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Attracted to the Earth

Feeling the Force:
Weight
o Force of gravity acting on a body
o w = mg
Special case of F = ma




Gravitational Field Strength (g)
o Force per unit mass (independent of magnitude of mass)
o Measured in Newtons or kg



Experiencing Weight
o We…

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Apparent Weight in Lifts

Suppose a person of mass `m' is standing on a weighing machine placed in a lift/elevator. The
actual weight of the man is `mg'. This weight presses against the floor. The floor also exerts
a reactionary force `R' on the person in upward direction where `R…

Page 3

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As
R=w
w = m.(g + a)
Thus, in this condition, the apparent weight `W' of the person is greater than the gravity
force `mg'. That is, the person will experience his weight to be increased than in the normal
state.




Now, suppose the lift is coming down with acceleration…

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Effect of the Atmosphere:

Viscosity
A measurement of a fluid's resistance to flow. When a fluid flows or an object moves
through a fluid the layers in the fluid exert resistive forces on each other, producing a stress
on the fluid

o Coefficient of Viscosity ()
Used to compare viscosities…

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Pulling Together:

Newton's Universal Law of Gravitation
Apples fall from trees no matter how tall the tree is. This requires a force which we call
gravity. If we fire a cannon ball from a tall tower, the ball will eventually fall to the ground.
Choosing a taller tower means that…

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Meaning the separation is equal to the radius of the earth ( Re )

Using M e to represent the mass of the earth

mg = GMRe2
m
e




g = GM2
Re
e



This shows that from the earth's surface g falls off with distance from the centre of…

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Circling the Earth:
(links with circular motion)

An object moving in a circle with a constant speed must have force acting on it, directed
towards the centre of the circle ­ centripetal force.
2 2
F = mv
r m r


o F = Centripetal Force

o m = Mass…

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The time period (T) of an oscillation is related to the frequency (f) by the equation
1
T= f


o T is time taken for full circle

Showing the objects steady angular speed can be found from

T = 2



Satellites in Orbit
o Gravity must provide the centripetal force…

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This would give the radius of a geostationary orbit
GM E

3
r= 2


Geosynchronous Orbit
Satellite with an orbital period of 24 hours but not above the equator

It will pass over the same points on the earth at the same time everyday



Rotation of the Earth and Apparent…

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