Forces and Motion
Scientists construct explanation of
phenomena (something that happens)
by analyzing a situation, applying ideas
and evaluating an explanation.
The analysis is good when the interaction is
correctly described and the energy diagram (or
force diagram) is correct.
Source
Water
Decrease in
Motion
Energy
Receiver
Mechanical
Energy
Water and String
Increase in
Thermal
Energy
What we know
An
______
applied
mechanical interaction
occurs when two non-elastic (rigid or
stiff) objects push or pull on each
other.
What we know
A
______
friction
mechanical interaction
occurs when two surfaces rub
against each other.
What we know
A
______
drag
mechanical interaction
occurs when an object moves
through fluid like a gas or a liquid,
and the fluid resists the object’s
motion.
What we know
An
_______
elastic
mechanical interaction
occurs when two objects push or pull
on each other and at least one of
them is stretchy.
What we know
During a mechanical interaction,
mechanical energy is transferred from
the
source
_____
to the
receiver.
______.
What we know
1. Interactions Ideas
2. Energy Ideas
3. Now we will layer
FORCE ideas to
explain those same
changes in motion
during an interaction .
What we want to know
 How
do forces affect motion?
 How does a constant forward force
affect motion?
 How can an arrow be used to
represent a force?
Background Information
Force: A force is a push or pull and
has both strength and direction.
This idea is represented by a force
arrow.
What Do You Think?
An air puck glides on a cushion of air. The
base of the puck has a stem with a small
hole drilled down the center. An air-filled
balloon fits over the stem. The air from the
balloon makes an air cushion for the puck to
float on. The air does not push the puck –
it just creates a frictionless surface for the
puck to glide on.
Think about it
What does the teacher do?
 What does the air puck do?

Write about it

While gliding on a cushion of air,
does the puck
– speed up
– slow down
– constant motion?
Read about it
Read the following thinking bubbles
among four students who are trying to
account for the puck’s motion as it
glides on the cushion of air.
The puck looks
like it moves with
constant speed,
but is really
slowing down
because the
hand is no longer
pushing it. To
keep it moving,
you would have
to keep pushing
it.
Write, in
Your own
Words,
Why you
Think the
Puck had
The motion
That you
Observed.
The puck looks
like it moves with
constant speed,
but is really
slowing down a
bit. Even if the air
in the balloon
never ran out, the
puck would
eventually stop
because
everything
eventually slows
down and stops.
The puck keeps
moving because
the quick shove
the teacher gave
it stays with it
even after the
puck is no
longer touching
her hand.
The puck
keeps moving
because
nothing is
stopping it.
1
2
3
4
What Do You Think?
If you used a hair dryer to exert a
constant force on a small ball, what kind
of motion would the ball have?
 Will it speed up?
 Or move at a constant speed?
 Why?

Predict and please answer these in
your notebook
For You To Do – Part A
Ball and Hair Dryer: (teacher demo)
1. What kind of motion did the ball
have? Did it speed up? Or move at
a constant speed?
2. Is this what you predicted in the
WDYT section?
3. Please answer in your notebook
What Do You Think?
Suppose you have a fan connected to a
car. (When the fan blades whirl around,
they push the air backwards. In return,
the air pushes steadily forward on the fan
unit and the car.) When the fan is on, the
air provides a constant forward force on
the car and fan unit. Would the car speed
up or have a constant speed? Why?
 Predict and please answer in your
notebook

For You To Do – Part A
Fan Car (teacher demo).
3. What kind of motion did the car
have? Did it speed up? Or move at
a constant speed?
4. Is this what you predicted in the
WDYT section?
Please write this in your notebook
Low Friction Car (YOU do it)
Now you come up with one or two
different ways to exert a constant forward
force on the car. This means that any
pushing or pulling on the car needs to
remain at constant strength. You wll use a
low-friction car that has very little friction
Between its moving parts.
Think about it
Describe how you will exert a
constant forward force on the low-friction
car.
Exert a constant forward force on the
Car and observe its motion.
6.
What kind of motion did the car
have? Did it speed up? Or move at
a constant speed?
5.