Section 3
Motion and Forces
Bellringer/10.3 Notes
The concept of force explains many occurrences in our everyday lives.
From your own experience, state what will happen in the following
situations:
1. A marble is placed at the top of a smooth ramp. What happens to
the marble? What force causes this?
2. A marble is rolling around in the back of a small toy wagon as the
wagon is pulled along the sidewalk. When the wagon is stopped
suddenly by a rock under one of the wheels, the marble rolls
towards the front of the wagon. Why does the marble keep going
when the wagon stops? (Hint: Consider what it takes to change the
velocity of the wagon and the marble.)
3. If you dropped a flat uncrumpled sheet of notebook paper and a
similar piece of notebook paper that was crushed into a ball from
the same height, which would reach the floor first? Why are the
forces on these two pieces of paper different?
What is FORCE?
Are these
people exerting
FORCE?
Which people are exerting a
balanced force?
WHY?
BALANCED
UNBALANCED
I. Unbalanced Forces
A. Net Force – Combination of ALL forces
acting on an object.
1. Net force greater than zero equals
acceleration of that object.
Why is there a net
force?
Are there unbalanced forces in the picture
below?
Unbalanced forces
are 2 & 4
Will he be able to
move the box?
II. Balanced Forces
The men are the same size and
have the same strength.
What will happen?
A. Balanced forces DO NOT change
motion
Man’s mass
pushing down
Force (Earth) pushing
up on the man
Are there balanced forces in the picture
below?
Balanced forces are
1&3
III. Friction
A. Friction is an unbalanced force
1. Friction is caused by tiny bumps
and ridges on a surface bumping
into the tiny bumps and ridges on
another surface.
What FORCE helps these cars going around
the corner?
The force of friction is greater than the
force of the car’s motion.
What FORCE prevents this car from rolling
down the hill?
Friction opposes the force of gravity
Why do they “sweep” the ice in front of
the stone?
IV. Static & Kinetic Friction
A. Static Friction – two objects not
moving.
1. Takes a greater force to oppose
static friction.
B. Kinetic Friction – friction between
moving surfaces.
1. Takes less force than static
friction.
2. Two kinds of kinetic friction:
a. Sliding friction
b. Rolling friction
3. Air resistance – friction between an
object and the air passing over it.
When the force of
gravity = the force of air,
no acceleration takes
place.
4. Air resistance increases as speed
increases.
a. More air per second must pass
by due to the increase of speed.
b. Size & shape also affects air
resistance.