Chapter 4

Newton’s Third Law of Motion Action and Reaction Presented by April Senger

Forces

     A speeding baseball does not have a force until it collides with something and interacts Every interaction requires a minimum of 2 forces Nonliving and living objects can both produce forces Two objects that collide exchange the same force and the same time in opposite directions You can not possess of force but can possess momentum and kinetic energy Presented by April Senger

Concept Check

 A car is accelerating along a horizontal road. What is pushing the car along?

 What direction is air drag acting?

 What direction is road friction acting?

Presented by April Senger

Newton’s Third Law

 Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first  Thus there is an action and reaction force  More simply stated…to every action there is an equal and opposite reaction Presented by April Senger

Concept Check

 Which exerts more force, the Earth pulling on the moon, or the moon pulling on the Earth  When a heavy football player and a light one run into each other, does the light player really exert as much force on the heavy player as the heavy one exerts on the light one?

 Is the damage to the heavy player the same as the damage to the light one?

Presented by April Senger

Action and Reaction Forces

 Stating action and reaction forces is simple  If I push on the wall I am the action force  The wall pushing back is the reaction force  To show this interaction, pictures are commonly drawn with two arrow indicating the direction of the action and reaction forces Presented by April Senger

Concept Check

 Can you identify the action and reaction forces in the case of an object that is falling in a vacuum? (A vacuum is a region of space that is completely empty with no air) Presented by April Senger

Mass and Actions

   If you feel a recoil or kick after firing a gun, you are feeling the push back on the weapon in reaction to the action of the weapon pushing forward The mass of the weapon and bullet are very different so you see different results depending on their mass A musket would have a different purpose than today’s hunting rifles Presented by April Senger

Example

 a = F / m of bullet is a large acceleratcion  Any time a large number is divided by a small number you get a big number  Ex: 10 / .1 is 100  a = F / M of weapon is smaller acceleration  Any time a large number is divided by a larger number you get a small number  Ex: 10 / 1 is 10 Presented by April Senger

     A space shuttle or rock pushes the Earth on ignition The Earth pushes back The rocket’s mass shows a greater impact and moves up The rocket doesn’t push down on the atm to go higher It climbs as a result of the first push

Rockets

   At one time, travel is space was thought to be impossible because the rockets couldn’t push against empty space It is actually easier for rocket travel in space because there is not air drag to create friction The rocket will continue to move forward unless acted on by an outside force Presented by April Senger

Concept Check

 A high-speed bus and an innocent bug have a head on collision. The force of the bus on the bug splatters the poor bug all over the windshield. Is the corresponding force of the bug on the bus greater, less, or the same?

 Is the resulting deceleration of the bus greater than, less than or the same as that of the bug?

Presented by April Senger

Adding It Up

    We usually don’t notice action reaction forces that are equal, such as a bike leaning against a tree, because they are not moving If you kick a football, your foot does slow due to the interaction but the football accelerates in the opposite direction The only way to have motion result in zero force is to have them be equal, in the opposite direction and at the same time Ex: Two people kick a football at the same time and force from opposite sides Presented by April Senger

Concept Check

 Why does a flower pot sitting on a shelf never accelerate “spontaneously” in response to the trillions of inter-atomic forces acting within it?

Presented by April Senger

Tug-A-War

 If we had a tug-a-war between boys and girls on a slippery polished floor with the boys wearing socks and the girls in rubber soled shoes…who would win  The winning force has more to do with the greatest net force with the ground than force pulling the rope  Why can a horse pull a heavy cart?

Presented by April Senger

Concept Check

 We said that a car accelerates along a road because the road pushes it. Can we say that a team wins in a tug-a-war when the ground pushes harder on them than the other team?

 Does the scale read 100 N, 200 N, or zero?

Scale 100 100 Presented by April Senger

So there’s a bird, plane and helicopter…

     What do all of these objects have in common?

They are all specially shaped so that they force air molecules down (action) and the reaction is air pushing up (reaction) When the lift up is greater than the lift down, the object takes flight Why do birds and sometimes planes fly in a V pattern?

They have an updraft that a second bird/plane can utilize for lift in flight Presented by April Senger

Follow Through

 If a person is boxing and hits a punching bag it is easy to exert a force of 20 N (about 4.5 lb) on the bag  It is impossible for a boxer to hit a piece of paper hanging in the air with the same punch and exert 20 N of force…Why?

Presented by April Senger