Class 9, Science, Chapter-8, Lecture-3, Newton's Third Law (Notes)
IMPULSE:
The product of the force and the time for which it acts in called impulse.
Impulse = Force x Time
Impulse is a VECTOR quantity
S.I.Unit – newton second ( Ns )
NEWTON’S THIRD LAW OF MOTION:
Whenever a body exerts a force on a second body, the second body exerts equal and opposite force on the first.
OR
“To every action there is an equal and opposite reaction.”
Important Points:
- Forces always occur in pairs
- Action and reaction act on different objects
EXPERIMENT TO DEMOSTRATE NEWTONS’S THIRD LAW:
Two spring balance A and B are joined hook to hook and B is fixed to the wall, as shown in the figure.
When spring A is pulled, both the balances show equal readings. This shows that the force exerted by A on B is equal to that exerted by B on A.
EXAMPLES OF NEWTON’S THIRD LAW:
- When a bullet is fired from a gun, the force exerted by the gun on the bullet is equal to the force exerted by the bullet on the gun due to which the gun RECOILS.
- JET PROPULSION THEORY
The jet exerts force on the gases to expel them the gases exert equal and opposite force on the jet which pushes the jet forward. - Swimming –
While swimming a person pushes the water backward and the water pushes the person forward. - Boat –
A boatman exerts a force on water with his a oars and the oars exert equal and opposite force on the boat. - Walking –
While walking, a person exert force on the earth in the backward direction and the earth pushes the body in forward direction. - Hosepipes –
While putting off a fire, the hosepipes exert force on water and the water pushes the pipe back due to which pipes need to be held strongly. - When a man jumps from a boat, he exerts a force (action) on the boat and the boat exerts equal and opposite force (Reaction). So the man moves in the forward direction and the boat in the backward direction.
- Lawn Sprinkler –
When water comes out from curved nozzles of a lawn sprinkler, it exerts an equal backward force on the sprinkler, due to which the sprinkler rotates and spreads water in all directions.
Action And Reaction Do not Cancel Each Other.
Reason :
Action and reaction, though equal and opposite, always act on different bodies and therefore can not produce equilibrium.
INTERACTION:
When one body influences another body by applying force, the first body is said to be interacting with the second body.
LAW OF CONSERVATION OF MOMENTUM:
When two bodies act upon one another, their total momentum remains conserved in the absence of an external force.
${{\rm{m}}_{\rm{1}}}{{\rm{u}}_{\rm{1}}}\,\,\,\,\,\,\,{\rm{ + }}\,\,\,\,\,\,\,{{\rm{m}}_{\rm{2}}}{{\rm{u}}_{\rm{2}}}\,\,\,\,\,\,\,\,\,\,{\rm{ = }}\,\,\,\,\,\,\,\,\,\,{{\rm{m}}_{\rm{1}}}{{\rm{v}}_{\rm{1}}}\,\,\,\,\,\,\,{\rm{ + }}\,\,\,\,\,\,\,{{\rm{m}}_{\rm{2}}}{{\rm{v}}_{\rm{2}}}$
Total momentum before interaction = Total momentum after interaction
Calculation of Recoil Velocity Of A Gun:
Let,
M = mass of the gun
m = mass of bullet
V = velocity of gun (Recoil Velocity)
v = velocity of bullet
Momentum before firing = 0
Momentum after firing = ${\rm{MV}}\,\,\,\,\,{\rm{ + }}\,\,\,\,\,{\rm{mv}}$
By Law of conservation of momentum,
$ \Rightarrow 0\,\,\,\,\,\,\, = \,\,\,\,\,\,\,MV\,\,\,\,\, + \,\,\,\,\,mv$
$ \Rightarrow MV\,\,\,\,\,\,\, = \,\,\,\,\,\,\, - mv$
$ \Rightarrow V = - \left( {{{mv} \over M}} \right)$
Momentum before firing = Momentum after firing