Presentation on theme: "Warm-up: Identify the action force and the corresponding reaction force for each event: A fish swims through water. A rocket accelerates in outer space."— Presentation transcript:
1 Warm-up:Identify the action force and the corresponding reaction force for each event:A fish swims through water.A rocket accelerates in outer space.If a soccer ball pushes back on your foot just as hard as your foot pushes on the ball, how can the soccer ball go anywhere?
2 Newton’s 3rd LawWith Newton’s 1st and 2nd Laws, we only talk about the forces on a single object.For N3L, we are now talking about the interaction between TWO objects.Symbols:To show the force that one object exerts on another, we use a symbol like this:F1-2 = the force that object 1 exerts on object 2
3 Newton’s 3rd LawExample: if you kick a soccer ball, the two forces would be written like this:Ff-s = the force of your foot on the soccer ballFs-f = the force of the soccer ball on your footSo, N3L says that Ff-s = Fs-f
4 Newton’s 3rd Law The Horse-and-Cart Problem: Problem: If the force of the cart on the horse (Fc-h) is just as strong as the force of the horse on the cart (Fh-c), how can the horse move the cart?Fh-cFc-h
5 Newton’s 3rd Law Think about each object separately! Fh-c Fc-h First, draw a force diagram for the cart:Then, draw a force diagram for the horse:FNFh-cFfFGFNFc-hFfFAPPLIEDFG
6 Newton’s 3rd LawAnswer: the horse can move because the force from the ground (forward) on the horse is even bigger than the force from the cart (backward)!!!IMPORTANT!!! -The “equal and opposite” forces don’t cancel each other out, because they are acting on different objects!
7 N3L: Force DiagramsYour assignment is to finish the N3L Force Diagrams worksheetWe’ll do an example together.