define Newton third law of motion.explain it with example?
Answers
Answer:
to an every action , there is equal and opposite reaction.
pushing a wall.
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Answer:
Slightly over half of the examples cited below are not examples of Newton’s Third Law. This is unhelpful!
When I stand on the ground, gravity pulls me downwards with 120kilograms-force or 1200 Newtons. The ground pushes back up on me at 1200 Newtons. This is not an example of Newton’s Third Law; it is an example of Newton’s First Law. Because I am not accelerating downwards, the two forces have to be equal and opposite to produce a resultant force = zero. In fact, I sink very slightly into the floor until the resultant force is equal to zero.
When the earth pulls me downwards with 1200 Newtons, the ‘3rd law reaction’ is that I pull upwards on the earth with 1200 Newtons. (The Earth doesn’t notice.) When my feet push on the floor with 1200 Newtons, the ‘3rd-law reaction’ is that the floor pushes on my feet with 1200 Newtons.
If the floor suddenly collapses, the force upwards on me disappears and I accelerate downwards towards the centre of the earth. Therefore the upwards force from the floor cannot be the ‘reaction’ to my weight; for that ‘action’ gravity force on me is still there, accelerating me remorselessly towards the concrete basement below.
To clarify those 4 different forces, suppose I jump into a boat, and for a few seconds the boat is accelerating downwards as it absorbs my impact.
Centre of earth on me (gravity) 1200 Newtons downwards
Me on centre of earth : 1200 Newtons upwards
Boat on me: 1400 Newtons upwards – so the resultant force on me is 200 Newtons upwards, which causes me to accelerate upwards, ie to decelerate my downward motion
Me on boat: 1400 Newtons downwards - causing the boat to accelerate downwards into the water
Okay, so some actual examples of the 3rd law: and note that the action force and the reaction force act on different bodies.
Fire a gun. Action: the forward force on the bullet. Reaction: the backwards force on the gun (recoil)
Shoot off a rocket. Action: the backwards force on the expelled gasses at the back. Reaction: the forward force on the rocket.
Jump in that boat. Action the upwards force of the boat on me, decelerating my fall. Reaction: the downwards force of me on the boat, pushing it down into the water.
Bang in a nail. Action: downwards force on the nail, driving it into the wood. Reaction: the upwards force on the hammer, causing it to bounce back up.
Sit in a boat and push on another boat. Action: force on other boat, pushing it away. Reaction: force on your boat, pushing you away backwards.