Unit Dynamics Module Newton s Three Laws page 1 of 2 Actions Reactions and Newton s Third Law Newton s third law states if object A exerts a force on object B then object B always exerts a force back on A The two forces are equal in magnitude but opposite in direction to each other It is sometimes useful to think of Newton s third law in terms of action and reaction pairs It is important to understand that the opposite forces in Newton s third law apply to different objects Newton s third law is used when comparing forces on objects Consider hammering a nail into a block of wood with a hammer First focus on the forces on the nail The nail is subject to a downward force from the hammer FNH The wood also exerts a force on the nail FNW in the upward direction Now focus on the hammer the hammer moves downwards until the nail stops it The force on the hammer by the nail is FHN and it is in the upward direction Newton s third law says that FHN is exactly equal in magnitude and opposite in direction to FNH The owner of a horse wants it to pull a cart The horse knows about Newton s third law and believes that pulling the cart won t accomplish anything For any force the horse applies to the cart the cart will apply an equal and opposite force on the horse The horse needs to consider the net force exerted on itself and on the cart When the force on the horse by the ground is greater than the force on the horse by the cart the cart will move forward When the force on the cart by the horse is greater than the force on the cart by the ground the cart will move forward It is important to understand that the equal and opposite forces in Newton s third law apply to forces on different objects www thinkwell com info thinkwell com Copyright 2001 Thinkwell Corp All Rights Reserved 1799 doc rev 03 29 2001 Unit Dynamics Module Newton s Three Laws page 2 of 2 Actions Reactions and Newton s Third Law Newton s third law is very useful when comparing forces on different objects If a car and a sport utility vehicle collide the force on the car by the SUV FCS and the force on the SUV by the car FSC will be equal in magnitude and opposite in direction Newton s second law states that F m a Combining Newton s second and third laws allows you to conclude that because the mass of the car is much smaller than that of the SUV the acceleration of the car will be much greater than that of the SUV www thinkwell com info thinkwell com Copyright 2001 Thinkwell Corp All Rights Reserved 1799 doc rev 03 29 2001
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