Week 2 - Chapter 2 - Kinematics Flashcards
What is defined as the study of motion without considering causes?
Kinematics
The word “kinematics” comes from a Greek term meaning motion and is related to other English words such as “cinema” (movies) and “kinesiology” (the study of human motion).
What is the simplest type of motion aka one-dimensional kinematics?
Motion along a straight line, or one-dimensional motion
In order to describe the motion of an object, what must you be first able to describe?
Its position; where it is at any particular time.
What is a change in position known as?
Displacement which implies that an object has moved, or has been displaced.
Concept Review
▲x = displacement, xf = final position, x0 is the initial position
▲x = xf - x0, this is the change in position of an object
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What does ▲(delta) always mean within the text?
“change in” whatever quantity follows it; thus, ▲x means change in position.
This is always solved by subtracting the initial position from the final position; xf - x0 = ▲x
Note that the SI unit for displacement is the meter (m), but sometimes kilometers, miles, feet, and other units of length are used. Keep in mind that when units other than the meter are used in a problem, you may need to convert them into meters to complete the calculation.
For x, it is “the location relative to the reference frame” (i.e., airplane, earth, etc.)
What does displacement have?
A direction AND magnitude.
In one-dimensional motion, direction can be specified with a plus or minus sign. When you begin a problem, you should select which direction is positive (usually that will be to the right or up, but you are free to select positive as being any direction).
What is distance defined as?
It is defined as the magnitude or size of displacement between two positions and has no direction.
For example, the professor’s ▲x = +2.0m (displacement)
For distance of what the professor walks = 2.0 m (distance is magnitude only with no sign)
Explain how distance or magnitude of displacement can be different than “distance traveled.”
For the professor problem, he could walk back and forth in front of the whiteboard accumulating 150 m traveled yet he may only have a magnitude of displacement from his starting position of 2 m.
For example, the professor could pace back and forth many times, perhaps walking a distance of 150 m during a lecture, yet still end up only 2.0 m to the right of her starting point.
In this case her displacement would be +2.0 m, the magnitude of her displacement would be 2.0 m, but the distance she traveled would be 150 m.
In kinematics we nearly always deal with displacement and magnitude of displacement, and almost never with distance traveled. One way to think about this is to assume you marked the start of the motion and the end of the motion. The displacement is simply the difference in the position of the two marks and is independent of the path taken in traveling between the two marks. The distance traveled, however, is the total length of the path taken between the two marks.
What is displacement and distance an example of when it comes to quantities?
Displacement is an example of vector quantity.
Distance is an example of a scalar quantity.
Since displacement is an example of a vector quantity, what is a vector quantity?
How do you annotate a vector in one-dimensional motion?
A vector is any quantity with both magnitude and direction. Other examples of vectors include a velocity of 90 km/h east and a force of 500 newtons straight down.
With a + or -
Vectors are represented graphically by arrows. An arrow used to represent a vector has a length proportional to the vector’s magnitude (e.g., the larger
the magnitude, the longer the length of the vector) and points in the same direction as the vector.
Since distance is an example of a scalar quantity, what is a scalar quantity?
A scalar is any quantity that has a magnitude, but no direction. For example, a 20 degree Celsius temperature, the 250 kilocalories of energy in a candy bar, a 90 km/h speed limit, a person’s 1.8m height, and a distance of 2.0 m are all scalars–quantities with no specified direction.
Note: A scalar can be negative, such as -20 degrees Celsius temperature. In this case, the minus sign indicates a point on a scale rather than a direction. Scalars are never represented with arrows.
What does every measurement of time involve?
A change in some physical quantity.
In physics, time is change or the interval over which change occurs.
Concept Review
▲t = elapsed time, tf = is the time at the end of motion, t0 = is the time as the start of motion.
▲t = tf - t0, this is the change in time or elapsed time
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What is average velocity and how is it calculated?
Average velocity is displacement (change in position; vector quantity) divided by the time of travel. See photo.
Notice that this definition indicates that velocity is a vector because displacement is a vector. It has both magnitude and
direction. The SI unit for velocity is meters per second or m/s, but many other units, such as km/h, mi/h (also written as mph),
and cm/s, are in common use.
Scenario for average velocity and average speed (No answer)
For example, if you drive to a store and return home in half an hour, and your car’s odometer shows the total distance traveled was 6 km, then your average speed was 12 km/h. Your average velocity, however, was zero, because your displacement for the round trip is zero. (Displacement is change in position and, thus, is zero for a round trip.) Thus average speed is not simply the magnitude of average velocity.
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What is the definition and equation of average acceleration?
It is the RATE at which velocity changes.
What does deceleration always refer to?
It refers to acceleration in the direction opposite to the direction of velocity.
Deceleration always reduces speed. Negative acceleration, however, is acceleration in the negative direction in the chosen coordinate system.
Negative acceleration may or may not be deceleration, and deceleration may or may not be considered negative acceleration.
See image for details.
Properly label the acceleration in respect to its coordinate system.
a - Positive or negative acceleration and why?
b -
c -
d -
a - The car is speeding up as it moves to the right therefore, it has positive acceleration in our coordinate system.
b - The car is slowing down as it moves to the right. Therefore, it has negative (left on the x axis) acceleration in our coordinate system, because its acceleration is toward the left. This is deceleration because the direction of acceleration is opposite to its direction of motion.
c - The car is moving toward the left, but slowing down over time. Therefore, its acceleration is positive in our coordinate system because it is toward the right. However, the car is decelerating because its acceleration is opposite to its motion.
d - This car is speeding up as it moves towards the left. It has negative acceleration because it is accelerating toward the left. However, because its acceleration is in the same direction as its motion, it is speeding up (not decelerating).
What is instantaneous acceleration a?
It is the acceleration at a specific instant in time, and is obtained by the same process as discussed for instantaneous velocity, that is, by considering an infinitesimally small interval of time.
