Module 9 Flashcards
Module 9 proficiency
If an object travels for 15 minutes with a constant velocity of 12 miles per hour west, what is the
acceleration?
Since the velocity is not changing, the acceleration is zero. The time was just put in there to fool
you. Remember, acceleration is the change in velocity. With no change in velocity, there is no
acceleration.
Acceleration
The time rate of change of an object’s velocity
Free fall
The motion of an object when it is falling solely under the influence of gravity
A long, vertical glass tube contains a feather and a penny. All the air is pumped out, and the tube
is inverted, causing the penny and the feather to fall. Which hits the bottom first, the feather or the
penny?
Neither will hit first. They both hit together. Remember, gravity accelerates all objects the same.
Without air, there is no air resistance, so both objects are in true free fall. As a result, they will fall at
exactly the same speed.
Scalar quantity
A physical measurement that does not contain directional information
Even though the physicist in question #12 is technically correct, why do we go ahead and assume
that heavy objects are in free fall when they fall near the surface of the earth?
Even though the physicist is technically correct, the effect of air resistance is so small on heavy
objects that it usually can be ignored.
Reference point
A point against which position is measured
A scientist decides to study the acceleration of an object moving in a straight line. He measures
the distance the object travels in 30-second time intervals. The scientist notices that in each interval,
the object travels a shorter distance than it did in the previous interval. Is the direction of the
acceleration the same as or opposite of the velocity?
Since the object covers less ground in each time interval, it is traveling slower in each time
interval. This clearly means that the object is slowing down. When an object is slowing, its
acceleration is in the opposite direction of its velocity.
Vector quantity
A physical measurement that contains directional information
A glass of water sits on a counter. Is it in motion?
The glass of water is moving relative to many reference points. To someone standing still in the
house, the glass is not moving. However, relative to any object not on earth, it is in motion. In fact, if
you walk towards the glass, the glass is in motion relative to you, because the glass’s position relative
to you changes!
A child is floating in an inner tube on a still lake. His position does not change relative to a tree on
the shore. He watches two girls jog along the shore of the lake. The girls are keeping perfect pace
with each other. Neither is pulling ahead of nor falling behind the other.
a. Relative to whom is the child in motion?
b. Relative to whom is the first girl in motion?
c. Relative to whom is the second girl not in motion?
a. The child is in motion relative to the two girls. Even though the child is floating motionless, his
position relative to the girls is changing. Thus, he is in motion relative to the girls.
b. The first girl is in motion relative to the child. Since the position of her relative to the child is
changing, she is in motion relative to him.
c. The second girl is motionless relative to the first girl. The girls are keeping perfect pace. Thus,
their positions
A car and a truck are traveling north on a highway. The truck has a speed of 45 miles per hour and
the car has a speed of 57 miles per hour. If the truck is ahead of the car, what is the relative velocity?
As the picture shows, the car is behind the truck, but they are both traveling in the same direction.
Thus, we get their relative velocity by subtracting their individual speeds:
relative velocity = 57 miles per hour - 45 miles per hour = 12 miles per hour
Since the car is traveling faster than the truck, it is catching up to the truck. Thus, the relative velocity
is 12 miles per hour towards each other.
If an object’s position does not change relative to a reference point, is it in motion relative to that?
In order for motion to occur, an object’s position must change. Since this object’s position is notchanging, it is not moving relative to the reference point.
Label each quantity as a vector or scalar quantity. Also, identify it as speed, distance, velocity, acceleration, or none of these. a. 10 meters b. 1.2 meters/second2 east c. 3.4 feet/hour and slowing d. 56 liters e. 2.2 miles/minute west f. 2.2 millimeters/year
a. 10 meters
b. 1.2 meters/second2 east
c. 3.4 feet/hour and slowing
d. 56 liters
e. 2.2 miles/minute west
f. 2.2 millimeters/year
————————————————
a. This is a scalar quantity since it has no direction. Meters is a distance unit.
b. This is a vector quantity, because it has direction in it. The units are distance over time squared,
which is acceleration.
c. This is a scalar quantity. It has no direction. The units indicate it is speed.
d. This is a scalar quantity. It has no direction. Liters is a volume unit, so it is none of these.
e. This is a vector quantity, because it has direction in it. The units are distance over time, which is
velocity. It is not speed because speed is not a vector quantity.
f. This is a scalar quantity. It has no direction. The units indicate it is speed.
A very picky physicist states that it is impossible for any object to experience free fall near the
earth’s surface. Why is the physicist technically correct?
The physicist is technically correct because for an object to be in free fall, it can only be
influenced by gravity. Air resistance is a second influence, and all objects experience air resistance.
