1Review: Newton’s 1st& 2ndLaws•1stlaw (Galileo’s principle of inertia)- no force is needed to keep an object moving with constant velocity•2ndlaw (law of dynamics) – a net force must be applied to change the velocity of an object.• A forceF (N) = m (kg)  a (m/s2) must be applied to produce an acceleration a for an object of mass m1L-7 Newton’s third law andconservation of momentum For every action there is anequal and opposite reaction.2For Newton: Action = ForceForces: pushes or pulls• If I push against a person, that person pushes back on me• If I push on the wall, the wall pushes back on me, otherwiseI fall over!• You can’t push on something unless it pushes back on you• Forces never occur alone, they occur in pairs• Every force is part of an interaction3A boxer’s punch• The boxer’s fist hits the bag and the bag hits back on the fist  a pair of BIG forces are involved in hitting the bag• When the boxer hits a piece of tissue paper, the boxer can only exert as much force on the tissue paper as the tissue paper can exert on his fist4Newton’s 3rdLawWhenever one object (A) exerts a force on a second object (B), the second object exerts an equal and opposite force on the first.AB5Force that A exerts on BForce that B exerts on AExample: static equilibrium• What keeps the box on the table?• The box exerts a force on the table due to its weight, and as result of the 3rdlaw the table exerts an equal and opposite (upward) force on the box.• If the table was not strong enough to support the weight of the box, the box would crash through it.62Example: The bouncing ball• Why does the ballbounce?• When the ball hits theground it exerts adownward force on it• By the 3rdlaw, theground must exert anequal and upward forceon the ball• The ball bounces because of the upward force exerted on it by the ground.7You can move the earth!• Since the earth exerts a downward forceon you, the 3rdlaw says that you exertan equal upward force on the earth. • The magnitude of the forces areequal:  FEarth= Fyou, but are theaccelerations equal?• NO, because the earth’s massand your mass are not the same!• Accelerations are the result of the 2ndlaw:FEarth=MEaEand Fyou=myouayou MEaE= myouayou• Therefore: aE= (myou/ ME) ayou the Earth’s acceleration is much less than yours, since(myou/ ME) is a very small number.8• Newton’s 3rdLaw plays an important role in everyday life, whether we realize it or not.• We will demonstrate this in the next few examples.9The donkey and 3rdlaw paradox• A man tries to make adonkey pull a cart butthe donkey argues:• Why should I even try?• No matter how hard I pull on the cart, the cart pulls back on me with an equal force, so I can never move it. What is the fallacy in the donkey’s argument?• The donkey forgot that action/reaction forces always act on different objects. As far as the cart is concerned, if the force the donkey exerts on it is large enough, it will move. The reaction force on him is irrelevant.10Friction is essential to movement• The tires push back on the road and the road pushes the tires forward.• If the road is slippery, the friction force between the tires and road is reduced, and the car does not move.. 11We could not walk without friction• When we walk, we push back on the ground• By the 3rdlaw the ground then pushes us forward.• If the ground is slippery, we cannot push back on it, so it cannot push forward on us  we go nowhere!123Ballthat bounces Non-bouncing ball13Force onThe groundForce onThe groundDo 2 balls released from the same height exert thesame force on the ground? It depends . . . The ball that bounces exerts alarger force on the ground.The two balls havethe same massBouncing and Non-bouncing balls• The ball that bounced exerted the larger force on the ground.• The force that the ball exerts on the ground is equal to and in the opposite direction as the force of the ground on the ball.• The ground must exert a force to bring the non-bouncy ball to rest• The ground must not only stop the bouncy ball, but must then project it back up  this requires more force!• Since the bouncy ball experiences a larger force from the ground, it must therefore by the 3rdLaw also exert a larger force ON the ground. • The next demo should convince you!14Knock the block over15The bouncy side knocks the block over but thenon-bouncy side doesn’t.The bouncy side exerts a LARGER force!We construct a ball with one side (red) bouncy, and the other side (black) non-bouncy. How do stunt actors survive falls?• Instead of actors we will use glass beakers• The beakers are dropped from same height so then have the same velocity when they reach the bottom.• One falls on a hard surface –a brick• The other falls on a soft cushion softhard16But why does the beaker break?• An object will break if a large enough force is exerted on it. Obviously, the beaker that hits the brick experiences the larger force, but can we explain this using Newton’s Laws?• Notice that in both cases, the beakers have the same velocity just before hitting the cushion or the brick. Also both beakers come to rest (one gently, the other violently) so their final velocities are both zero.• Both beakers therefore experience the same change in velocity = v (delta means change), so that the change in vv= vf–vi= 0 – vi= – vi• What about their acceleration?•a = v/ t, where t is the time interval over which the velocity changes (the time to stop)17Continued from previous slide• The stopping time t is the important parameter here because it is not the same in both cases.• The beaker falling on the cushion takes longer to stop than the one falling on the brick. The cushion allows the beaker to stop gently, while the brick stops it abruptly• Both beakers have the same velocity just before hitting the bottom, and both come to rest, so the change in velocity is the same in both cases.• However, the beaker that hits the brick, is stopped suddenly and thus experiences a greater acceleration and a greater force which cause it to shatter.184Stunt actors and air bags• The same reasoning applies to stunt actors and air bags in an automobile. Video• Air bags deploy very quickly, triggered when an unusually large acceleration is detected• They provide protection by allowing you to stop more slowly, as compared to the case where you