Gravitational Fields

Newton (4 January 1643 – 31 March 1727) Strikes again:

Besides developing his three law of motion he also examined the motion of the planets.

He wondered,

"What force keeps the moon in orbit around the Earth?"

He was also thinking about gravity

-To make things more difficult, there were no words to describe

- the amount of matter an object occupies.

- the relationship between the change in velocity in a given time

In the End

Since falling objects accelerate, they must have some force acting on them.

F= ma

We now call this force gravity.

Where a = acceleration due to gravity.

Recall:

-If an object has a force exerted on it, that force must be applied or exerted by another object.

-Newton concluded that the Earth itself must be the mass that exerts gravitational force on objects at or on its surface.

-He also showed that this force decreases with distance.

-bigger the mass the greater the force

r is the distance from the center of a mass

Once again everybody thought Newton was crazy!

- No one was comfortable with the idea of a force acting without physical contact.

His ideas worked for any two objects.

Recall

Non-contact force acts at a distance

Gravity is felt at a distance from the object.

A field is a region in which a force is felt.

So a gravitational field is the region around an object where another object experiences a force.

gravity = long range attraction

Gravitational Field Lines

Michael Faraday (22 September 1791 – 25 August 1867)

- introduces a concept of field lines to show both the strength and direction of a force.

-the number of field lines per unit area shows the strength of the force.

ie. lots of lines = lots of force few lines = small force

Force is a vector so you must indicate the direction of the force

Attractive force (-ve) forcegravity is attractive

Notice how the field lines have more space between as you move further away from the earth, this represents a weaker force.

Gravitational Field Strength

The gravitational field strength is defined as the gravitational force that a "test mass" would experience at some point in the field.

- the test mass is usually 1 kg.

g

Units = N/kg or _____

=Fg____m

Near the surface of the earth the magnitude of the gravitational field strength is 9.8 N/kg

field strength → Fg = mg = ma

# Of N of force per kg of mass

Weight = the force of gravity of a massive object on a second object

weight = Fg = mg

A spring scale measures weight, by seeing how much the object pushes on a spring (inside the device). On the Moon, an object would give a lower reading.

A balance scale measures mass, by comparing an object to references. On the Moon, an object would give the same reading, because the object and references would both become lighter.

http://en.wikipedia.org/wiki/Weight

http://www.astronomynotes.com/gravappl/s4.htm