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

3-A Bed of nails

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 )

4- The Ballon

5- The rocket

thrust exerts a force upon the rocket , the rocket answers by a force equal in magnitude and opposite in direction

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.