Download Chapter 2 Forces in Motion

Wile E. Coyote
If Wile E. Coyote and a bolder fall off a cliff
at the same time which do you think will
hit the ground first?

Objects fall to the ground at same rate
› Acceleration due to gravity = same for all
objects

All objects accelerate toward earth at a
rate of 9.8 meters per second
› 9.8 m/s/s
Using the figure pg 37
Math Break pg 37

What is more affected by air resistance a
school bus or a race car?

Fluid friction opposes motion of objects
› Also known as air resistance

Amount of air resistance depends on:
› Size of object
› Shape of object
Self Check
Figure 3
Net force does not equal 0 = object
accelerates downward
 Air resistance increases as speed of
object increases
 Upward force of air resistance increases
until it exactly matches the downward
force of gravity
 This causes net force of 0= Terminal
Velocity (Constant Velocity)

Pg 38 Figure 4
Simulation:
galileo_pisa_finalv1.2

Free Fall= No air resistance

Free fall can only happen where there is
no air
› In a vacuum
› In space
Pg 39 Figure 5
What problem might you have if you
were to sky dive on the moon?

Orbiting objects are in free fall

An orbiting object is going forward and
falling- It is falling around the earth
Pg 40 Using the figure Figure 7
›
Projectile Motion
The curved path an object follows when
thrown
 Thrown objects and falling objects have
the same acceleration
 Orbiting objects are examples of
projectiles
 Projectile objects have two componentshorizontal and vertical
 Both components are independent of
each other

 “An
object at rest remains at
rest and an object in motion
remains in motion at constant
speed and in a straight line
unless acted on by an
unbalanced force”

Part 1: Objects at Rest
› Object will not move until a push or pull is
exerted on them
 Ex. Plane won’t fly unless pushed by exhaust
from engine

Part 2: Object in Motion
› Object will move forever at the same speed
and in same direction unless some
unbalanced force acts on it
 Ex. Bumper car stops but you continue to
move forward until your seat belt stops you
Pg 44 Apply

Friction: Makes it difficult to observance
of the first law on everyday objects
› Ex. Grass causes a rolling ball to stop

Inertia: Tendency of all objects to resist
any change in motion
› Ex. Slide toward side of car when driver
makes a sharp turn

Mass: Smaller mass has less inertia than
object with large mass
› Ex. Push car vs. bike
Pg 45 Self Check
“The
acceleration of an
object depends on the
mass of the object and the
amount of forced applied”

Part 1: Acceleration Depends on Mass
› Same force= objects acceleration
decreases as its mass increases and its
acceleration increases as its mass decreases
 Ex. Shopping Cart

Part 2: Accelerating Depends on Force
› An objects acceleration:
 Increases as force increases
 Decreases as force decreases



a= F/m
F= m X a
M= F/a

Math Break Pg 47
“
Whenever one object
exerts a force on a second
object, the second object
exerts an equal and
opposite force on the first.”

All forces act in pairs: Actions and
Reactions

Can occur when there is no motion
› Ex: Action Force= Body exert force on chair
Reaction force= force exerted by the
chair that pushed up on your body
› No movement

Force Pairs Do Not Act on the Same
Object
› Ex. Swimming
 Action force = exerted on the water by
swimmer’s hands and feet
 Reaction force = exerted on the swimmer’s
hands and feet by water

The Effect of a Reaction Can be Hard to
See
› The force of gravity between Earth and a
falling object is a force pair
› Pg 49 examples
› Pg 49 real world connections
Momentum= Property of a moving
object that depends on the object’s
mass and velocity
 P= m X v

› P= Momentum
› m=mass
› V=velocity

More momentum= harder to stop or
change direction

Law of Conservation of Momentum:
› Two or more objects interact, may exchange
momentum, but total amount stays the same
› Ex. Billiards or Bowling

Momentum and Newton’s 3rd Law
› Action force= billiard ball moves
› Reaction force= stops cue ball’s motion