Download Forces - Physics

Forces
What is a Force?
Force = a push or pull one body exerts on
another
Can cause acceleration or deceleration
Friction & Gravity are 2 main types of
force
Balanced Forces = opposing forces
balance out
Unbalanced Forces = one force is greater
than an opposing force and will cause
movement
Static Friction
Static friction = force that
acts on objects that are not
moving
Ex: When you step
forward, your shoe doesn’t
slide backwards b/c of
static friction
Sliding Friction
Sliding = force that
opposes an object’s
motion as it slides
across a surface
Ex: Pushing a box
across the floor/ground
Rolling Friction
Rolling = force that acts against rolling objects
Ex: easier to move heavy
items w/ wheels or
a hand truck
Fluid Friction
Fluid = force that
opposes motion
of an object
through a fluid
Ex: stirring brownie
mix or
swinging a bat
through the air
https://www.youtube.com/watch?v=U6K-F9Bjl8s Bolt
Gravity
Force exerted by
every object in the
universe on every
other object
Gravity pulls down on
the object (toward the
center of the Earth)
and adds more
resistance.
https://www.youtube.com/watch?v=w6aaJ_gQuWY Zero G plane
https://www.youtube.com/watch?v=IMIPTtvAuUU Ping Pong Teachers
Newton
F = ma
Force = mass x
acceleration
SI unit for Force = N
(Newton)
1 N = amt of force
needed to accelerate a
1 kg object at a rate of
1 meter/second2
N = kg • m/s2
According to Newton's first law...
An object at rest will stay at rest unless
acted on by an unbalanced force.
An object in motion will stay in motion
(with the same speed and in the same direction)
unless acted upon by an unbalanced
force.
This law is often called
"the law of inertia".
Eventually Friction & Gravity will act on an object & caused them to stop moving (except in
space – no gravity, no friction)
What does this mean?
This means that there is a natural
tendency of objects to keep on doing what
they're doing. All objects resist changes in
their state of motion. In the absence of an
unbalanced force, an object in motion will
maintain this state of motion.
Let's study the "skater" to
understand this a little better.
What is the motion in this picture?
What is the unbalanced force in this picture?
What happened to the skater in this picture?
This is the reason you should
always wear your seatbelt.
According to Newton's second law...
Acceleration is produced when a force
acts on a mass. The greater the mass
(of the object being accelerated) the greater the
amount of force needed (to accelerate the
object).
What does this mean?
Everyone unconsciously knows the
Second Law.
Everyone knows that heavier objects
require more force to move the same
distance as lighter objects.
The Second Law gives us an exact
relationship between force, mass, and
acceleration.
FORCE = MASS times ACCELERATION
An example of how Newton's Second Law works:
Mike's car, which weighs 1,000 kg, is out of gas.
Mike is trying to push the car to a gas station, and he
accelerates the car at 0.05 m/s/s. Using Newton's
Second Law, you can compute how much force Mike
is applying to the car.
F=ma F=1000kg • .05 m/s/s = 50 kg•m/s/s
1 kg•m/s/s = 1 Newton
50 kg•m/s/s = 50 Newtons
Momentum
Momentum = mass • velocity
p = mv
Ex. - Newton’s Cradle
Determine the momentum of a ...
a. 60-kg halfback moving eastward at 9 m/s.
p = m • v = 60 kg • 9 m/s
p = 540 kg•m/s, east
b. 1000-kg car moving northward at 20 m/s.
p = m • v = 1000 kg • 20 m/s
p = 20 000 kg•m/s, north
c. 40-kg freshman moving southward at 2 m/s.
p = m • v = 40 kg • 2 m/s
p = 80 kg•m/s, south
According to Newton's third law...
For every action there is an equal and
opposite re-action.
Ex. = Your desk & You
What does this mean?
This means that for every force there is
an equal and opposite reaction.
When an object pushes another object it
gets pushed back in the opposite direction
equally hard.
How a rocket works
The rocket's action is to
push down on the ground
with the force of its powerful
engines, and the reaction is
that the ground pushes the
rocket upwards with an
equal force.
UP,
UP,
and
AWAY!
Quiz Time
1. Who was the scientist who gave us the Laws of Motion?
Answer: Sir Isaac Newton
2. How many Laws of Motion are there?
Answer: three
3. What is another name for the first law of motion?
Answer: Law of Inertia
4. Which law explains why we need to wear seat belts?
Answer: First Law of Motion
5. Which law says that force is equal to mass times acceleration (F=MA)?
Answer: Second Law of Motion
6. Which law says that heavier objects require more force than lighter
objects to move or accelerate them?
Answer: Second Law of Motion
7. Which law explains how rockets are launched into space?
Answer: Third Law of Motion
8. Which law says that for every action there is an equal and opposite
reaction?
Answer: Third Law of Motion
Newton’s Cradle
Add to your notes
Scalar – anything with a value (a number)
i.e. – age, cost, speed, etc.
Distance can be scalar
Vector – anything with a value & direction
i.e. – velocity, acceleration, displacement
(should be)
Distance can be a vector (include direction)
Fg = mg
Force (with gravity applied) = mass • gravitational
acceleration
Fg = weight
http://www.iit.edu/~kallend/skydive
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