It is all in Newton's third law: as your legs propel your body towards the dock, they also apply to the boat an equal force in the opposite direction, which pushes it away from the dock. Otherwise, even as you jump, the boat "magically" moves away from the dock, possibly making you miss your leap or pushing the boat out of reach. Those familiar with small boats know that before jumping from a boat to the dock, it is wise to tie the boat to the dock first, and to grab a handhold on the dock before jumping. In a similar way, the forward motion of a rocket comes from the reaction of the fast jet of hot gas shooting out from its rear. Rotating garden sprinklers work by the same principle.The nozzle of a big firehose has handles which firefighters must grasp firmly, because as the jet of water shoots out of it, the hose itself is forcibly pushed back.By Newton's law, the gun itself recoils backwards. When a gun is fired, the force of the gas produced by burning gunpowder hurls out the bullet.In contrast, Newton's 3rd law always involves more than one object. Between the floors, as long as the elevator does not accelerate, the net force must be zero, hence the two forces must be equal and opposite ( v>0, a=0). It senses two forces: downwards, its weight and that of the people in it, and upwards the pull of the cable which holds it up. An elevator is pulled up from the street level to the 5th floor.Obviously, the floor is exerting on it an equal and opposite force -m g (velocity v=0, acceleration a=0). However, it does not move in that direction, because the floor stops it. A heavy object stands on the floor, pulled down by the Earth with a force m g (drawing).This is not an example of the third law, but of equilibrium between forces. Note the important provision: two objects must be involved! There exists a whole set of situations where two equal and opposite forces act on the same object, canceling each other so that no acceleration (or even no motion) occurs. "Every action has an equal and opposite reaction" Then object B exerts an equal and opposite force –F on object A" ![]() ![]() If object A exerts a force F on object B,
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