Copyright © 2012 Pearson Education Inc. Gravitation Physics 7C lecture 17 Tuesday December 3, 8:00 AM – 9:20 AM Engineering Hall 1200

Copyright © 2012 Pearson Education Inc.

Gravitation

Physics 7C lecture 17

Tuesday December 3, 8:00 AM – 9:20 AMEngineering Hall 1200


Introduction

• What can we say about the motion of the particles that make up Saturn’s rings?

• Why doesn’t the moon fall to earth, or the earth into the sun?

• By studying gravitation and celestial mechanics, we will be able to answer these and other questions.


Newton’s law of gravitation

• Law of gravitation: Every particle of matter attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.


Newton’s law of gravitation

• The gravitational force can be

expressed mathematically as

• Fg = Gm1m2/r2,

• where G is the gravitational

constant. Note G is different from

g.

• G = 6.67 E -11 N m2/kg2


Gravitation and spherically symmetric bodies

• The gravitational interaction of bodies having spherically symmetric mass distributions is the same as if all their mass were concentrated at their centers.

• This is exact, not approximation! Let’s prove it mathematically.








Determining the value of G

• In 1798 Henry Cavendish made the first measurement of the value of G. Figure below illustrates his method.

• G = 6.67 E -11 N m2/kg2


Some gravitational calculations

• Fg = Gm1m2/r2,


The sphere on the right has twice the mass and twice the radius of the sphere on the left.

Compared to the sphere on the left, the larger sphere on the right has

A. twice the density.

B. the same density.

C. 1/2 the density.

D. 1/4 the density.

E. 1/8 the density.

Q13.1

mass mradius R

mass 2mradius 2R


The sphere on the right has twice the mass and twice the radius of the sphere on the left.

Compared to the sphere on the left, the larger sphere on the right has

A. twice the density.

B. the same density.

C. 1/2 the density.

D. 1/4 the density.

E. 1/8 the density.

A13.1

mass mradius R

mass 2mradius 2R


Why gravity is important?

• It is the dominant force on astronomical scale.


Why gravity is important?

• Anomalous gravity at micron scale can be evidence for extra dimension of our universe.


Weight

• The weight of a body is the total gravitational force exerted on it by all other bodies in the universe.

• At the surface of the earth, we can neglect all other gravitational forces, so a body’s weight is w = GmEm/RE

2.

• The acceleration due to gravity at the earth’s surface is g = GmE/RE

2.


Weight

• The weight of a body decreases with its distance from the earth’s center, as shown in Figure below.


Weight

• We can use weight to measure earth’s mass!

g = GmE/RE2

We know the radius of earth = 6300 km from satellite/astronomical observations. Thus mE = G/g RE

2 = 5.98 E 24 kg

The average density of earth is then 5500 kg/m3, 5.5 times heavier than water!


Interior of the earth

• The earth is approximately spherically symmetric, but it is not uniform throughout its volume.

• The inner core is supposed to be made of iron and rotates at high speed, giving earth’s magnetic field.


Gravitational potential energy

• The gravitational potential energy of a system consisting of a particle of mass m and the earth is U = –GmEm/r.

• This assumes zero energy at infinite distance.


Gravitational potential energy

• U = –GmEm/r.

• Proof:


Gravitational potential energy depends on distance

• The gravitational potential energy of the earth-astronaut system increases (becomes less negative) as the astronaut moves away from the earth.


From the earth to the moon

• To escape from the earth, an object must have the escape speed.


The motion of satellites

• The trajectory of a projectile fired from A toward B depends on its initial speed. If it is fired fast enough, it goes into a closed elliptical orbit (trajectories 3, 4, and 5).


Circular satellite orbits • For a circular orbit, the speed of a satellite is just right to keep its distance

from the center of the earth constant. (See Figure below.)

• A satellite is constantly falling around the earth. Astronauts inside the satellite in orbit are in a state of apparent weightlessness because they are falling with the satellite. (See Figure below.)

Documents

Copyright © 2012 Pearson Education Inc. Gravitation Physics 7C lecture 17 Tuesday December 3, 8:00 AM – 9:20 AM Engineering Hall 1200