Transcript Forces

Section 1: The Nature of Force
What is a force?
 Force
A push or pull on an object
Has both Size & Direction
 Size:
Measured in SI units called
newtons (N)
Spring Scale
How do you combine forces?
 Direction:
Same
direction: Add (+)
Diff. direction: Subtract (-)
Net force = combination of all
forces acting on object
Unbalanced Forces
Result in motion
What does balanced forces mean?
Balanced
Forces
Net force = 0
No motion
Lab: The Nail Challenge!
 Objective:
Balance
nails on
single nail head
Work in pairs
Section 2: Friction and Gravity
What is friction?
 Friction
Resistance
to motion
Opposite direction of travel
Caused when 2 surfaces rub
together
resistive force (slows down
objects)
What does Friction depend on?
 Friction
depends on…
Types of surfaces
How hard surfaces push
together
What are sliding and rolling friction?
 Types
of Friction
Sliding Friction: solid
surfaces slide over
each other
Rolling Friction:
object rolls over
surface
What are fluid and static friction?
 Types
of Friction
Fluid Friction:
object moves
through fluid (or
air)
 Static Friction:
objects not
moving
What are some uses for friction?
 Is
Friction
harmful or
helpful?
 Ways to reduce
friction
 Ways to
increase
friction
What is a gravitational
force?
 Gravitational
Force
Force of attraction between 2
objects
Pulls things toward each other
Depends on:
Mass
Distance
What is the difference between
mass and 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
What is Free fall?
 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)
Gravity & Freefall
What is Air resistance?
 Air
resistance
Type of fluid friction
Opposes motion of objects
through air
Depends on:
Size, Shape, Speed
What is 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!
Terminal Velocity
Section 3: Newton’s First and Second Laws
What is Inertia?
 Inertia
Tendency
of
object to
resist a
change in
it’s motion
What does inertia depend on?
 Inertia
depends on
Mass
“Amount” of
inertia
depends on
objects’
mass
st
1
What is Newton’s
Law?
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.
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
What is Newton’s 2nd law of motion?

Newton’s 2nd law of Motion
Force, Mass & Acceleration are
related
= Mass X Acceleration OR
Acceleration = Force ÷Mass
Force
FYI,
1 N = 1kg X 1 m/s2
Force, Mass & Acceleration
Force, Mass & Acceleration
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
2
2
= 104 kg x m/s = 104 kg*m/s
= 104 N
Force, Mass & Acceleration
 What is the force on a 1000 kg
2
elevator accelerating at 2 m/s ?
1000
kg X 2 m/s2 = 2000 N
 How
much force is needed to
accelerate a 55 kg cart at 15
2
m/s ?
55
kg X 15m/s2 = 825 N
Section 4: Newton’s Third Law
What is Newton’s 3rd Law?
rd
 Newton’s 3 law of Motion
For
every action, there is an
equal and opposite reaction
 “Action” & “Reaction” are
names of
forces
How do forces always occur?
 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”
What do equal and opposite mean?
 In
Newton’s Third Law,
“equal” means:
Equal in size
Equal in time
 “Opposite”
Means:
Opposite in direction
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
Try These!!
 If
forces 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? (Remember F=ma)
 Different accelerations are
obtained when the objects have
different masses.