1. Chapter 4: Newton and Universal Motion

2. Mechanics Mechanics = laws of motion Aristotle Galileo
Rest = Natural State of Motion
Heavy objects fall faster
Galileo
Object continues in motion unless something pushes on it
Heavy and light objects fall at same rate

3. Study of Motion (Mechanics)
Velocity
Change in location
Speed (mph) and direction (NE)
Acceleration
Change in velocity (speed and/or direction)
Force
Push or pull (pounds)
Mass
How much stuff (grams, kilograms)

4. Mass vs Weight Mass Produces Gravity Weight = Force of Gravity
Mass intrinsic to object
(never changes)
Gravity proportional to mass
Weight = Force of Gravity
Stand on scale
scale pushes back with equal force
Weight proportional to mass
Free-Fall (falling elevator, astronauts)
Acceleration of gravity = weight / mass
All objects fall at same rate
Objects appear “weightless”
mass on Moon =
mass on Earth
weight on Moon =
1/6 weight on Earth
in space, force of gravity is not zero

5. Newton Laws of Motion Moving object keeps moving
Same speed
Same direction
Objects want to move in straight line
Change in motion (speed or direction)
caused by force
acceleration = force / mass
Equal, but opposite, forces between pairs of objects
Push on object; it pushes back (just as hard)

6. Newton Invents mathematics (calculus) Circular motion
Used to solve force equations
Circular motion
Direction of motion changes
Requires force
Force changes direction; speed unaltered
Force points toward center of circle

7. Newton Gravity Orbits similar to circles Newton’s Hypothesis
Pulls apple toward earth
makes apple fall
Weight = force of gravity
Orbits similar to circles
Newton’s Hypothesis
All objects produce gravity
Sun’s gravity
planets orbit sun
Planet’s gravity
moon orbits planet
Gravity
Sun

8. Launching Rockets Fire Cannon Sideways; keep increasing velocity
Rocket moves sideways; offsets falling
Circular Orbit Speed =17,000 mph
Escape Speed = 25,000 mph

9. Newton Law of Gravity Force = G M1M2 / R2
Double either mass: force increases by 2
Double distance: force decreases by 4
M1 = mass 1st object (sun)
M2 = mass 2nd object (planet)
R = distance between them
G = Newton’s constant
(a number)
Larger (smaller) mass causes larger (smaller) gravitational force.
Larger (smaller) distance causes smaller (larger) gravitational force.

10. Newton and Planets Law of Gravity Force = G MsunMplanet / R2
Acceleration = Force / Mplanet = G Msun / R2
Planet motion:
independent of planet mass depends on: mass of sun distance

11. Newton and Planets Laws of motion + Gravity Predicts Kepler’s Laws:
1st Law (orbits are ellipses)
2nd Law (equal area in equal time)
conservation of angular momentum
Skater pulls arms in; spins faster
Planet gets closer to sun; goes faster
Extended 3rd Law
a3 = M P2
use to measure mass M (of central body)
M in solar masses

12. Consider a planet orbiting the Sun
Consider a planet orbiting the Sun. If the mass of the planet doubled but the planet stayed at the same orbital distance, then the planet would take
a) more than twice as long to orbit the Sun.
b) exactly twice as long to orbit the Sun.
c) the same amount of time to orbit the Sun.
d) exactly half as long to orbit the Sun.
e) less than half as long to orbit the Sun.

13. Workbook Exercise: Orbital Period and Orbital Distance
(pages in workbook)

14. Which of the following best describes what would happen if Mercury and Jupiter were to
switch places in their orbits about the Sun?
a) Jupiter, the larger planet, would have a shorter orbital period than before.
b) Mercury, the smaller planet, would have a shorter orbital period than before.
c) Neither of the two planets would have any change in their orbital periods.

15. Imagine a new planet in our solar system located 3 AU from the Sun
Imagine a new planet in our solar system located 3 AU from the Sun. Which of the following best approximates the orbital period of this planet?
a) 1 year
b) 3 years
c) 5 years
d) 9 years
P2=a3, so if a=3, then a3=3x3x3=27; then P2=27, so P~5 (since 5x5=25)

16. Ocean Tides
Moon’s Gravity
Tides

17. Spring / Neap Tides Tidal effect of Sun new/full moon
Tides larger = Spring Tides
1st/3rd Quarter moon
Tides smaller = Neap Tides