Physics 104 - How Things Work.
Greg Sullivan
Fall, 2000
Introduction of who I am.
· Professor here
· Research in Particle Astrophysics
o Elementary particles & fields
Purpose of course:
· How things work
o Scientific bases of how things work
o From outside to inside and the underlying scientific principles
Grading:
· Homework 15%
· 2 - 1 Hr exams 20% each
· Term paper 20%
· Final exam 25%
o Comprehensive
Homework:
· Practice
o OK to help each other but….
Term papers:
· Look at the web page
Exams:
· Closed book ~75 minutes
· Stress concepts
o Some short answer
o Some numerical
Don’t want you to remember a bunch of formulas (some). I want you to know how things work not remember ho they work.
Ask Questions
· Cliché – The only stupid question is the one you don’t ask.
TA:
Yung-Fu Chen
physics 1322
Book:
· How Things Work – The Physics of Everyday Life
o Louis A. Bloomfield
Hand out the syllabus, policy, & schedule.
Physics.
What is Physics??
· Try to break down the complexity of the world into simple rules.
· A few basic laws
o Periodic table
o Example of the soccer game
§ Watch the game and try to figure out the rules from what happens on the field
DEMO – C3-02: Table Cloth trick
· Why do the dishes stay there when I pull out the table cloth?
o What are the rules?
o Do they apply everywhere?
Issue:
· Language
o Often physics terms have a meaning different then in normal usage. (e.g. conservation of energy)
So, in order to set a foundation for the entire course, and for your basic understanding, we will start with
The Laws of Motion
Begin Section 1.1
DEMO – Throw tennis balls around room
Falling Balls
· What are the rules?
· Are there rules?
Questions:
· Does tennis ball & baseball follow the same rules?
· Under what conditions do they have the same path?
· Horizontal motion vs. falling motion?
· How does weight affect the motion?
History:
· Aristotle 350BC
o V µ F
§ Heavier fall faster
· Galileo 1600
o All fall equal (tower of Pisa)
DEMO
- C4-33: FREE FALL IN VACUUM - FEATHER AND
BALL
- C4-34: GALILEO'S EXPERIMENT - MASSES IN FREE FALL
The motion of an isolated ball
· No gravity
· Correct answer eluded people for thousands of years
· Galileo’s law of inertia
Inertia: A Body in motion tends to remain in motion; a body at rest tends to remain at rest.
Falling ball more complicated
· Accurate description need several physical quantities
o Position
o Speed
o Velocity
o Mass
o Acceleration
o Force
Location of ball:
· Position
o Distance & direction from reference point
· 3 spatial coords wrt a reference
DEMO - A2-01: CARTESIAN COORDINATE AXES
o e.g. 30 miles north of Campus
o 10 miles east of my house
· could both be same location!
Position is an example of a vector quantity.
· Both magnitude and direction
If a ball is moving, then its position is changing.
· Velocity
o How quickly the position is changing
o Speed the ball is moving & direction it is heading.
o Velocity is vector quantity with speed & direction
· 50mph due north
Newton - ~1660
Armed with this terminology:
Inertia stated as
Newton’s 1st Law:
An object that is not subject to any outside force moves at a constant velocity, covering equal distances in equal times (speed) along a straight line path(direction).
DEMO - C4-04: F=MA WITH ULI AND FORCE PROBE
use demo to show graph of constant velocity motion
Why is dV/dt = 0?
· Because of mass
o Inertial Mass
· What takes more force?
o To stop a heavy or a light object?
DEMO - C3-04: INERTIA - LEAD BRICK AND HAND
C3-12: PENCIL AND PLYWOOD
OK Back to falling balls again
DEMO - C2-06: BALL DROP ON ROPE - EQUAL AND UNEQUAL INTERVALS
What happens?
· Are equal distances covered in equal times?
· What does the 1st Law tell us?
o There must be an out side force!
§ Gravity pulling down!
DEMO - C2-07: FREE FALL - EQUAL TIME INTERVALS
When something pushes on the ball, its velocity changes
· It accelerates
Acceleration:
· Change in velocity per unit time
· Vector quantity like position & velocity
o Magnitude & direction
§ Direction along the direction of push (force)
DEMO - C4-04: F=MA WITH ULI AND FORCE PROBE
show position vs velocity vs acceleration graphs
What objects are accelerating?
1. car going straight down the road
2. constant speed on the beltway
3. object in orbit (satellite)
4. putting breaks on at 55mph
5. falling ball
6. thrown baseball
If we push an object its velocity changes, that is it accelerates
Direction of the change in velocity is along the pusj.
A Simple rule that relates the push (force), the acceleration, and the inertia (mass).
Newton’s 2nd Law
The force exerted on an object is equal to the product of that object’s mass times its acceleration. The acceleration is in the same direction as the force.
or by rearranging the formula:
acceleration is proportional to Force divided by the inertia(mass) and is along the direction of the force.
DEMO - C4-02: AIR TRACK - A = F/M (with ULI)
NOTE: at start of 2nd lecture use computer with internet connection displayed on projector to access the phys 104 class website http://umdgrb.umd.edu/sullivan/physics104.html and to join hypernews.
Units:
SI Units (metric)
· length (distance) meters , m
· time seconds, s
· mass kilograms. Kg
So, for example
· velocity = m/s
· acceleration = m/s/s or m/s2
· Force – kg m/s2 = 1 Newton ,N
o 1 N ~ force created by 10 US quarters in your hand
Weight & Gravity:
Why does an object weight something?
· Gravity
· Pull of gravity from earth
o Moon & sun too far away
§ More subtle – ocean tides
· Remarkable thing about gravity
o Weight is proportional to mass
Where g is a constant that is determined by the local strength of gravity. The constant g is called the acceleration due to gravity.
· Determined by the properties of Earth.
o Radius, mass
o Doesn’t depend on the object being considered
g = 9.8 m/s2 (32 ft/s2) on earth’s surface
Objects will weigh something different on another planet or moon
On moon weight = 1/6 that of earth
gmoon = 1/6 gearth
Back again to the Falling Balls:
Only force on the falling ball is its weight.
· How much will it accelerate?
The acceleration of the object due to the force of gravity is independent of the mass of the object.
· Remember the demo with the feather & ball
· Newton & Einstein got famous for this
Consider a baseball and a bowling ball:
Although the bowling ball weighs more (has more force due to gravity), it also has more mass (inertia) which resists the force more. So, the greater inertia exactly cancels the greater force and they both end up with the same acceleration, and therefore the same motion.
Now we can examine the motion of any falling ball near the earth’s surface. Any ball will accelerate downward at a constant rate of
9.8 m/s2.
· What about velocity & position of falling ball? (1D)
Velocity
V = V0 + a t
Example: throw a ball up at 20 m/s (~45mph).
V = 20m/s –9.8m/s2 x t
When does it stop?
V=0 = 20 –9.8 x t t ~ 2s
What does this mean? What is a at this point?
What about position?
Distance = <vel> x time
<vel> = ½(V0 + Vf) = ½ (V0 + (V0 + at))
<vel> = initial velocity + ½ acceleration x time
present position = initial position + Distance
X = X0 + V0 * t + ½ * a * t2
Second order in time
· Parabola
Summarize falling ball: (draw this geometrically as in book)
T(s) / A(m/s2) / V(m/s) / P(m)0 / -9.8 / 0 / 0
1 / -9.8 / -9.8 / -4.9
2 / -9.8 / -19.6 / -19.6
3 / -9.8 / -29.4 / -44.1
Recall V & X graphs from Air-track demo with ULI
Projectile Motion:
DEMO – Throw the ball around
Ball has 2-D motion
· Up & down (called this X above, now call this Y)
· Direction it is thrown (X)
Gravity only acts in Y direction
· ax = 0 , ay= -9.8 m/s2 = g
X = X0 + V0x * t
Y= Y0 + V0y * t + ½ * g * t2
DEMO - C2-25: FUNNEL CART
DEMO - C2-21: BALLS DROPPED AND SHOT
DEMO - C2-22: MONKEY AND HUNTER
DEMO - C2-24: WATER DROP PARABOLA
End Section 1.1
RAMPS:
Try pushing something up a ramp.
· Heavy object sitting on table
o Large force
· Put it on an inclined plane
o Less force to move it!
DEMO: B2-03: EQUILIBRIUM OF FORCES - INCLINED PLANE
You can use a ramp to lift very heavy objects with relatively little force!
· Mechanical advantage.
How does this work?
Need to understand addition of forces.
· What forces are acting me as I stand here?
o Gravity down
o Why don’t I accelerate?
§ Force of floor acting up
Is the force always the same?
· No, it always just cancels out weight
· Reaction force
Newton’s 3rd law:
For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object.
DEMO: C5-19: ACTION AND REACTION - INSTRUCTOR AND CART
Examples
· Boat on water
· Person on ice
· Recoil of a gun
We’ll come back to this later in Rockets.
Consider a collision between a car and a truck:
Mcar = 1 ton Mtruck = 10 ton
Ft®c = -Fc® t
Mcac = Mtat
ac = -Mt/Mc * at ac = -10 at
Summarize Newton’s 3 Laws.
Addition of Forces:
Draw figure of object with forces from gravity(weight) and the reaction force of the floor.
· Force is a vector
DEMO: B2-02: SUM OF FORCES - SPRING SCALES
Give examples of summing of forces.
· Pushing you east and north, you go northeast
DEMO: B2-16: VECTOR ADDITION WITH ROPE AND STUDENTS
DEMO: C5-31: AIR TRACK - SAILING UPWIND
Work & Energy:
The capacity to make things happen is energy, and the process of making them happen is called work.
Work & Energy are physical quantities
· They are measurable!
Physical definitions are different then common English usage.
· Energy
o Is not exuberance of a 5yr old
o Capacity to do work
· Work
o Not the activities you do to get money
o Process of transferring energy
Energy is what is transferred, and work does the tyransferring.
Important:
ENERGY IS A CONSERVED QUANTITY!!
We will talk about this a lot all semester.
What is work?
· You do work on an object by exerting a force on it, as it moves in the direction of that force.
· As you lift a rock you do work on the rock.
In both cases you are transferring energy to the object.
Sometimes Work (transferring energy) makes an obvious change.
· Throw the ball
o Picks up speed, energy increases
§ Kinetic Energy
o Pick up the rock
§ It can do work on the objects beneath it if it drops
· Potential energy
· Energy stored in the forces between things
§ Gravitational Potential Energy
DEMO: C8-04: HILL TRACK
DEMO: C8-11: INTERNAL VS EXTERNAL ENERGY - SPRING-COUPLED SUPERBALLS
DEMO: C8-12: JUMPING MASSES WITH INTERNAL SPRINGS
The amount of work you do is determined by how hard you push(force) and the how much distance you push it.
Work = Force x Distance
How much work do you do in lifting the piano? The work is transferred energy into the piano in the form of Gravitational Potential energy.
Gravitational Potential Energy of any object:
Net amount of work you do in lifting the piano is mgh, it doesn’t matter how you get there!
If you lift 100kg to a height of 10m
U = m g h = 100kg x 9.8 m/s2 x 10m
~ 10,000 kg m/s2 x m = 10,000 Nm
= 10,000 Joules (J)
Now we can examine how a ramp allows to lift the piano with a little force by giving us a mechanical advantage.
Small residual net force
· Ramp supplies most of force needed to keep it from accelerating
If you have a 50m ramp go up 5m. for every 10m along ramp you go up 1m. You can push a 2000N weight up this 10 to 1 grade with only 200N of force.
The total work is 50m x 200N = 10,000 J
It doesn’t matter if you go up the ladder
2000N x 5m = 10,000J
or slide it up the ramp with only 200N of force. Either way the final energy (mgh) is the same.
Work = LARGE FORCE x small distance
= small force x LARGE DISTANCE
This ramp gives us a mechanical advantage.
SEESAWS:
Another form of mechanical advantage using torque.
We all now how a seesaw works. Need equal masses.
What if we have different weights?
· Balance it by changing distance from the pivot point.
DEMO: B2-32: EQUILIBRIUM OF TORQUES – LARGE
DEMO: B3-03: LEVER - WRECKING BAR
DEMO: B3-11: PULLEY - HUMAN LIFT
Wheels:
If we have a heavy object (book uses file cabinet) what happens when we push on it?
We start pushing on the file cabinet, but it doesn’t budge.
· Why Not?
· Newton’s 2nd Law says if we push it should accelerate
Something must be pushing back harder as we push harder to cancel the force.
· Net force = 0
· No acceleration
ANSWER:
FRICTION – a force that opposes the relative motion of two surfaces in contact with one another.
Friction always opposes the relative motion.