2Newton’s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.ANOTHER WAY TO SAY IT….For every action there is an equal and opposite reaction.
3Think about it… How is the gymnast able to flip over? When the gymnast the vault, the vaultagainst the gymnast.The of the force arrows are thebut the direction isPUSHESPUSHES BACKLENGTHSAMEOPPOSITE
4Force Pair The forces two objects apply to each other. Force pairs will always act inThe girls feet act onThe boat acts onOPPOSITEDIRECTIONSTHE BOATTHE GIRLS FEET
5Do Action-Reaction Forces Cancel? If 2 equal act in opposite directions they are balanced and cancel each other out. There is no movementAction – Reaction forces cancel out because they are acting onDO NOT2 DIFFERENT OBJECTS
6FORCE PAIRIf one of the pairs is much more you will only see the objectExample – When youpush down on the Earth,you won’t see the Earthmove, only you jumpingin the airMASSIVELESS MASSIVEMOVE
7ACTION AND REACTION FORCE PAIR called the ACTION FORCE In a one force is called the and the other force iscalled theFor every there is athat is in strength, but in direction.Action – Reaction forces don’t cancel because theyFORCE PAIRACTION FORCEREACTION FORCEACTION FORCEREACTION FORCEEQUALOPPOSITEACT ON DIFFERENT OBJECTS
8How do Action-Reaction forces work when you are… JUMPINGWhen you jump, you push down on the ground.The ground then pushes up on you. It is this upward force that pushes you into the air.
9How do Action-Reaction forces work when… A rocket launches*When the rocket fuel is ignited, a hot gas is produced. As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.The action force is -THE DOWNWARD PUSHThe reaction force is -THE UPWARD PUSH ON THE ROCKET ENGINE BY GAS MOLECULES
10Why doesn’t the bowling ball move backward if the pin is pushing on it equally? The bowling ball has moreMOMENTUM
11Momentum The measure of how hard it is to stop a moving object. Momentum = xP = xMomentum is a vector quantityUnits:Mass = Velocity = Momentum =MASSVELOCITYmvkgm/skg * m/s
12Momentum More = more More = more More = needed to an object. Which object would have more momentum?MASSMOMENTUMVELOCITYMOMENTUMMOMENTUMGREATER FORCESTOP
13Practice Problem 1What is the momentum of a bird with a mass of kg flying at 15 m/s?
14Practice Problem 2A golf ball travels at 16m/s, while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is kg. Which has greater momentum?
15Law of Conservation of Momentum The total momentum of a group of objects stays the same unless outside forces act on the objects.The cue ball has because it has mass and velocity.When it hits the other balls , the cue ballsdecrease.The other balls start moving (and have mass) which mean they now haveMOMENTUMVELOCITY& MOMENTUMMOMENTUM
16Stopping an object with momentum To stop an object we need to apply a force over a certain period of time. We call this impulseImpulse – the force acting on an object in a specific amount of time.IMPULSE = F x tForce (F) = NChange in time ( t) = sImpulse (I) = N s
17Using Newton’s 2nd Law to determine how hard it is to stop a moving object. Newton’s 2nd Law tells us that F=maWe could also look at this as:And if we were to manipulate this formula by multiplying both sides by time it would end up looking like:Impulse = change in momentum
18Why does an egg break or not break? An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why?FΔt= mΔvIn both cases, m and Δv are the same.If Δt goes up, what happens to the force?Right! Force goes down. When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops.
19Practice ProblemA net force of 100 N is applied to a 20kg cart that is already moving at 3m/s. The final speed of the cart was 8m/s. For how long was the force applied?Identify the variablesMass = 20kgΔvelocity = 8m/s-3m/s = 5m/sTime = ?Force = 100N
20Practice ProblemA .057 kg tennis ball falls on a tile floor. The ball changes velocity from -1.2 m/s to +1.2 m/s in 0.02 s. What is the average force on the ball?Identify the variables:Mass = kgΔvelocity = +1.2 – (-1.2) = 2.4 m/sTime = 0.02 susing FΔt= mΔvF x (0.02 s) = (0.057 kg)(2.4 m/s)F= 6.8 N
21Collisions Objects collide in two different ways: Elastic collision – Inelastic collision –The amount of momentum involved before the collision willWhen colliding objects bounce off each otherWhen objects collide and stick together.always be the same after the collision.
22Collisions with 2 moving objects When two objects are moving in the and a occurs, the momentum of the slower object and the momentum of the faster objectSAMEDIRECTIONCOLLISIONSPEEDS UPSLOWS DOWNBEFORE COLLISION4 m/s2 m/sAFTER COLLISION
23Collisions with 1 moving object When object is moving and into a nonmoving object, all the is to the object.ONECOLLIDESMOMENTUMTRANSFERREDNONMOVINGBEFORE COLLISION4 m/s0 m/sAFTER COLLISION
24Collisions with connected objects When one object is moving and , but to a nonmoving object, the gets evenly split between theCOLLIDESCONNECTSMOMENTUMTWO MOVING OBJECTSBEFORE COLLISION4 m/s0 m/sAFTER COLLISION