Transcript Ex. 1 - Mr. Schroeder

1) Forces of 3.0 N east and 7.0 N north act on an object. What is the net force on the object?
7.6 N 67º N of E
2) Forces of 8.0 N 30º north of east and 3.5 N 15º west of north act on an object. What is the net
force on the object?
9.5 N 51º N of E
Remember to draw
vector diagrams with
arrows, eh.
“If I have seen
further it is by
standing on the
shoulders of
giants.”
The
“Giants” that Newton’s quote refers to
are Galileo and Copernicus.
Both contributed ideas that proved central to our
modern-day understanding of DYNAMICS.
Nicolaus Copernicus
(1473 - 1543)
Galileo Galilei
(1564 - 1642)
Dynamics is the study of forces, the cause of motion.
Remember that a force can be a
or a
KINEMATICS + DYNAMICS = MECHANICS
Greeks (like Aristotle) had noticed 2500 years ago that in order to
keep something moving you had to keep pushing on it.
If you stopped pushing, it stopped moving!
This seems to be true from our everyday experience.
Aristotle (384 -322 B.C.)
Aristotle believed that any object’s
“natural state” is to be AT REST.
Do you agree with his view?
Can you think of something that the Aristotle didn’t account for?
Galileo’s Experiments
Galileo came up with two “thought experiments” (they could
only be imagined, not done in real life).
Galileo Galilei
He imagined a world where there was no
friction.
No, he didn’t think that this was the way the world actually IS, but he believed friction
complicated his study of dynamics, and by ignoring it, he could simplify his theories
about the cause of motion.
Let’s take a look at what he was thinking . . . .
Experiment Number 1
Ignoring everything else, an object rolling down a slope will speed up.
The only reason its velocity will increase is because it is going down a
slope while gravity is pulling down on it.
Ignoring everything else, an object rolling up a
slope will slow down. The only reason its velocity will decrease is because
it is going up a slope while gravity is pulling down on it.
If the ball is moving on a level surface it has no reason to speed up or
slow down! It will move at a constant velocity forever.
Experiment Number 2
A ball rolls down a slope on one side, then rolls just as
high up a similar slope on the other side.
The ball now has to roll up a slope that is not
as steep, but rolls to the same height. Notice
the distance it must go along the slope is
greater.
Since it has no slope to go up, it should
keep moving forever along the level
surface at a constant velocity. Galileo said that this
is the “natural state” of the object.
In these two “experiments,” Galileo was saying that without friction, an
object moving at a constant velocity forever is just as natural as an object
staying at rest forever.
Here’s another way to look at Galileo’s ideas in a more modern way…
What do you have to do to keep the box moving at a constant velocity of 2.0 m/s
in each of these situations?
1)
3)
2)
4)
So in the real world, the constant pushing we do to keep something moving at a
constant speed is just to overcome friction!
In short their contributions were:
Nicolaus Copernicus
Our planetary system is heliocentric,
meaning that the planets, including
earth, rotate about the sun. The sun
does NOT rotate around earth.
It takes a force to stop a moving object,
that is an object does not have a
tendency to just come to rest.
Gravity accelerates all
objects at the same rate,
regardless of their mass.
Galileo Galilei
Newton’s genius was in unifying these ideas into three concise statements explaining why objects move
the way they do under the action of a force. The scope of Newton’s work was so broad and impressive, he
gets a little bit of extra credit.
Any object will remain at a constant velocity
unless acted upon by an unbalanced force.
Newton’s first law is called the LAW OF INERTIA.
The tendency of matter to continue in its current state of motion (whether it is moving or at rest) is called
inertia.
When moving difficult to stop
When stationary difficult to push
An unbalanced force is the vector sum of all the
forces acting on an object. It is also called the net
force.
The symbol for net
force is:
Fnet
Imagine that you are a passenger in a car . . . . .
Ex. 1:
Question:When the car approaches a red light and brakes, your body lurches forward. Why does
this happen?
Ex. 2:
Question:When the car makes a sharp right-hand turn which way does your body lurch?
Ex 3.
Question: It is easier to push and stop a child on a swing compared to an adult?
Ex 4.
Question: Why are lids helpful for pop and coffee when you go drive thru?
Ex 5.
Question: Why do headrests in cars help to elleviate the severity of whiplash?
Both you (the dot) and the car (the box) are originally moving with the same velocity (red
vector), together.
The force that made the body of the car turn has not
acted on you. Your body’s tendency is to keep moving
in its original direction. With respect to the car, it
seems you are pushed left.
If you are wearing a seat belt, you will still feel the sensation, but you won’t slide to the
left very much.
Your velocity will, after a short time interval, be in the same direction as the car’s
velocity, because the seat and seat belt exert unbalanced forces on you.
Check out some car crashes
of your choice!
Check out some
stat’s that will
make you want
to buckle up!
To Summarize Newton’s 1st law!
 when forces are balanced the net force is zero,
the resultant acceleration is zero, there is no
change in velocity- inertia!
 an object in motion will remain in motion unless
acted upon by an outside (unbalanced) force
 an object at rest will remain at rest unless acted
upon by an outside (unbalanced) force
 inertia is the property of an object to remain in
motion (or remain at rest)
Bart and the Case of Inertia…
1. Bart is using a force of 60 N west to ride his skateboard, the force of friction on
the skateboard is 60 N east. Bart and the skateboard have a mass of 45 kg.
a. draw a free body diagram
b. describe the motion of Bart and the skateboard
FN
Fapp = 60 N
Ff = 60 N
Fg