9/26 2nd and 3rd Law Text does not stress the 3rd Law but we will HW 9/26 “Three Blocks” due Monday 9/30 Two Handouts on 2nd and 3rd Laws up front One is a worksheet we will do today Today we will practice using the 2nd and 3rd Laws (worksheet) Friday we will do a worksheet on friction

Newton’s Third Law: All forces come in pairs. Here the car pushes on the truck and the truck pushes the opposite direction on the car, with the same size force! NT,C NC,T subscripts are the key!!! truck on car car on truck

Mike is stuck in the middle of a frozen lake holding a large bag of gold. The surface is perfectly frictionless so you can’t walk. How does Mike get off the ice? gold Mike Throw the gold in some horizontal direction! NG,M NM,G Equal and opposite, TWO SEPARATE FBD’s!!!, and switched subscripts 3rd Law

What THING is pushing on the rocket? Well, what is the rocket pushing on? What THING is pushing on the rocket? Is this a contact or non-contact force? The rocket pushes the exhaust gasses down so... The exhaust gasses push the rocket up. Contact

Example: Elevator moving down and slowing down. The magnitude of the elevator’s acceleration is 2m/s2. Let g = 10N/kg. Find the normal force that the floor puts up on the block But first find the direction of acceleration Draw FBD Adjust the arrow length(s) Apply 2nd law NF,B N - W = ma 5kg El N - 50 = 5(2) = 10 WE,B N = 60N a = = mg = 5(10) = 50N Fnet =

Example: The magnitude of the elevator’s acceleration is 2m/s2. Let g = 10N/kg. Note the danger in calling g an acceleration!!!! g tells us the Weight force on an object and is not an acceleration. But first find the direction of acceleration NF,B = 60N El 5kg WE,B a = = mg = 5(10) = 50N Fnet =

Example: Now find the Tension force that the rope puts up on the elevator, using both the 2nd and 3rd Laws. Elevator moving down and slowing down. The magnitude of the elevator’s acceleration is 2m/s2. Let g = 10N/kg. Fnet,El = mEla = 200N T-N-W = 200 TR.El = ? mEl = 100kg T-60-1000 = 200 NEl,B = 60N 1260N 5kg El WE,El = 1000N NB,El = 60N B a = WE,B = 50N Fnet =

Example: Try this Moving up and slowing down Draw FBD,s of both blocks consistent with the 2nd and 3rd Laws. Then rank the magnitudes of all the forces. 2m m