2 Introduction If you lean over – you fall If you lean over with your hands against the wall – you don’t fallWhy?Because the wall is pushing on you and holds you in place
3 Forces and Interactions Force – a push or pullNewton saw an interactionMutual action between forcesHammer and nailHammer moves nailNail halts the hammer
4 Newton’s Third LawWhenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object
5 Third Law Forces Action force – Reaction force – It doesn’t matter which is whichPartners in the interactionEqual and oppositeNeither can exist without the other
6 Examples You push on floor, the floor pushes on you Car pushes on road, the road pushes on the carSwimming – you push water backward, the water pushes you forwardsMore friction – more force
7 Questions Does a stick of dynamite contain force? No, force is an interaction between two objects. An object may be capable of creating a force, but it can’t possess it. Dynamite possesses ENERGY
8 QuestionsA car accelerates along the road. Strictly speaking, what is the force that moves the car?The road pushes the car. It provides a horizontal force.
9 Identifying Action and Reaction Sometimes it is hard to find the action/reaction pairsWhat are the forces on a falling boulder?Gravity?Weight?Ground?No – none of these
10 Recipe for finding forces Identify the interactionAction: Object A exerts force on object BReaction: Object B exerts force on object AFalling boulder – Earth exerts force on boulder, Boulder exerts force on Earth
11 Action and Reaction on Different Masses Boulder-Earth interaction – the forces are EQUALDoes the earth fall into the boulder?Yes, but not as farThe reactions are equal and oppositeBecause Earth is so large, we can’t sense its very small acceleration
12 QuestionWe know the Earth pulls on the moon. Does the moon also pull on the earth? If so, which pull is stronger?Yes, the Earth and moon both pull on each other. Both pulls are equal and opposite.
13 Cannon/Cannon ballWhen fired, the cannon ball exerts a force on the cannonThe cannon recoils or “kicks”Why doesn’t the cannon move as fast as the cannon ball?A given force exerted on a small mass will result in a greater acceleration than on a larger mass
14 Rocket propulsion Air escaping from a balloon The balloon accelerates the opposite direction as the escaping air.Rocket – each molecule of escaping gas acts like the cannon ball
15 Rocket in air and spaceOnce though the rocket needed air to “push” against.Actually, rockets work better without air drag (in space)
16 Momentum How can a karate expert break a stack of bricks? Why does it hurt more to fall on concrete than a wooden floor?Why do you follow through in golf, baseball or bowling?
17 Momentum Momentum = mass x velocity p = m x v How much momentum does a 400 kg bike moving 40 m/s have?p= mv = (400 kg) ( 40 m/s) = 16,000 kgm/s
18 Helicopters Helicopters – blades push air down. Air pushes chopper up. Lift – upward reaction forceWhen lift equals weight of chopper, it can hoverWhen lift is greater, the chopper rises
19 Birds and Airplanes Fly due to action/reaction forces Wings of bird reflect air downward. The air then pushes upwardAirplanes must have a continuous flow of air in order to have lift.The engines push air back. The air pushes the jet forward
21 Do Action and Reaction Forces Cancel ? Why do the equal forces not cancel to zeroHow can there be acceleration?There are MANY equal and opposite forces in a system.If two people kick the ball at the same time – together they cancel out
22 QuestionSuppose a friend who hears about Newton’s law say that you can’t move a football by kicking it because the reaction force by the kicked ball would be equal and opposite to your kicking force. The net force would be zero, so no matter how hard you kick, the ball won’t move! What do you say to your friend?
23 AnswerObviously kicking the ball will accelerate it. Your kick acts on the ball. It accelerates the ball. The reaction force acts on your foot – it decelerates your foot.
24 The horse-cart problem If a horse pulls on a cart, doesn’t the cart pull back equally?Three viewsFarmer – get the cart to marketHorseHorse-cart system
25 Farmer Only concerned about force exerted on the cart Force on cart/mass of cart = accelerationDoesn’t care about the reaction on the horse
26 Horse system Opposite reaction force by the cart restrains the horse So how can the horse move forward?The horse must push on the GROUND – which pushes back.
27 Horse-Cart systemThe pull of the horse on the cart and the reaction of the cart on the horse are internal forces.They contribute nothing to the accelerationThe acceleration comes from the horse-cart system and the ground
28 Stalled carYou can’t move the car by sitting in your seat and pushing against the dashboardYou must get outside and push against the ground.Don’t worry if you get confused, Newton struggled with his law too.
29 Action Equal Reaction What if you hit a wall? What if you hit paper? It hits backYou can’t hit it harder than it can hit you backWhat if you hit paper?It can not hit back as hard
30 Conclusion Push the world hard – it pushes back hard Push the world gentle – it pushes back gentleYou can not touch without being touched!!!