Forces and friction

Force
A push or pull on an object
Has both Size & Direction
Size:
Measured in SI units called
newtons (N) with a
spring scale

Direction:
Same direction: Add (+)
Diff. direction:Subtract (-)
Net force = combination of all
forces acting on object
Unbalanced Forces
Result in motion

Section 1: The Nature of Force
Balanced Forces
Net force = 0
No motion
Practice Calculating Net Force

Lab: The Nail Challenge!
Objective:
Balance nails
on single nail
head
1st & 2nd highest #
for each period
gets bonus
coupon


Section 1: Newton’s 1 Law
st

Inertia
Tendency of
object to
resist a
change in
it’s motion

st

Inertia
depends on
Mass
“Amount” of
inertia
depends on
objects’
mass

st

Newton’s 1st Law of Motion
Object at rest will remain at
rest
Object in motion will remain
in motion unless acted on by
an unbalanced
force.

Practice Problem 1
Imagine a place in the cosmos
far from all gravitational and
frictional influences. Suppose
an astronaut in that place
throws a rock. The rock will:
a) gradually stop.
b) continue in motion in the
same direction at constant
speed.

Practice Problem 2
An 2-kg object is moving
horizontally with a speed of 4
m/s. How much net force is
required to keep the object
moving with the same speed
and in the same direction?

 0 N (no force)

Practice Problem 3
Ben Tooclose is being chased through
the woods by a bull moose which he
was attempting to photograph. The
enormous mass of the bull moose is
extremely intimidating. Yet, if Ben
makes a zigzag pattern through the
woods, he will be able to use the large
mass of the moose to his own
advantage. Explain this in terms of
inertia and Newton's first law of
motion.

2-1: Newton’s 1st Law Review
Unbalanced
force from
another car
changes your
CAR’s motion
You continue
as before until
your seatbelt
changes
YOUR motion

2-2: Force, Mass & Acceleration
 Newton’s 2 law of Motion
nd

Force, Mass & Acceleration
are related
Force = Mass X Acceleration

OR Acceleration = Force
Mass
FYI, 1 N = 1kg X 1 m/s2

A 52 kg water skier is being
pulled by a speedboat. The
force causes her to accelerate
@ 2 m/s2. Calculate the FORCE
that causes this acceleration.
F = 52 kg x 2 m/s2
= 104 kg x m/s2 = 104 kg*m/s2
= 104 N

What is the force on a 1000 kg
elevator accelerating at 2 m/s2?
1000 kg X 2 m/s2 = 2000 N

How much force is needed to
accelerate a 55 kg cart at 15
m/s2?
55 kg X 15m/s2 = 825 N

2-4: Newton’s 3 Law
rd

 Newton’s 3rd law of Motion
For every action, there is an
equal and opposite reaction
“Action” & “Reaction” are
names of
forces

2-4: Action & Reaction Forces
Forces ALWAYS occur in
pairs.
Single forces NEVER
happen
2 objects are involved in
every force
Action force: “A pushes B”
Reaction force: “B pushes
A”

2-4: “Equal” Forces
In Newton’s Third
Law, “equal” means:
Equal in size
Equal in time

“Opposite” Means:
Opposite in direction

2-4: Possible Problems
 Don’t Action & Reaction
forces cancel each other?
 Action & Reaction forces
act on DIFFERENT objects
 In “Net force” problems,
we are talking about
opposing forces acting on
the SAME object

2-4: Try These!!
Ifforces are equal and
opposite why don't they
cancel each other out?
They occur on two different
objects. Forces can only
cancel out when the forces are
acting on the same object.

If the forces are equal and
opposite how do two
different objects obtain
different accelerations in
the same interaction?
Different accelerations are
obtained when the objects have
different masses.

When a small bug is
splattered across a fast
moving windshield what
experiences more force-
the bug or the windshield ?
Some what of a tricky question,
they both experience the SAME
force.

Why does the force have a
greater effect on the bug?
Because the bug's mass is
much much smaller than the
car's, it will experience a much
greater change in acceleration
than the car. This change in
acceleration over a very small
fraction of time is why the bug
experiences a greater effect.

2-3: Friction
Friction
Resistance to motion
Opposite direction of travel
Caused when 2 surfaces rub
together
resistive force (slows down
objects)

2-3: Friction
Friction depends on…
Types of surfaces
How hard surfaces push
together

2-3: Types of Friction
Types of Friction
Sliding Friction:
solid surfaces slide
over each other
Rolling Friction:
object rolls over
surface

2-3: Types of Friction
Types of Friction
Fluid Friction:
object moves
through fluid (or
air)
Static Friction:
objects not
moving

2-3: Uses of Friction
Is Friction
harmful or
helpful?
Ways to reduce
friction
Ways to
increase
friction

2.3: Gravitational
Force
Gravitational Force
Force of attraction between 2
objects
Pulls things toward each
other
Depends on:
Mass

Mass vs. Weight
Mass
Amount of matter
Same no matter where you are
SI units = kilograms (kg)
1 kg = 1000 grams (g)

Weight
Force of gravity
SI units = newtons (N)
Depends on where you are

2.3: Gravity & Freefall
Acceleration
Free Fall
Only force acting on an
object is gravity
Objects in free fall accelerate
as they fall
All objects free fall at the
same rate (9.8 m/s2)

2.3: Free Fall
Acceleration
For every
second an
object falls
its
downward
velocity
increases
by 9.8 m/s

2.3: Air Resistance
Air resistance
Type of fluid friction
Opposes motion of objects
through air
Depends on:
Size, Shape, Speed

2.3: Terminal Velocity
Terminal Velocity
As an object falls it picks up
speed
Increased speed 
increased air resistance
Eventually force of air
resistance = force of gravity
 TERMINAL VELOCITY
Object stops accelerating!

Forces and friction

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