Download Formal Demonstration_Miha

Miha Lee
SED 525
Professor Herr
20 March 2007
Physics Laboratory Demonstration
(1) Title: Newton’s First Law of Motion
(2) Principle(s) Investigated: This activity demonstrates the following:
 What is the Newton’s first law of motion?
 How is it related to our life?
Learning objectives of the demonstration are:
 State and apply Newton’s first law of motion
 Name a force that slows down or stops moving objects.
 Observe the effects of seat belts on passengers in vehicle.
(3) California content standards. (The 8th grade Physical Science)
Forces
2. Unbalanced forces cause changes in velocity. As a basis for
understanding this concept:
c. Students know when the forces on an object are balanced, the
motion of the object does not change.
d. Students know how to identify separately the two or more forces
that are acting on a single static object, including gravity, elastic
forces due to tension or compression in matter, and friction.
(4) Materials:
Activity 1: A bottle with a lid that is so stuck that it is hard to open it.
Activity 2: A hooked mass (about 500g) hanged from a ring stand with
a string and another string at the bottom of the mass
Activity 3: A cup with a sheet of paper, a coin.
Activity 5: a toy car or dynamic cart with an object.
(5) Procedure
 Activity 1: A bottle with a lid that is so
stuck that it is hard to open it.
=> To open a bottle or a volumetric flask
that has a stuck lid, you can hit it
against a wall.
 Activity 2: A hooked mass (about 500g) hung from a ring stand
with a string and another string at the bottom of the mass.


When you pull down the
slowly, the string will be
the A point.
But if you pull down the
fast, the string will be cut
B point.
string
cut at
string
at the

 Activity 3: A cup with a sheet of paper and a coin.


If you pull fast the sheet of paper above
the cup, the coin will fall down into the
cup.
When you pull slowly the sheet of
paper above the cup, the coin will move
following the sheet.
 Activity 4: Watch a flash movie about Galileo’s
experiment about a ball on inclined plane on the website.
thought
http://user.chollian.net/~knuephy/physics/chap2/phy2-14.htm
 The ball rolling down inclined plane speeds up;
 The ball rolling up slows down;
 Rate of slowing down depends on steepness of incline: less
steep _ longer distance traveled;
 Extrapolation to zero slope of incline: ball will go on forever
at a constant speed.
 Activity 5: a dynamic cart with a wood brick.
 A dynamic car holds a wood brick above it.
 When the cart starts to run, the brick will fall back of the cart.
 When the cart suddenly stops, the brick will fall forward.
 When you tape the brick to the cart, although the cart starts
or stops suddenly, the brick will remain above the cart.
(6) Student prior knowledge
 Students know a force has both direction and magnitude.
 Students think any force can change the motion of an object.
 Students think keeping moving at a constant velocity
requires a force.
(7) Explanation:
Newton’s first law of motion deals with balanced forces. Equal forces
acting in opposite directions cancel each other. Such forces are balanced.
So, an object at rest does not mean that there are no forces acting on it,
but means that all the forces acting on it are balanced or cancel each
other.
Newton’s first law of motion states that as long as the forces on an
object balance each other, the object’s motion will not change. If an
object is at rest, it will remain at rest. If an object is moving, it will not
change its velocity. In other words, objects will keep doing what they
have been doing as long as the forces are balanced.
In my activities, the forces are exerted not on the objects (the bottle,
the coin, the hooked mass and the wood block), the but directly on the
other objects (the lid, the sheet of paper, the string, and the dynamic
cart) that are contact with the objects. In those cases, the forces that are
exerted on the other objects too fast to act on the objects don’t have any
effects on the objects, and thus the motions of the objects don’t change.
However, it looks like those objects resist internally the force that
we exert on the other objects. Newton named that internal resistance
inertia. So, if we try to move a motionless object, we feel like exerting a
force against the inertial force of the object. But the inertial force is not
real force. It’s a fictitious force.
Some people insist that the first law of motion is just one case of the
second law of motion when the force is 0.
F = ma (m is inertial mass)
Here, F is the net force that acts on an object. If all forces that act on an
object are balanced, that is the net force is 0, then the acceleration is 0.
Zero acceleration means that the object is at the constant velocity. When
the initial velocity is 0, the object which was at rest remains at rest.
However, if an external unbalanced force acts on abject, the object
changes its motion, and the resistance against the force is felt. The force
produced by the reaction of a body to an accelerating force, equal in
magnitude and opposite in direction to the accelerating force. An inertial
force lasts only as long as the accelerating force does.
(8) Questions & Answers: Give three thought-provoking questions and
provide detailed answers.
1. Evaluate the following assertion.
If there is no frictional force on the road, cars can move
without consumption of energy by the Newton’s first law of
motion. => Cars can move by the reaction of the frictional
force to the force that car acts on the road. So, without friction,
car can’t start move no mater how much energy is supplied.
2. What does the magnitude of inertial force depend on?
=> Obviously, inertia increases with the mass. A bowling ball is
harder to get moving and harder to stop than a hollow rubber
ball of the same size.
3. Seat belts protect us from being hurt by accidents because it
helps us to keep in touch with the car seat. Without seat belts,
according to the Newton’s first law, we will move forward
toward the window even though the car has stopped. How does
a seat belt keep us tied to the seat?
=> In the seat belt system, there is a retractor that locks the
webbing in the case of changes in the car motion. See the
below section for detailed explanation.
(9) Applications to Everyday Life:
1. Open the bottle with a stuck lid.
2. Getting rid of dusts from your clothes.
3. The seat belt of car: Automatic-emergency locking retractor for
seat belts
RETRACTOR
Also known as Roll-up device, Retracting, Inertia Reel, Automatic Reel
and Automatic Seat Belt. Designed to stow webbing not in use and lock in
a predetermined situation. Types frequently used are:
1. 'Pendulum' based - Known as vehicle sensitive - lock when the
'pendulum' moves because of sudden vehicle movement. Technically
known as Emergency Locking Retractor (ELR).
2. Webbing acceleration based - Known as snatch sensitive i.e. lock
when webbing is snatched. Technically known as Emergency
Locking Retractor (ELR).
3. A combination of 1 & 2 - Known as Dual Sensitive ELR.
4. Automatic Locking - i.e. lock when webbing is extracted and
fastened, unlock when webbing is fully retracted.
known as Automatic Locking Retractor (ALR).
Technically
4. Seismometer
Seismometers have:
1. A frame securely affixed to the earth. The foundation is critical, and often the
most expensive part of a seismic station.
2. An inertial mass suspended in the frame by some method, using springs or
gravity to establish a steady-state reference position.
3. A damper system to prevent long term oscillations in response to an event.
4. A means of recording the motion or force of the mass relative to the frame.
Passing seismic waves move the frame, while the mass tends to stay in a
fixed position due to its inertia. The seismometer measures the relative
motion between the frame and the suspended mass.
Early seismometers used optics, or motion-amplifying mechanical
linkages. The motion was recorded as scratches on smoked glass, or
exposures of light beams on photographic paper. Modern instruments use
electronics. Usually, the proof mass is held motionless by an electronic
negative feedback loop that drives a coil. The distance moved, speed and
acceleration of the mass are directly measured. The measurements are
often digitized and stored using a computer, and then are sometimes
automatically interpreted by computer programs to locate earthquakes.
(10) Picture
(11) References
Carle, M., Sarquis, M., and Nolan, L., (1991) Physical Science, the
challenge of discovery, D. C. Heath and company, Lexington, MA.
http://www.phy6.org/stargaze/Snewton.htm
Seismometers: http://en.wikipedia.org/wiki/Seismograph
Seat belt: http://www.securon.co.uk/seat_belts_components.htm
Inertial force: www.daviddarling.info/encyclopedia/I/inertial_force.html