.
Answer to Q10
c) is the correct statement.
9.8 m/s2 is always the same (close to
the surface) and directed to the
center of the Earth.
Question 11
• What is the law of conservation of
mechanical energy?
Answer to Q11
The total mechanical energy in a
system remains constant as long as
the only forces acting are
conservative forces.
Question 12
• A 3.0-kg object is placed on
horizontal and rough surface. If the
coefficient of static friction is 0.20,
what is the force to move the
object?
Answer to Q12
5.88 N
The frictional force is the normal
force  the coefficient of friction.
The normal force in this case is mg.
Question 13
•
a)
b)
c)
d)
e)
What is Newton’s Law of Universal
Gravitation? Choose one related equation.
W = mg
F=GmM/r2
F=mv2/r
F=ma
V=2r/T
Answer to Q13
b) is the correct equation.
The law to describe the force between
masses is Newton’s Law of Universal
Gravitation.
Question 14
• Here is an equation:
ΔQ/ΔT = e σ A (T14 - T24) .
What are e and σ called? Which
situation can you use this equation?
Answer to Q14
e is emissivity.
σ is Stefan-Boltzmann constant,
5.670373 × 10-8 kg s-3 K-4.
This equation is used for radiation of
heat.
Question 15
• What is the apparent weight?
a) It is always that the mass of the
object times gravity.
b) It is equal to weightlessness.
c) It is the net mass of the object.
d) It is the net weight you can feel
how heavy it is.
Answer to Q15
d) is the correct description of
apparent weight.
In this sense, the apparent weight can
be zero (weightlessness ) due to an
inertial force.
Question 16
•
a)
b)
c)
d)
e)
What is the work-(kinetic) energy
theorem? Choose one of the best
equation to describe it.
KEf-KEi=W
KEf=KEi
fk=kn
pf-pi=Fs
Qgained=Qlost
Answer to Q16
a) is the correct expression to
describe the work-(kinetic) energy
theorem.
You should be able to explain it in your
own words.
Question 17
• Find the impulse from the graph.
F (N)
60
40
20
0.02
0.04
0.06
0.08
0.10 0.12
t (s)
Answer to Q17
0.9 Ns
You can find the area, 60 N  (0.09 –
0.06) s  ½. (The area of the
triangle)
Question 18
• What is the following equation mean?
Q/t=kA(T1-T2)/
What is a condition that the equation
can be valid?
Answer to Q18
• That is used for conduction of heat.
Q/t represents the rate of the heat
transfer. The k is conductivity, A is
area,  is the length to conduct, etc.
• The equation assumes that the heat
does not go away from the side of
the object (conducting only one way).
Are you ready for the
final?
• Ask yourself why you have to use the specific
equation to solve the problem.
• There are always concepts behind physics
problems, so don’t pick up numbers and plug in
equations you randomly find from the equation
sheet!!
• Make sure the definition of each physics term.