NATS 1945: Chapter 3: Gravity, Force, and Space
October 8, 2015
Gravity, mass, and weight
Force: a push or pull exerted on an object, either from contact or
a force 'eld.
oContact force: eg. a pull from a horse, there is some sort of
physical contact with an object.
Tension force: a force through a cable, example of a
Friction force: the force of a surface on top of another
surface. The heavier the mass, the larger the friction.
Depends on the surface. Contact force?
The normal force arises from the forces between
matters. Only exists when it is in contact.
oForce 'eld: eg. Magnetic 'eld
Mass: a measure of an object’s amount of atoms
oMass does not change.
Gravity: an attractive force (a pull) exerted by all objects with
mass; the greater the mass, the stronger its pull.
oCan be napped by a gravimeter (produces a density map)
Doesn’t necessary have to be in contact with the
ground. It can be airborne.
Weight: the strength of gravity exerted by a planet (or moon,
start, etc) or an object in the planet’s gravity 'eld.
oThe pull towards the ground changes.
o1kg=10N….on earth. 1/2kg=1lb
odepends on object’s mass; the more massive the object,
the greater its weight on the planet
oalso depends on planet’s mass & size; the smaller & more
massive the planet, the greater the object’s weight on the
Newton’s Law of Gravity: the strength of an object’s gravity
decreases by the inverse-square of distance from the object.
oDensity, the bigger the weaker the force?
Newton’s Law of Motion:
o1st Law: Objects in motion maintain their motion unless
acted upon by an external force.
Will continue its motion without increasing its speed
or direction. If there is a change in speed or
direction, there must be an external force present.
Orbital motion results from constant tug by a central
source of gravity
Circular motion has a external force acting upon it.