Transcript Lecture02
Reading Quiz
Which of the following statements is FALSE
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•
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Speed, velocity, and acceleration are each defined as vectors
"A car moving in a circle at a constant speed of 30 mph is
accelerating
"The instantaneous speed of a moving car can be different from
its average speed
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PHYS 214: The Nature of Physics
Newtonian Revolution
The Force equals the Motion ( F = ma )
Newton’s Laws
1. An object remains at rest, or in uniform motion in a
straight line, unless it is compelled to change by an
externally imposed force.
2. The acceleration of an object is directly proportional
to the magnitude of the imposed force and inversely
proportional to the mass of the object.
3. For every action there is an equal and opposite
reaction.
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PHYS 214: The Nature of Physics
Academic Integrity
• Never give your HIP password to anybody else.
• All CHIP Reading Quiz and HW answers must be your own.
• Consulting with others on HW is fine, as long as you are not
just being given the answers.
• When permitted, Lecture Quiz answers may follow
consultation with others.
• Any cheating on exams will earn an F in the class, and formal
disciplinary proceedings.
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PHYS 214: The Nature of Physics
Classroom Demeanor
• Come to lectures to listen and participate.
• No personal conversations with friends during lecture!
• Don’t get up to leave before 8:20 AM if lecture has not
ended.
• Join in on the Lecture Quiz discussions.
• Interrupt if you have a question.
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PHYS 214: The Nature of Physics
SPEED
Speed equals distance covered divided by the time needed to
move that distance :
S=d/t.
Average speed refers to a large time interval:
If it takes three hours for a race car to complete the
Indianapolis 500 race, its average speed was s=500 miles / 3
hours = 166.7 mph.
Its fastest speed was probably more than 200 mph. Its
slowest speed was exactly 0 mph, when the engine was
started. These are examples of instantaneous speeds. These
correspond to speeds over very short time intervals.
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PHYS 214: The Nature of Physics
Instantaneous Speed
S=Dd/Dt.
A very small time
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PHYS 214: The Nature of Physics
Velocity
Velocity describes the speed of an object and the direction it
travels. Such a quantity is called a vector.
Direction can be described as North, South, etc., or it can be
described mathematically, e.g. along the x-axis, the y-axis,
etc. Vectors are often drawn as arrows. A larger velocity
would be given a longer arrow.
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PHYS 214: The Nature of Physics
Question 5: Consider various types of weather maps and
navigational maps of the North Atlantic, that plot out
temperatures, storm conditions, etc. Which of the following
quantities might best be represented as vectors on the map?
A. Air temperature
B. Water temperature
C. Ocean currents
D. Barometric pressure
E. None of the above
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PHYS 214: The Nature of Physics
Figure 2.5
The direction of the velocity changes as the car moves
around the curve, so that the velocity v2 is not the same as
the velocity v1 even though the speed has not changed.
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PHYS 214: The Nature of Physics
Figure 2.6
The direction of the velocity changes when
a ball bounces from a wall. The wall exerts a force
on the ball in order to produce this change.
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PHYS 214: The Nature of Physics
Acceleration
Acceleration is the change in the velocity of an object
during a certain time interval.
[If a car is moving in the positive direction]
A car that speeds up has a positive acceleration.
A car that slows down has a negative acceleration.
Acceleration = {final velocity - initial velocity}/time
Because Acceleration is a vector; a negative
acceleration can mean that the car is speeding up in the
negative direction!
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PHYS 214: The Nature of Physics
Summary of Chapter 2:
1. Speed is distance divided by time. “Average” speed is
over a longer time interval; “instantaneous” is over the
shortest possible time.
d
speed
t
2. Velocity is a vector which denotes both the size of the
speed, as well as its direction.
3. Acceleration is how much the velocity changes during
a specified time interval:
Dv
acceleration
t
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PHYS 214: The Nature of Physics
Question 5:
Consider what happens when the far end of the track is
lifted up. The ball will move up, then roll back down.
Which of these is true:
A. First the acceleration is positive, then it is negative.
B. The acceleration is constant.
C. The velocity is constant.
D. The speed is constant, but the velocity changes.
E. None of the above.
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PHYS 214: The Nature of Physics