Newtons Third Law of Motion Action and Reaction

1
Chapter 4

• Newtons Third Law of Motion Action and Reaction

2
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

3
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?

4
Newtons 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 statedto every action there is an
  equal and opposite reaction

5
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?

6
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

7
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)

8
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
  todays hunting rifles

9
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

10
Rockets

• A space shuttle or rock pushes the Earth on
  ignition
• The Earth pushes back
• The rockets mass shows a greater impact and
  moves up
• The rocket doesnt push down on the atm to go
  higher
• It climbs as a result of the first push

• At one time, travel is space was thought to be
  impossible because the rockets couldnt 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

11
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?

12
Adding It Up

• We usually dont 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

13
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?

14
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 shoeswho
  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?

15
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
16
So theres 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

17
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 forceWhy?