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INTRO TO PHYSICS MIDTERM EXAM REVIEW 2024
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INTRO TO PHYSICS MIDTERM EXAM REVIEW 2024INTRO TO PHYSICS MIDTERM EXAM REVIEW 2024INTRO TO PHYSICS MIDTERM EXAM REVIEW 2024
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INTRO TO PHYSICS MIDTERM EXAM REVIEW2024
What is Physics?
Physics attempts to explain the universe and how it works by breaking it into its smallest
possible components and, given the interactions among them, deduce how the universe works at
all levels.
For us for the time being: Physics is the study of motion.
How physics (and science) is done.
The Scientific Method.
1. Observation — Newton sees an apple fall
2. Hypothesis — A force called gravity pulled the apple to the Earth
3. Prediction — Same force of gravity holds the Moon in orbit about the Earth
4. Experiment — Measure the motion of the Moon
5. Conclusion — If the prediction and the result of the experiment disagree within experimental
uncertainty then the hypothesis is rejected. If the prediction and the experimental result agree
within the uncertainty, the hypothesis is supported.
The ultimate decision as to the validity of a scientific idea is how well it predicts the results of
experiments.
Note: The scientific method as described above is an ideal. No single scientist or group of
scientists follow the method as described. However science follows the spirit of the scientific
method—no idea is accepted unless it is supported by experimental evidence.
International System of Units (SI)
Unit of Distance: meter
Originally: One ten-millionth of the distance from north pole to equator
Original Standard - platinum-iridium bar with two finely engraved lines one meter apart.
Current: the meter is defined so that the speed of light is
,299,792, 458 m/s
Unit for Time: second
Originally based on the length of the year.
Current Standard – atomic clock — so many oscillations of cesium atoms.
Chapter 2 — Newton’s First Law of Motion — Inertia
Ancient Greeks: Natural state of motion is one of rest.
Galileo tested this idea. He applied a given impetus to a block and measured how far it would
slide along a horizontal surface.
Found that a body given the same impetus would slide different distances before coming to rest
depending on how lubricated the surface was. Note: the pendulum below is used to provide the
same impetus to the block in each trial.
He concluded that a body sliding on a perfectly frictionless surface would slide at constant speed
in a straight line forever.
Galileo: Natural state of motion is constant speed in a straight line.
Why don’t all bodies maintain constant speed in a straight line? Forces!
Force: A physical push or pull that can cause the motion (speed and/or direction) of a body to
change. Think of pushing or pulling a grocery cart.
Note that the word force has some different meanings in everyday life: the bank robber forces the
teller to give him money by pointing a gun at her. This is not a physics force.
A body without forces applied to it will not change its motion.
,If the net or total force on a body is zero—that is, if all the forces add to zero—the motion of the
body won’t change either.
Newton took Galileo’s idea and added force to produce:
Newton’s 1st Law of Motion (N1): If the net force on a body is zero, the body, if at rest, will
remain at rest and, if in motion, will continue in motion with constant speed in a straight line.
If the net force on a body is zero, we say that the body is in equilibrium. Static equilibrium
means that the body is at rest. Dynamic equilibrium means that the body is in motion, but
motion with constant speed in a straight line.
What causes a body to continue moving with constant speed in a straight line: Inertia!
Inertia: The characteristic of a body that keeps it moving with constant speed in a straight line.
It resists change in motion.
Some bodies have more inertial than others — but we’ll discuss that with Newton’s 2nd Law of
motion.
Note: N1 is often called the “law of inertia.”
Chapter 3 — Linear Motion
Linear motion is motion in a straight line.
Use a coordinate system or reference frame to numerically define the location or position of a
particle.
In one dimension:
Set up a real number line.
Choose an arbitrary origin.
Calibrate the line with numbers in units of length — meters, centimeters, etc
Coordinates are positive to the right of the origin and negative to the left.
If we place a particle on this
line, the number at its location
is its coordinate. The
coordinate of the particle in the
figure is roughly x = 2.4 m.
Motion — a particle moves
from one coordinate to the next in some time as shown in the figure below. The figure shows to
“snapshots” of the coordinate system, one at time t1 with the coordinate of the particle being x1
and the other at time t2 with the coordinate of the particle being x2.
, Note that the quantity Δx is the change in coordinate of the particle and is just the difference
between x2 and x1.
This motion can be described in terms of the average velocity. The average velocity is defined as
the ratio of the change in coordinate to the time needed for the change in coordinate to take
place. Here, that time is the difference between t2 and t1, which we will designate by Δt. The
formula for the average velocity is
The bar above the v symbol means “average.” Note that we don’t know from our snapshots how
the particle gets from its initial coordinate to its final coordinate. It could travel past its final
coordinate and then come back. Or it could have started moving to the left from its initia l
coordinate and than turned around and moved to its final coordinate. And its speed might not
have been constant during the trip.
Important: In physics speed and velocity are NOT the same.
Average speed is defined in terms of the total distance traveled — the distance traveled divided
by the time to travel the distance.
Note that average speed is always positive. Average velocity will be negative if the particle ends
up to the left of its starting point.
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