Newton’s Third Law of Motion

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Newton’s Third Law of Motion:

Newton’s Third Law of Motion By, Sameer D. Devidas


Introduction If you lean over – you fall If you lean over with your hands against the wall – you don’t fall Because the wall is pushing on you and holds you in place

Forces and Interactions :

Forces and Interactions Force – a push or pull Newton saw an interaction Mutual action between forces Hammer and nail Hammer moves nail Nail halts the hammer

Newton’s Third Law states that :- :

Newton’s Third Law states that :- Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. For every action, there is an equal and opposite reaction . R

Third Law Forces :

Third Law Forces Action force Reaction force It doesn’t matter which is which Neither can exist without the other Equal and opposite



Balanced force acting on you:

Balanced force acting on you gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving.


What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).


Sometimes it is hard to find the action/reaction pairs What are the forces on a falling rock? Gravity? Weight? Ground?

for finding forces :

for finding forces Identify the interaction Action: Object A exerts force on object B Reaction: Object B exerts force on object A Falling rock – Earth exerts force on rock, rock exerts force on Earth

Action and Reaction on Different Masses:

Action and Reaction on Different Masses Big rock-Earth interaction – the forces are EQUAL Does the earth fall into the rock? Yes, but not as far The reactions are equal and opposite Because Earth is so large, we can’t sense its very small acceleration


Which is stronger, the Earth’s pull on an orbiting space shuttle or the space shuttle’s pull on the earth?


According to Newton’s Third Law, the two forces are equal and opposite. Because of the huge difference in masses, however the space shuttle accelerates much more towards the Earth than the Earth accelerates toward the space shuttle . a = F/m


Flying of Rocket When the rocket fuel is ignited, a hot gas is produced. As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.


Reaction: gases push on rocket Action: rocket pushes on gases =


Suppose you are taking a space walk near the space shuttle, and your safety line breaks. How would you get back to the shuttle?


The thing to do would be to take one of the tools from your tool belt and throw it is hard as you can directly away from the shuttle. Then, with the help of Newton's second and third laws, you will accelerate back towards the shuttle. As you throw the tool, you push against it, causing it to accelerate. At the same time, by Newton's third law, the tool is pushing back against you in the opposite direction, which causes you to accelerate back towards the shuttle, as desired.

Newton’s 3rd Law in Nature:

Newton’s 3rd Law in Nature Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).


Flying through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.


The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird ( upwards). Action-reaction force pairs make it possible for birds to fly.


If forces are “equal but opposite,” why don’t action-reaction pairs cancel? Equal and opposite forces acting on the same object would cancel. Action-reaction pairs don’t cancel because they act on different objects, not on the same object.


Conclusion Push the world hard – it pushes back hard Push the world gentle – it pushes back gentle You can not touch without being touched !!!!


References Science and Technology class IX By Anupam Dikshit , R.D Shukla Ultimate visual Dictionary of sccience Publisher- Dorling kindersley Advanced physics by Keith Gibbs Mechanics part I by D.C.Pandey

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