Physics 101THursday 10/27/11Class 19

Chapter 12.2 – 12.3

Gravitational attraction of spherical bodies

Keplers laws

Gravity

The space shuttle orbits about 200 km above the earths surface. The shuttle's "pre-launch weight" is its weight measured on the ground. (Neglecting the fuel) The magnitude of the force of gravity on the shuttle while it is in orbit is (the earth’s radius is 6370 km): A: Slightly greater than pre-launch.B: Equal to pre-launchC: Slightly less than pre-launchD: Lots less than pre-launch, almost (but not quite) zero. E: Precisely zero.

Gravity

Gravity

Gravity

Gravity

An astronaut is floating around in the space shuttle's cabin. Her acceleration, as measured from the earth's surface, is A: zero - she's floatingB: very small, in some random directionC: quite large, nearly g, directed towards

the center of the earthD: quite large, nearly g, directed along the line

of travel of the shuttle

gravity

Astronaut Dave Bowman is standing in the centrifuge of the spaceship Discovery. He drops his pen and observes it fall to the floor. Which statement below is most accurate? A: After he releases the pen, the net force on the pen is zero.B: The pen falls because the centrifugal force pulls it toward the floor.C: The pen falls because the artificial gravity pulls it toward the floor.   

If you weigh yourself at

the equator of Earth,

would you get a bigger,

smaller, or similar value

than if you weigh yourself

at one of the poles

(assume the Earth is

completely spherical, but

spinning)?

a) bigger value

b) smaller

value

c) same value

You weigh yourself on a

scale inside an airplane that

is flying with constant

speed at an altitude of

20,000 feet. How does your

measured weight in the

airplane compare with your

weight as measured on the

surface of the Earth?

a) greater than

b) less than

c) same

Gravity

At a new moon the sun, moon and earth are aligned as shown below. The force between the moon and Sun is:

A) 4 x 1026 NB) 4 x 1020 NC) 7 x 1031 ND) 7 x 1037 N

G = 6.67x 10-11 N m2/kgMe = 6 x 1024 kgMs = 2 x 1030 kgMm= 7 x 1022 kgRearth-moon = 4 x 105 kmRearth-sun = 1.5 x 108 kmNote: km not m

Also note F due to gravity between earth and moon is 1.8 x 1020 N

Sun Moon earth

A planet of mass m is a distance d from Earth. Another planet of mass 2m is a distance 2d from Earth. Which force vector best represents the direction of the total gravitation

force on Earth?

a bc

d

e

2d

d

2m

m

Earth

Gravity

At what point above the earths surface does the gravitation acceleration of the earth decrease to 1/4 the value at the surface? (note: the radius of the earth is 6 x 103 km)

A) 3 x 103 kmB) 6 x 103 kmC) 12 x 103 kmD) 24 x 103 km

Keplers laws

Orbits

A satellite is in circular orbit around a planet that has a very tenuous atmosphere extending up to the altitude of the satellite. Due to atmospheric drag, the speed of the satellite… A: increasesB: decreasesC: remains constant

Orbits

Keplers laws

Kepler's 3rd law: T^2/R^3=constant for all planets. The period T for the earth is 1 year (!) The distance of the earth to the sun is called 1 A.U. Suppose an asteroid is in circular orbit around the sun at a distance of 2 A.U. How long does it take the asteroid to orbit the sun once?A 2 yearsB: 3 yearsC: 2^(3/2) = 2.83 yearsD: 2^(2/3) = 1.59 yearsE: Not enough information (An asteroid is not a planet - can't figure it out from Kepler's third law.) 

Orbits

Kepler's 3rd law: T^2/R^3=constant for all planets. The period T for the earth is 1 year. The distance of the earth to the sun is called 1 A.U. Suppose a satellite orbits the earth at a distance of about 1/1000 AU. How long does it take the satellite to orbit the earth once? A: Not enough information given.B: 1/1000 year.C: 1000 years.D: (1/1000)^(3/2) yearsE: (1/1000)^(2/3) years 

Orbits

Two communications satellites are in orbit at the same height, but one weighs twice as much as the other. The speed of the heavier satellite is A: Less than B: Equal to C: Greater thanD: (need more information) the speed of the lighter one. 

Orbits

Orbits

Astronauts in the space shuttle float because:

a) they are so far from Earth that Earth’s gravity doesn’t act any more

b) gravity’s force pulling them inward is cancelled by the centripetal force pushing them outward

c) while gravity is trying to pull them inward, they are trying to continue on a straight-line path

d) their weight is reduced in space so the force of gravity is much weaker

Keplers 3rd law

Weighing planets:Astronomers can use the Keplers 3rd law to weigh a planet, by measuring the orbit of a moon of the planet (knowing T and radius then:

If Mercury orbits the sun at a radius of 1011m, with a period of 90 days (8x106s), the mass of the sun is (close to what order of magnitude)?

A) 1010 kg B) 1020 kg C) 1030 kg D) 1040kg

Reading Assignment

Tuesday 11/1 12.4 – 13.3

Thursday 11/3 13.4 – 13.7

Conservation of mechanical work

Problems 12.8, 12.15, 12.30, 12.32