Download A. Speed

STUDY GUIDE FOR TEST #2
MOTION, SPEED, AND FORCE
I. MEASURING MOTION
A. What is Motion?
1. Motion is a Change in Position
2. Position just means Where You Are at a given time
3. Distance is the Amount of Space Between Two Points
B. Describing and Measuring Motion
1. To describe motion and position, you need to have…
a) A reference point
(1) The origin is the point where you start.
b) A set of reference directions
(1) If one direction (North or right) is positive, then
its opposite (South or left) is negative
2. You can measure position and motion on a number
line or on a graph
3. If you use a graph, then you have two coordinates (x,y)
a) The x coordinate is for right (+) and left (-) and the
y coordinate is for up (+) and down (-)
b) Example: (3,-5) is three units to the right and five
units down – or three units East and five units down
C. Measurement and the Metric System
1. Meters (m) – length (A meterstick is 1m)
a) Centimeter (cm) – About the width of your pinky
finger 100 cm=1 m
b) Millimeter (mm) – 1/10 of a Centimeter 10 mm=1
cm, 1000 mm=1m
c) Kilometer (mm) – 1000 meters (Here to
McDonald’s). 1000m = 1 km
2. Grams (g) – Mass
a) Mass is the measure of how much matter you have.
In other words, how many atoms
b) You use a triple beam balance to measure mass.
c) Mass is measured in grams and kilograms. 1000
g=1 kg
3. Converting – You go from one unit in metrics to
another by moving the decimal place.
a) To go to bigger units, move the decimal left (The
number gets smaller)
b) To go to smaller units, move the decimal right
(The number gets bigger)
c) For milli-centi, move 1 place
d) For centi-unit, move 2 places
e) For mill-unit and unit-kilo, move 3 places
II. Speed and Velocity
A. Speed
1. Speed is the rate of change of position over time.
2. To calculate speed, divide distance by time (s=d/t)
3. s=d/t is the same as d=s*t and t=d/s
4. In metrics, speed is measured in meters per second
(m/s) 1 m/s = 2.25 mph
B. Types of Speed
1. Average Speed is speed over a long time, like for a trip
that takes an hour. (s=d/t)
2. Instantaneous Speed is your speed right this second,
like what you see on the speedometer.
3. If you keep the same speed, we call that Constant
Speed
C. Velocity
1. Velocity is speed with a direction (25 mph North)
2. To change velocity, you can
a) Speed up (Change Speed)
b) Slow down (Change Speed) or…
c) Turn (Change Direction)
3. Speed is always a positive number, but velocity can be
negative. -10 mph is just +10 mph in the other direction
III. Graphing Position, Speed and Velocity
A. Position vs. Time
1. In a position vs time graph, you always put time on the
x-axis and position on the y-axis.
2. If you are not going anywhere (s=0), then the graph is a
flat line.
3. If you are moving at a constant speed (ex: s=2 m/s),
then the graph is a straight line.
a) The slope of a line is the amount you go up or
down divided by the amount you go right (rise over
run)
b) The steeper the slope of the line, the faster
your speed.
4. If you are changing speed, then the graph is a curve.
B. Speed vs. Time
1. In a speed vs time graph, you always put speed on the
x-axis and position on the y-axis.
2. If you are moving at a constant speed (the same speed
all the time), then the graph is a flat line.
3. If you are speeding up then the graph goes up and
right.
4. If you are slowing down then the graph goes down and
right.
IV.
Acceleration
A. Acceleration is the rate of change of velocity over time.
1. This means that you can accelerate by speeding up,
slowing down, or changing direction.
2. If Acceleration is positive, you are speeding up.
3. If Acceleration is negative, you are slowing down.
4. If Acceleration is zero, you are keeping a constant
speed.
B. Calculating Acceleration
1. To find acceleration, divide the change in speed by the
time it took to change
2. Acceleration is (New Speed - Old Speed)/time
a) If you go from 10 mph to 20 mph in two seconds,
then a=(20-10)/2 = 10/2 = 5 mph/sec
3. Acceleration is measured in meters per second
squared (m/s2). This means how many meters per second
faster you go each second.
a) Example – If you accelerate from a stop at
10 m/s2, then after one second, your velocity is 10 m/s,
after two it’s 20 m/s, after three it’s 30 m/s and so on.
b) We call it meters per second squared because it’s
meters per second per second.
V. Force
A. A force is a push or a pull or anything that changes
motion
1. In order to accelerate (change speed or direction) you
need a force.
2. Forces have two parts:
a) Magnitude - The size or strength of a force
b) Direction
c) Ex – A force is 300 N up!
d) We often draw forces as vectors (arrows with
numbers)
3. Different forces acting on the same can combine
a) If the forces are in the same direction, they add
together
b) If forces are in opposite directions, they cancel
out
c) If forces are at right angles, treat them separately.
d) When we put forces together, we call the answer
the net force
e) If forces add up to zero, we call that balanced or
in equilibrium. When forces don’t add up to zero,
they are unbalanced.
f) If forces on an object are unbalanced, the object
accelerates in the direction or directions of the
larger net forces.
B. Kinds of Forces
1. Gravity pulls downward on an object (see below for
more)
2. Friction slows an object down when it rubs against
another object.
a) Friction always pushes against the direction of
motion.
b) Static friction is when friction keeps an object
from moving at all.
c) Sliding friction is when something is moving
d) Rolling friction is friction on wheels. It
reduces the effect of friction so you can go faster.
e) Fluid friction is friction in or over a liquid or gas.
Usually, fluid friction is less than sliding friction, so it
reduces it.
3. Tension is the force pulling back against you when
you pull on a string or wire. Think tug of war.
4. Compression is the force of a spring or rubber band
trying to get back to where it was. Think squeezing and
stretching.
VI.
Newton’s Laws of Motion
A. Newton’s First Law of Motion – An object at rest remains
at rest and an object in motion remains in motion at
constant speed and in a straight line unless acted upon by
an unbalanced force.
1. In other words, if you don’t have a force, or net force
is zero, you can’t have acceleration.
2. If you’re stopped, you stay stopped. If you’re moving,
you stay moving at the same speed in the same
direction.
3. So, if you’re going 50 mph, you keep going 50 mph
unless there’s a force on you.
4. The reason stuff slows down on Earth is because
friction is slowing it down.
5. The tendency of stopped objects to stay stopped and
moving objects to keep moving is called inertia.
B. Newton’s Second Law of Motion – The acceleration of
an object depends on the mass of the object and the
amount of force applied.
1. Force=Mass*Acceleration (F=ma, a=F/m, m=F/a)
2. The harder you push something, the more it
accelerates.
3. The more mass something has, the harder it is to
accelerate.
4. These relationships are proportional. 2x Force means
2x acceleration. 2x mass means ½x acceleration.
5. Force is measured in Newtons. One Newton is the
amount of force it takes to accelerate one kilogram by one
meter per second squared (1N = 1 kg * 1 m/s2)
C. Newton’s Third Law of Motion – Whenever one object
exerts a force on a second object, the second object exerts
an equal and opposite force on the first.
1. In other words, for every action, there is an equal and
opposite reaction.
2. Example – Mr. Castroll is moving on his skateboard at
10 m/sec when he hits a wall. When he hits the wall, he
does so with a force of about 7000 N. Since the wall isn’t
going anywhere, it pushes back at him at a force of 7000
N. Ouch!
3. Normal force is the force of the floor pressing up
against you equal to the force of gravity pushing down.
D. Gravity
1. Gravity is the force that all pieces of matter pull
against each other.
a) The gravitational pull of one object on another
depends on their mass and the distance between them.
(1) Gravity increases with mass. Bigger objects pull
harder and bigger objects are pulled harder
(2) Gravity decreases with distance.
b) On Earth, gravity accelerates all objects regardless
of mass at a rate of 9.8 m/sec2. (I usually round to 10
m/sec2.)
c) Because air resistance pushes back upwards,
objects do not accelerate at the full 9.8 m/sec2.
d) Objects in orbit try to move in a straight line, but
the Earth’s (or Sun’s or Whatever’s) gravity pulls
them downward, making them turn in a circle. This is
what makes objects go in orbits.
e) The force of gravity is called weight. Note that
weight changes on other planets but mass doesn’t.