Newton's Third Law

Newton’s Third Law

• when two objects interact, the force exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 on object 1

• F1 on 2 = - F2 on 1

  • forces always come in action/reaction pairs

  • action and reaction forces are equal and opposite

  • action and reaction forces are of the same type and act on different objects

forces always come in action/reaction pairs

remember: forces always come in action/reaction pairs

when two objects interact, the force exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 on object 1

Remember: when two objects interact, the force exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 on object 1

when two objects interact, the force exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 on object 1

Newton’s third law

walking is an action/reaction pair

action/reaction pair ex:

• when a person walks

  • action: they push backward against the surface

  • reaction: this surface pushes forward on the person

• these two forces have the same length (magnitude) but in opposite directions 

• these forces dont cancel each other out because they are applied to different objects 

driving is an action/reaction pair

action/reaction pair ex:

• when a car moves forward

  • action: the tire pushes backward against the road

  • reaction: the road pushes forward on the tire

• these two forces have the same length (magnitude) but in opposite directions 

• these forces dont cancel each other out because they are applied to different objects 

rocket take off/ flying is an action/reaction pair

action/reaction pair ex:

• when a rocket takes off/flys 

  • action: the rocket pushes the hot gases backward 

  • reaction: the gases push the rocket forward 

• these two forces have the same length (magnitude) but in opposite directions

• these forces dont cancel each other out because they are applied to different objects 

a hammer hitting a nail is an action/reaction pair

action/reaction pair ex:

• when a hammer hits a nail

  • action: hammer exerts a force on the nail

  • reaction: the nail also exerts a force on the hammer

• these two forces have the same length (magnitude) but in opposite directions

• these forces dont cancel each other out because they are applied to different objects 

horse pulling cart is an action/reaction pair (contradiction?)

action/reaction pair ex:

• when a horse pulls a cart 

  • action: horse pulls on the cart

  • reaction: the cart pulls back on the horse with the same magnitude force

• these two forces have the same length (magnitude) but in opposite directions

• these forces dont cancel each other out because they are applied to different objects 

  • + horse is digging into the ground and pushing back on the ground, so the ground is pushing the horse forward

• Fnet on horse and Fnet on cart are both forward

problem solving strategy 

• when two or more objects are connected or in contact, apply Newton’s laws to each individual object

  • draw a free-body diagram for each diff object separately

  • only include forces acting on the specific object currently of interest

  • do the objects move together? if so, they must have the same acceleration

if objects are connected & move together then they have the same acceleration

remember: if objects are connected & move together then they have the same acceleration