A little bit of 32 and 33: Newtons Third Law of Motion

1
A little bit of 3-2 and 3-3 Newtons Third Law
of Motion

• 12 opportunities minimum

2
3-3 Rough Outline

• Newtons Third Law?
• Action/Reaction
• Action/Reaction forces dont cancel
• Rocket Propulsion
• VII. Momentum
• Formula P mv
• Force and changing momentum F (mvf-mvi)/t
• Law of conservation of momentum
• When objects collide

3
Centripetal acceleration

• Objects moving in a circular path tend to
  accelerate towards the center of the circle.
  This is called
• Centripetal acceleration
• Centripetal acceleration is very evident in
  motorcycles. When you turn, you put your brakes
  on going into the turn and then give the bike
  more gas to exit. The bikes forward
  acceleration balances the centripetal
  acceleration moving it to the center of its
  circular path.

4
Centripetal Acceleration, cont.

• Gravity also exerts a centripetal force on
  objects that orbit the Earth, for example. If
  gravity DIDNT pull on an object orbiting, what
  would the object do?
• Leave orbit in a horizontal direction
• If the object lost all horizontal velocity, what
  would it do?
• Centripetal acceleration would pull it to the
  center of the its circle (crash).

5
Newtons Third law

• What is a two-word way of remembering Newtons
  Third Law?
• Action-Reaction
• This basically means that for every force, there
  is an equal and opposite force that balances it
  out.
• In a rocket, the rocket engine pushes against the
  gases as they explode. They escape out the
  rocket nozzle. What is the reaction?
• The gas exerts a force on the rocket, and it
  shoots forward.

6
Momentum

• What is the formula for momentum?
• P mv
• What do the variables stand for?
• P momentum
• M mass
• V velocity
• There are no official units for momentum.
  However, what units would be left after you do
  the math?
• Kilogram meters/second.

7
Force and changing momentum

• Force is also related to changing momenta. Watch
  on the board as I show you how to relate force,
  acceleration, and momentum.

8
Law of Conservation of Momentum

• Momentum can be transferred from one object to
  another. How did we show this in last classs
  lab?
• When the rolling ball hit the stationary ball, it
  caused the stationary ball to start moving.
  Thus, the momentum of the first was transferred
  to the momentum of the second.\
• The Law of Conservation of Momentum says that the
  momentum going in will be EXACTLY equal to the
  momentum going out, unless there are. . .
• Other forces acting on the system
• Our stationary ball didnt keep going after it
  picked up the momentum from the rolling ball.
  So, there must have been other forces in the
  system that we created. Can you think of some?
• Rolling friction
• Air resistance
• Deformation of the balls and loss of energy as
  heat