# AQA A2 Physics - Attracted to Earth Revision Notes

Notes for the unit 4 physics aqa A2 exam for those studying spec b: physics in context

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• Created by: tom
• Created on: 27-04-11 15:46

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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 experience the sensation of weight from the reaction force of whatever you are
standing/sitting on ("every action has an equal and opposite reaction")
o When objects are not moving vertically the reaction force is equal to the weight of
the object
When object is human body it is the reaction that is felt
Not the pull of gravity
o A person free falling would not experience their weight
As there is no reaction force
The gravitational attraction is still there though
Weightlessness
o Two reasons why an object would be weightlessness
No mass
No net gravitational field
o Everywhere in the universe is subject to a gravitational field
Meaning weightlessness can only be achieved when two identical gravitational
forces act on an object and cancel the effects of each other out
This point is called a neutral point

## Other pages in this set

### Page 2

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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 = W' (Newton's Third
Law). Thus, two forces are acting on the person; gravity force `mg' and reactionary force `R'.…read more

### 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 `a' (fig (c)).…read more

### Page 4

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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 of fluids
Measured in Pascal Seconds (Pa s)
Viscosity arises in gases from the molecular diffusion that carries momentum between the
different layers in a moving gas.…read more

### Page 5

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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 the distance travelled by the ball before it falls to the
ground will increase, with the curvature of the earth eventually meaning that it will fall below

### Page 6

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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
the earth as an inverse square relationship
Inside the earth the gravitational field strength would be linear
Close to the earth's surface it is very close to being constant, but further…read more

### Page 7

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Circling the Earth:
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.…read more

### Page 8

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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 for the satellite to orbit the earth
Gravitational Force = Centripetal Force
M 1M 2
G R = mv2
r
GM E
v= r
This shows that the velocity needed by a…read more

### Page 9

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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 Weight
The difference between our weight and the reaction of the earth provides the centripetal
force need to keep us from flying off at a tangent
o Since mg - R = m2r