Transcript Chapter 11 -- Motion

Section 1—Measuring Motion
Section 2—Acceleration
Section 3—Motion and Force
Measuring Motion
 Why it Matters
Rescue workers can use the last-known velocity of a lost
airplane to determine where to look for survivors.
 We are surrounded by moving things. From a car
moving in a straight line to a satellite traveling in a
circle around the Earth, objects move in many ways.
In everyday life, motion is so common that it seems
very simple. But describing motion scientifically calls
for careful use of definitions.
Observing Motion
 You may think that the motion of an object is easy to
detect—just look at the object. But you really must
observe the object in relation to other objects that stay
in place, called reference points.
 A frame of reference is used to describe the motion of
an object relative to these reference points.
 See figure 1 on page 365 in your text.
 When an object changes position with respect to a
frame of reference, the object is in motion.
 Motion—change in position relative to a reference
point.
 Frame of reference—a system for specifying the
precise location of objects in space and time.
 Distance measures the path taken
 In addition to knowing the direction, you need to
know how far an object moves if you want to correctly
describe its motion.
 To measure distance, you measure the length of the
path that object took.
 Displacement is the change of the objects position.
 The distance traveled is found by measuring the whole
path. The line from the starting point to the ending
point is called displacement. (see figure 2 on page
366).
 Displacement--the change in position of an object.
 Distance measures how far an object moves along a
path.
 Displacement measures how far it is between the
starting and ending points.
 Displacement is often shorter than the distance
traveled unless the motion is all in a straight line.
 The direction of displacement must also be given.
Example the distance between your home and the
school might be 12 blocks, but this does not tell us if
you are going towards or away from the school.
 Displacement must always indicate the direction, such
as 12 blocks toward school.
Speed and Velocity
 Speed describes how fast an object moves.
 Speed—is the distance traveled divided by the time
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interval during which the motion occurred.
Sometimes you may need to know the direction on
which an object is moving. This is velocity.
Velocity—the speed of an object in a particular
direction.
See figures 3 and 4 in your text.
What example did your text book give?
 A lion that escaped from a zoo.
 They used a helicopter crew to guide the ground
searchers. Without knowing the direction of the lions
motion searches could not have predicted its position.
 Speed tells us how fast an object moves, and
velocity tells us both the speed and direction that
the object moves.
 Velocity is described relative to a reference point.
 The direction of motion can be described in different
ways, such as north, south, east, or west of a fixed
point. Or it can be an angle from a fixed line.
 Direction is described as a positive or a negative along
the line of motion.
 If a body is moving in one direction, it has positive
velocity. If it is moving in the opposite direction it has
negative velocity.
 By convention up and right are usually positive, left
and down are negative.
 Combined velocities determines the resultant velocity.
 If you are riding in a bus traveling at 15 m/s, you and all
the other passengers are traveling at a velocity of 15
m/s east relative to the street.
 If you stand up and walk at 1 m/s toward the back of
the bus. Are you still moving at the same velocity as
the bus relative to the street? No, but your new
velocity can be easily calculated. Your new velocity is
equal to 15 m/s east + (-1 m/s east) = 14 m/s east.
Calculating Speed
 Speed describes distance traveled over a specific time.
 To calculate speed you must measure two
quantities: the distance traveled and the time it
took to travel that distance.
 The SI unit for speed is meters per second (m/s).
 When an object covers equal distances in equal
amounts of time, it is moving at a constant speed.
 Average speed is calculated as distance divided by
time.
 Most objects do not move at a constant speed but
change speed from one instance to another. One way
to make it easier to describe the motion of an object is
to use average speed.
 Average speed is the distance traveled by an object
divided by the time the object takes to travel that
distance.
 Speed= distance
v=d
time
t
 Instantaneous speed is the speed at a given time.
 You can measure the distance traveled in a shorter
period of time interval. The smaller the time interval,
the more accurate the measurement of speed will be.
 speed measured in an infinitely small time interval is
called instantaneous speed.
 Practically speaking, a car’s speedometer gives the
instantaneous speed of the car.
Graphing Motion
 You can investigate the relationship between distance
and time in many ways. You can use mathematical
equations and calculations.
 You can plot a graph showing distance on the
vertical axis and time on the horizontal axis.
 You measure either distance or, if you know the
direction, displacement and the time interval during
which the distances or displacements take place.
 Motion can be studied using a distance vs. time graph.
 In a distance vs. time graph, the distance covered by an
object is noted at equal intervals of time.
 As a rule, line graphs are made with the x-axis
(horizontal axis) representing the independent
variable and the y-axis (vertical axis) representing the
dependant variable.
 Time is the independent variable because time will
pass whether the object moves or not.
 Distance is the dependant variable because the
distance depends upon the amount of time that the
object is moving.
 The slope of a distance vs. time graph equals speed.
 For a car moving at a constant speed, the distance vs.
time graph a straight line. See figure 6 on page 371.
 The speed of the car can be found by calculating the
slope of the line. The slope of any distance vs. time
graph gives the speed of the object.
 If the car is stopped it has a speed of 0 m/s. Its
position does not change as time goes by. So the
distance vs. time graph of a resting object is a flat line
with the slope at zero.
Acceleration
 Why it matters:
Acceleration is calculated by reconstructionists
investigating automobile accidents.
 Imagine that you are a race-car driver. You push your
accelerator. The car goes forward, moving faster and
faster. As you come up to the curve in the track, you
remove your foot from the accelerator to make the
turn. In both situations, your velocity changes. When
you increase your speed, your velocity changes. Your
velocity also changes if you decrease your speed or if
your motion changes direction.
Acceleration and Motion
 Velocity has both speed and direction. Like velocity,
acceleration has a value and a direction.
 When an object undergoes acceleration, its
velocity changes.
 Positive acceleration is in the same direction as the
motion and increases velocity.
 Acceleration--the rate at which velocity changes over
time; an object accelerates if its speed, direction, or
both change.
 Acceleration can be a change in speed.
 If you start moving south on your bicycle and speed as
you go. Every second, your velocity increases by 1 m/s.
After 1 s, your velocity is 1 m/s south. After 2 s, your
velocity is 2 m/s south. Your velocity after 5 s is 5 m/s
south. Our acceleration can be stated as an increase of
one meter per second (1 m/s/s) or 1 m/s² south.
 Acceleration can also change in direction.
 Why do you accelerate when changing direction? It’s
because acceleration is defined as the rate at which
velocity changes with time.
 Velocity includes both speed and direction, so the
object accelerates if its speed, direction, or both
change.
 So…..you can constantly accelerate while never
speeding up or slowing down.
 Uniform circular motion has centripetal acceleration.
 If you move at a constant speed in a circle, even though
your speed is, never changing your direction is always
changing. So, you are always accelerating.
 Examples: the moon is constantly accelerating around
the Earth. When you ride the Ferris wheel at a park,
you are accelerating.
 When you stand still on the Earth’s surface you are
accelerating. You are not changing speed, but you are
moving in a circle as Earth revolves.
 An object moving in a circular motion is always
changing its direction. So velocity is always changing
even if its speed does not change.
 The acceleration that occurs in circular motion is
known a centripetal acceleration.
Calculating Acceleration
 To find the acceleration of an object moving in a
straight line you need to measure the object’s velocity
at different times.
 The average acceleration over a given time
interval can be calculated by dividing the change
in the object’s velocity by the time over which the
change occurs.
 The change in an object’s velocity is symbolized by
∆v.
 Average acceleration (for a straight line of motion)
 Acceleration = final velocity-initial velocity
time
a=∆v
t
If the acceleration is small, the velocity is increasing very
gradually. If the acceleration is large, the velocity is
increasing more rapidly.
 Acceleration is the rate at which velocity changes.
 Look at figure 4 and discuss.
 In science, acceleration describes any change in
velocity, not just “speeding up”. When you slow down,
your acceleration is negative because it is opposite the
direction of motion.
 Acceleration is negative when slowing down.
 When the driver pushes on the gas pedal in a car, the
car speeds up. The acceleration is in the direction of
the motion and therefore is positive. When the driver
pushes on the brake pedal, the acceleration is opposite
the direction of motion. The car slows down, and its
acceleration is negative. When the driver turns the
steering wheel, the velocity changes because the car is
changing direction.
Graphing Accelerated Motion
 You can find acceleration by making a speed vs. time
graph.
 Plot speed on the vertical axis and time on the
horizontal axis.
 A straight line on a speed vs. time graph means that
the speed changes by the same amount over each time
interval. This is called constant acceleration.
 The slope if a straight line on a speed vs. time
graph is equal to the acceleration.
 You can look at a speed vs. time graph and easily see if
an object is speeding or slowing down.
 A line with a positive slope represents an object that is
speeding up. A line with a negative slope represents
an object that is slowing down.
 Acceleration can be seen on a distance vs. time graph.
 Look at figure 5 on page 377—imagine that one of the
riders is slowing uniformly from 10.0 m/s to a complete
stop over a period of 5.0 s. A speed vs. time graph of
this motion is a straight line with a negative slope.
This straight line indicates that the acceleration is
constant. You can find the acceleration by finding the
slope of the line.
 The distance vs. time graph, (fig 5) is not a straight line
when the rider’s velocity is not constant. This curved
line indicates the object is under acceleration.
Motion and Force
Why it Matters
The force of friction is essential to making automobile
breaks work properly—and essential to making a car
move forward.
Fundamental Forces
 In science force is defined as any action that can
change the state of motion of an object.
 Force—the action extended on a body in order to
change the body’s state of rest or motion; force has
magnitude and direction.
 Scientist identify four fundamental forces in nature.
 These forces are the force of gravity, the
electromagnetic force, the strong nuclear force,
and the weak nuclear force.
 The strong and weak nuclear forces act only over a
short distance, so you do not experience them directly
in everyday life.
 The force of gravity, is a force that you feel every day.
 Other everyday forces, such as friction, are a result of
the electromagnetic force.
 Fundamental forces vary in strength.
 The fundamental forces vary widely in strength and
the distance over which they act.
 Strong nuclear forces hold the protons and neutrons
together in an atoms nucleus and is the strongest of all
forces. It is negligible over the distances greater than
the size of an atomic nucleus.
 The gravitational and electromagnetic force is about
1/100 the strength of the strong force.
 The gravitational force is very much weaker than the
electromagnetic force.
 Forces can act through contact or at a distance.
 If you push a cart, the cart moves. When you catch a
ball, it stops moving.
 These pushes and pulls are examples of contact forces.
 There is another class of forces –called field forces—
that do not require the objects to touch each other.
 The attraction or gravity or repulsion between two
north poles of a magnet are examples of a field force.
 Both contact and field forces can cause an object to
move or stop moving.
Balanced and Unbalanced Forces
 See figure 2 on page 381
 Moving a heavy sofa—will you and a friend push from
opposite sides or push in the same direction?
 The net force, the combination of all the forces acting
on the sofa, determines if the sofa will change its
motion.
 Whenever there is a net force acting on an object,
the object accelerates in the direction of the net
force.
 Balanced forces do not change motion.
 When the forces applied to an object produce a net
force of zero, the forces are balanced.
 Balanced forces do not cause objects at rest to start
moving.
 Balanced forces do not cause a change in the motion of
moving objects.
 Unbalanced forces do not cancel completely.
 If 2 people push a box in one direction and 1 person
pushes in the opposite the direction –then the object
will accelerate in the direction of the greater force.
 And if pushed in different direction such as 1 pushes
north and another pushes east the object will move in
a northeasterly direction.
The Force of Friction
 A car rolling along a flat, evenly paved street.
Experience tells you that the car will keep slowing
down until finally it stops.
 The unbalanced forces that acts against a car’s
direction of motion is friction.
 The force of friction always opposes the motion.
 Friction--a force that oppose motion between two
surfaces that are in contact.
 Friction occurs because the surface of any object is
rough. The rubbing together of two rough surfaces
creates heat.
 Surfaces that look and feel smooth are covered with
hills and valleys. So when the two surfaces are
touching the hills and valleys of one surface stick to
the hills and valleys of the other surface.
 Static friction is greater that kinetic friction.
 The friction between surfaces that are stationary is
called static friction and the friction between moving
surfaces is called kinetic friction.
 Static friction is usually greater than kinetic friction,
because the force required to make a stationary object
start moving is usually greater than the force necessary
to keep it moving.
 Static friction—the force that resist the initiation of
sliding motion between two surfaces that are in
contact and at rest.
 Kinetic friction—the force that opposes the movement
of two surfaces that are in contact and are moving over
each other.
 Not all kinetic friction is the same.
 Sliding friction— when objects slide past each other.
 Rolling friction— if a rounded object rolls over a flat
surface.
Friction and Motion
 Without friction, the tires of a car would not be able to
push against the ground and move the car forward.
 You would not be able to walk without falling down.
 Friction is necessary for many everyday tasks to
work correctly.
 Unwanted friction can be lowered.
 Sometimes we must lower unwanted friction. We can
use low-friction materials, such as nonstick coatings
on cooking pans.
 Another way is to use lubricants, these are substances
applied to surfaces to lower the friction between them.
 Examples: motor oil, wax, & grease.
 Helpful friction may be increased.
 Helpful friction is increased by making surfaces
rougher.
 Example: sand scattered on icy road
 Cars could not move without friction.
 See figure 5
 Without friction between the tires and the road, the
tires would not be able to push against the road and
the car would move forward.
 The force pushing the car forward must be greater that
the force of the friction that opposes the car’s motion.
 Friction also affects object that are not moving.
 Example: when a truck is parked on a hill and its
brakes are set, friction opposes the force of gravity
down the hill and stops the truck from sliding.