Abeka Physics Section 8.5 Flashcards
Knowing Newton’s law of gravitation allows us to derive another expression for weight. If an object of mass is at a distance of r from the earth’s center, the gravitational force on the object is specified as follows by what equation?
F = G (Mem/r^2) where Me is the mass of the earth, G is the constant of gravitation, and r is the distance between their centers
Give the formula that gives the value of g at locations above the earth’s surface? Identify each variable.
g=GMe/r^2
G: constant of gravitation, Me: mass of the earth, r: the distance from the earth’s center of mass
Explain why an average person weighs about a pound more at the poles than at the equator.
An average person weights about a pound more at the poles than at the equator >because a person at the equator has a slight inertial tendency to fall outward from the rotating earth beneath his feet
>because the earth is not perfectly round: because the earth’s center of mass is closer to the flattened poles than to the bulging equator, r (the distance from the earth’s center of mass) is lower at the poles; consequently, g (acceleration due to gravity) is greater; but at both locations on the earth’s surface you are pulled downward by the total mass of the earth
If you were to break through the earth’s surface and descend through a tunnel leading to the center of the earth, you would lose weight all along the way until, at the very center, you were weightless. Explain why.
The reason is that as you moved ever deeper into the earth, progressively more of the earth’s mass would lie above you, pulling you away from earth’s center.
At about 300 miles above the earth’s surface (in what we call “space”), an astronaut is often described as being “weightless.” This is not because there is not effect of gravity at this point; rather what is the reason?
The astronaut is free-falling as he orbits the earth. Since his rate of free-fall is approximately equal to the acceleration due to gravity, the astronaut feels weightless when in reality he is not.
Describe static equilibrium
> an object that remains at rest within earth’s gravitational field is said to be in static equilibrium
there are three kinds of static equilibrium: stable, unstable, and neutral
Describe the center of gravity
> the one spot that averages the effect of gravity’s influence on all the particles is called the center of gravity
it represents the combined effect of gravity on all of the particles in an object
Describe stable equilibrium
> an object in stable equilibrium is not easily tipped over
in order to move an object in stable equilibrium you must raise its center of gravity by tipping the solid up on an edge
tipping the solid up on an edge allows some particles of the solid to shift closer to the earth, elevating the majority of particles to positions farther away from the earth
to upset any object in stable equilibrium requires the lifting of mass against the force of gravity
“changing the orientation of a stable object requires work”
Describe unstable equilibrium
> an object in unstable equilibrium is upset at the slightest disturbance
the reason is that if the center of gravity does not stay precisely above the point of support, the center of gravity is free to move downward to a lower position in response to the force of weight
once the delicate balance is lost, the force of gravity overturns the object
Describe neutral equilibrium
> an object in neutral equilibrium can be overturned without changing the height of it center of gravity
rolling objects like balls have neutral equilibrium
Describe instability
> results from the center of gravity being unsupported, and the point where the force of gravity is effectively applied being free to move downward, in the direction of the applied force
Describe the difference between being unstable and being in unstable equilibrium
> the net force of an object in unstable equilibrium is zero, and the object maintains its precarious balance indefinitely–if protected from any disturbing force
an unstable object is subjected to the unbalanced force of weight, and the object falls immediately
generally speaking, an object is unstable if its center of gravity outside its base of support (the base is found by tracing the shortest possible perimeter around all the members of the object in contact with the ground)
Describe how to determine the center of gravity
> for any object of regular shape, the center of gravity can be computed by means of a formula
for irregular objects, the simplest way of determining the center of gravity is by multiple suspensions
center of gravity lies directly above any single point of balance and directly below any single point of suspension
Describe the center of mass
> the center of gravity has meaning only when an object is under the pull of gravitational force; otherwise, that same point is called the center of mass
center of mass - the point in an object that describes the combined effect of all the individual particles of mass
If the net force of an extended object is zero, how does the center of mass move?
If the net force of an extended object is zero, the center of mass moves at a constant speed in a straight line.
