Transcript Do Now: - Baltimore Polytechnic Institute

Take out your lab paper from last week. You may
discuss with a partner but you must write down
your own responses
 How did you find the acceleration of the cart in this
lab?
 The acceleration of the cart was the same as what
other object in the lab?
 How did you calculate Fnet?
 Which mass did you use to find Fnet? Why?

Reminder: Newton’s Second Law Labs will be due
Tuesday October 26. Please come in for coach
class if you need help!!!
1. What
does the word
“force” make you think
of?
2. What are some ways you
have used the word
“force” in your life?
 An
object at rest will stay at
rest or in constant straight
line motion (a=0) UNLESS
acted on by a nonzero net
force
 An
object is accelerating
if it is EITHER changing
speed and/or changing
direction (ex. A car
making a right turn but
keeping the same speed)
 the
tendency of an object to
resist a change in motion.
This is quantified by its
mass.
Imagine
you are
standing in a stationary
bus. Suddenly the bus
starts moving. What
happens?
You fall backwards! This is
due to the inertia of your body.
When the bus was at rest, you were
also at rest. When the bus started
moving, your legs went with the
floor of the bus due to friction. But
the upper part of your body tends to
continue in its state of rest (inertia of
rest). As a result you fall backwards.


Braking in a car: Hopefully you'd be wearing
your seat belt because that would be the
outside force. The seat belt stops you from
flying through the windshield. If not, then the
windshield is the outside force slowing you
down.
Turning in a car: That is inertia because your
body wants to go straight but the outside force
(aka the car) is turning you.
 An
object which is
accelerating has a nonzero
net force acting on it equal
to the object’s mass times
its acceleration.
 i.e.
F = ma

The same force exerted on a larger mass produces
a smaller acceleration.
is measured in Newtons,
represented by a capital N
 Force
1
N = 1 kg x m/s2
 One Newton is approximately
7 Fig Newtons
 a=5
m/s2, m = 4 kg
F = ?
F = ma = (5 m/s2)(4 kg) = 20 N
A golfer applies a force of 6 N to a 0.5
kg golf ball. What is the ball’s
acceleration?
F = ma  a = F/m
a =F/m=(6 N)/(0.5 kg)=12 m/s2
Use
the rest of class to
work on the worksheet
“Newton’s Laws.” It will
be due on Monday
11/21/11


Read the article: “As Injuries Rise, Scant
Oversight of Helmet Safety” on
www.nytimes.com (you may have to search
for it)
Write as detailed a description of the article
as possible explaining how concepts we have
talked about in physics class appear in this
investigation. The longer and better the
description, the more points!!!
1.
2.
A baseball pitcher throws a ball
by applying a force of 25 N to it.
The ball accelerates at 50 m/s2 .
What is the ball’s mass?
You are standing on a moving bus
when it suddenly brakes. Describe
what happens and why.




A force is any influence that
causes an object to
accelerate (usually a push
or pull)
Forces are vector quantities
Represented by a force
vector
Every force vector should
have an arrowhead, a tail,
and a label


Symbol: Ff
Definition: The contact force that opposes
sliding motion between surfaces
Ff


Symbol: FN
Definition: the contact force exerted by a
surface on an object; acts perpendicular to the
surface
FN


Symbol: FT
Definition: the
pull exerted
by a string,
rope, or cable
when attached
to a body and
pulled taut
FT


Symbol: Fthrust
Definition: a general term for the forces that
move objects such as rockets, planes, cars
and people


Symbol: Fg
Definition: a long-range force due to gravitational
attraction between two objects, generally earth and
an object
Fg
 Forces
come from the
CENTER of the object!
 Work
with your groups to
complete STEP 1 on your
paper (add the correct
forces to your selected
diagram). You may draw
them right onto the paper.
 BALANCED
FORCES: If net
force is 0, there is no
acceleration (v is either 0 or
constant)
 UNBALANCED FORCES: If
net force is nonzero, there
is acceleration.
 Work
with your groups to
complete STEP 2 on your
paper (determine whether
the forces are balanced or
unbalanced). When you
finish, tape your paper to
the blackboard in the back.
 Use
the rest of class to
work on the paper
“Balanced and Unbalanced
Forces.” It will be due at the
end of class.


Use the rest of class to fill in your chart. You
may use classroom textbooks (p. 123) to help
you.
After you have finished your chart, you may
continue working on your homework
(Newton’s Laws Worksheet)

Fill in the force vectors in the following
diagrams. Be sure to include ALL forces!!!
1.
A car
traveling at
60 mph
2.
(a ball rolling down a
hill)

Weight is the force of gravity (represented by
Fg)
Fg=mg
where m=mass
and g=acceleration
due to gravity (on
2
earth: -9.8 m/s )
Step 1: Write down your weight in
pounds. If you don’t know your weight
or don’t want to write it, make
something up.
 Step 2: To convert your weight into
Newtons, multiply your weight in
pounds by 4.448221 or 4.45
 Step 3: Write that weight in the weight
column next to “Earth”
 Step 4: Follow the directions on the
rest of the worksheet. Happy Friday 
