Newton’s Third Law
Week of 2/17/15
Integrated Chemistry and Physics
Bell Work
1) SEATING CHART!!! Get in your seat.
2) GRAB A WORKSHEET AT THE FRONT OF THE ROOM. Start working on it.
A force is part of an Interaction
Force is part of a mutual action
applies to both parties involved
When a truck crashes into a car
Part of the interaction is the truck exerting a force on the car
The other part is the car exerting a force on the truck
Forces always occur in pairs
This is called a force pair.
Newton’s Third Law
The every action there is an equal and opposite reaction
OR
Whenever one object exerts a force on a second object, the second object exerts an
equal and opposite force on the first.
SAME THING, REALLY.
Which one is action or reaction?
It doesn’t matter. The important thing is that they are co-parts of a single
interaction and that neither force exists without the other
Newton’s Third Law Checkpoint
Which exerts more force: Earth pulling on the Moon, or the Moon pulling on 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 player exerts on the light
one?
Would the damage to the heavy player be the same as the damage to the light one?
A Simple Rule
Action: Object A exerts a force on
Object B.
Reaction: Object B exerts a force on
Object A.
Action and Reaction for Different
Masses
What is Newton’s formula for acceleration?
a=F/m
A given force everted on a small mass produces a large acceleration, while the same
force exerted on a large mass produces a small acceleration
Checkpoint
What is the acceleration of a 10 kg box if a 150 kg human pushes against it with a
force of 12 N? What is the acceleration of the human?
A bus and a bug have a head-on collision. The force of the bus on the bug splatters
the bug all over the windshield (RIP). 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 that, or the same as that of the bug?
Systems
System: any object or collection of
objects that you are studying
We always need a force OUTSIDE
of the system for movement
The Horse-Cart Problem
How does a horse pull the cart?
If it pulls on the cart, the cart pulls back, right?
Which one weighs more?
At the same time the horse pushes backward against the ground, the ground pushes
forward on the horse.
This outside force—the ground—is what moves the horse-cart
system
Action Equals Reaction
How does tug-of-war work?
It isn’t which team provides the
greatest force on the rope, but
which team exerts the greatest
force on the ground!
Animal Locomotion
How animals move
How does a fish swim?
How do giraffes run?
How do birds fly?
Bell Work
1. Please get out you Newton’s Second Law Practice Problems and your Newton’s
Second Law Graphing Worksheet and prepare to turn them in.
2. On a sheet of paper, answer the following questions:
a) What does Newton’s Third Law state?
b) If an 2000 kg elephant pushes on a 0.1 kg mouse with 50 N of pressure, how
much pressure would the mouse exert on the elephant (assuming the
mouse survives)?
c) Also assuming the mouse survives, how much would he accelerate?
d) How much would the elephant accelerate?