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Newton’s Third Law
For every action there is a reaction equal in magnitude and opposite in direction .
F1 = – F2
Applications:
1- When the gun is fired from a gun forward , the gun is rebounded backward
also..
2- When a hammer hits a nail by a force the nail reacts by equal opposite force
But why is the nail driven in the wood then ??!!
there are Two forces are acting on the nail :
The first one is the hammer hitting the nail the nail answers by an equal force
The wall is affected by hitting the hammer so the wall has a reaction on the nail which is smaller than the original force acting on the nail (hammer hitting) so the nail can be driven in the wood and the net force becomes in the direction of the greater force which is the action of the hammer
Magicians use a bed of nails with a heavy block on their chests but the nails don’t penetrate their bodies and they are not harmed why??
Because the force is distributed over a large area of nail tips , so the force of each nail is so small and the body’s reaction can resist it (body’s reaction is equilibrium with the pressing force of each nail )
Joule’s Experiment
Mechanical work and thermal energy
James houle performed this experiment to show that the mechanical energy can be transformed into internal energy in water producing a rise in temperature.
The appratus used :
The structure :
Two heavy lead masses are attached to a string wounded around the spindle of an eight vanes paddle wheel.
Insulated copper vessel containig water including four fixed vanes to prevent the circulation of water (you will know why do we want to do so).
A thermometer to record the increase of temperature of water
The Experiment :
we wound the masses to the maximum height
when the masses fall freely , this leads to spining paddles(the potential energy stored in the masses is converted into kinetic energy in the paddles , i.e :the decrease in KE = increase in PE).
the turning paddles do work on water (churning it up)equal to force of gravity times L(the potential energy).
by the time the masses stop , all of the potential energy has been transfered to work in water (minus some friction in ropes).
the motion of paddle wheels cause friction between water molecules leading to an increase in the temperature of water.
the fixed vanes are used to increase the collisions between water molecules , because if water rotated uniformly , the water temperature won’t change much.
General Gravitational Law
It was said that newton thought of gravitational forces when he noticed an apple falling down from a tree towards the ground , so Newton suggested some basic concepts which helped up in setting the general gravitational law
1- The Apple fell down due to the gravitation of Earth and in turn , it attracted Earth towards it (According to Newton’s third Law).
2- The Gravitational attraction between diffrent bodies occurs even if they are far apart from each other such as : the attraction between moon and earth and the attraction between sun and planets.
3- The gravitational attraction between bodies depends on The mass of bodies and the distance between them.
Deduction of general Gravitational law :
| It could be verified practically and theoritically that the gravitational attraction force between two bodies is:
Directly proportional to the product of their mass F ∝m1*m2——————(1) Inversly Proportional to the square of distance between them.
F ∝ 1/d^2—————-(2) |
 |
From (1) and (2) :
F ∝ m1*m2/d^2
therefore : F = G *m1*m2 / d^2
where G is the universal gravitational constant
G=6.67 * 10^-11 Nm^2/Kg^2
The universal gravitational constant is the attractive force between two 1 Kg spherical bodies whose centers are one meter apart .
G = F*d^2 /m1m2
Why are astronauts weightless in space?
imagine a girl in an elevator so :
the gravity pulls down the girl , the girl pulls on the elevator’s surface
the elevator has a reaction opposite in direction which is the tension
force.
1- When the elevator is at rest :
since the acceleration= zero
therefore :
F net =Ft-Fg = ma
where Ft is the tention force
(normal force in an elevator)
F net = Ft-Fg= zero
therefore Fg=Ft.
2- When the elevator moves upwards by
acceleration (a) :
since the elevator is moving upwards
therefore the force of gravity has a negative
sign
F net =Ft-Fg = ma
therefore
Ft = Fg + ma
and since the tension force is greater than gravity so the girl feels heavier in this case.
3- When the elevator is moving downwards by acceleration (a) :
since the motion is upwards , therefore the force of gravity has a positive sign while the tension force – which is opposite to gravity- has a negative sign thus we have :
F net =Fg-Ft = ma
therefore -Ft=-Fg+ma
therefore Ft = Fg-ma
therefore tension force is smaller than gravity so the girl feels lighter .
4- Imagine that the cable of elevator is cut and the elevator is falling freely :
Ft = Fg-ma
and since the elevator is moving with the same acceleration due to gravity :
therefore Ft = Fg-mg
and since Fg=mg
Therefore Ft = Zero
and since the the tension force is zero therefore the girl feels weightless
this is the same reason for why are astronauts are weightless in space , they feel weighless not because gravity doesn’t exist in space (of course gravity does exist in space), but because they are falling freely towards earth’s center.
The normal force is the responsible for the feeling of weight and in the absence of this force we feel weighless.
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