Download Chapter 6

10/10/2013
Weight
w = mg
In orbit, in the space shuttle, an astronaut is
Chapter 6
A.
B.
C.
D.
10/11/13
Apparent Weight = normal force
• Weight is force of gravity w = mg
• Even if you’re falling
• Astronauts are not weightless, they are
apparently weightless
A car is rolling over the top of a hill at a constant
speed. At this instant,
A. n > w
B. n < w
C. n = w
D. we can’t tell about n
without knowing speed.
Weightless
The same weight as on earth
Less than on earth but not weightless
More than on earth
What is your true weight on the
moon?
Say you have a mass of 60kg
Your weight on the moon is
A. 60 kg 9.8 m/s2 = 590 N
B. 60 kg 1.6 m/s2 = 98N
C. 0 N
A car is rolling through the bottom of a valley at
a constant speed. At this instant,
A. n > w
B. n < w
C. n = w
D. we can’t tell about n
without knowing speed.
a
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10/10/2013
Weight
w = mg
In orbit, in the space shuttle, an astronaut is
Gravity and Orbits
A.
B.
C.
D.
Weightless
The same weight as on earth
Less than on earth but not weightless
More than on earth
Newton’s law of gravity
Orbits
F = Gm1m2/r2
Free fall
Newton’s Mountain thought problem
G – Gravitational Constant (ALWAYS the same)
G = 6.67 x 10-11 N m2/kg2
PhET: Gravity and Orbits
Acceleration in Orbit
What is the direction of acceleration for the
Earth when in orbit around the Sun?
A.
B.
C.
D.
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10/10/2013
PhET: Gravity and Orbits
Force Arrows
Planet Mass
What will happen if the mass of the Planet
increases?
A.
B.
C.
D.
Nothing
The planet’s angular velocity will increase
The planet’s angular velocity will decrease
The planet will crash into the sun!
No gravity
Star Mass
What will happen if the mass of the Star
increases?
What happens if gravity no longer exists?
A. Fall into the sun
A.
B.
C.
D.
Nothing
The planet’s angular velocity will increase
The planet’s angular velocity will decrease
The planet will crash into the sun!
B. Continue in a straight line
C. Fly directly away from the sun
D. None of the above
Moon’s Orbit
Which of the following best depicts the path of
the moon around the sun?
A.
C.
B.
PhET: Gravity and Orbits
To Scale
D.
E. The moon does not orbit the sun.
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Ellipses – My Solar System
When moving in an ellipse, a satellite's speed is
A. Largest when it is closest to the sun
B. Largest when it is furthest from the sun
C. Equal during the entire orbit
Ellipses – My Solar System
Does the sun move? Is it attracted to the Earth?
A. Sun is stationary in space
B. Sun moves as the planets orbit
4-star ballet
Slingshot
Rotational motion Problems
Determine the speed of a satellite of mass m
orbiting a body of mass M at a distance r.
Approach:
Solving Force Equations
1. Givens
2. Draw situation – identify object of interest & forces on it.
3. Draw a Free Body Diagram (FBD)
4. Identify if in equilibrium or dynamic.
Does SF = ma or SF = 0?
5. Apply Newton’s 2nd law which means find the net
force in the y and find the net force in the x. SFx
and Sfy
6. Solve equations you got from 4 and 5
7. Assess
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1. Givens
M – mass of the planet
m – mass of the satellite
r – distance from center to center
2. Draw situation and identify forces
on the object of interest
m
Gravity F
r
v=?
M
3. Draw free body diagram
F
4. Identify if in equilibrium or dynamic
Does SF = ma or SF = 0?
F
a
5. Apply Newton’s Laws
6. Solve for unknown
GmM/r2 = mv2/r
F
v2 = GM/r
a
SFx = F = ma
SFx = GmM/r2 = ma
ma = mv2/r
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7. Assess
Kepler’s Law
v = √GM/r
Does not depend on the mass of the planet
v = √GM/r = 2pr/T
T2 = (4p2/GM)r3
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