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27 Cards in this Set
- Front
- Back
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speed
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How fast something moves; the distance traveled per unit of time. Speed = distance/time. (p.36)
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instantaneous speed
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The speed at any instant. (p.37)
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average speed
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The total distance traveled divided by the time of travel. Average speed = total distance covered/time interval. (p.37)
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velocity
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The speed of an object and a specification of its direction of motion. (p.38)
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vector quantity
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Quantity in physics that has both magnitude and direction. (p.38)
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scalar quantity
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Quantity that can be described by magnitude without direction. (p.38)
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acceleration
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The rate at which velocity changes with time; the change in velocity may be in magnitude, or direction, or both. Acceleration = change of velocity/time interval. (p.39)
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free fall
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Motion under influence of gravity only. On Earth, free-fall acceleration is approximately 9.8 m/s2. (p.41)
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total distance covered
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average speed x time
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"She moves at a constant speed in a constant direction." Rephrase the same sentence in fewer words.
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"She moves at constant velocity."
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The speedometer of a car moving to the east reads 100 km/h. It passes another car that moves to the west at 100 km/h. Do both cars have the same speed? Do they have the same velocity?
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Both cars have the same speed but they have opposite velocities because they're moving in opposite directions.
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During a certain period of time, the speedometer of a car reads a constant 60 km/h. Does this indicate a constant speed? A constant velocity?
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The constant speedometer reading indicates a constant speed but not a constant velocity because the car may not be moving along a straight-line path, in which case it is accelerating.
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A hungry mosquito sees you resting in a hammock in a 3-m/s breeze. how fast and in what direct should the mosquito fly in order to hover above you for lunch?
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The mosquito should fly toward you into the breeze. When just above you, it should fly at 3 m/s in order to hover at rest. Unless its grip on your skin is strong enough after landing, it must continue flying 3 m/s to keep from being blown off. That's why a breeze is an effective deterrent to mosquito bites.
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What is the average speed of a cheetah that sprints 100 meters in 4 seconds? If it springs 50 m in 2 s?
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25 m/s
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If a car moves with an average speed of 60 km/h for an hour, it will travel a distance of 60 km.
a. How far would it travel if it moved at this rate for 4 h? b. For 10 h? |
a. 240 km
b. 600 km |
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In addition to the speedometer on the dashboard of every car is an odometer, which records the distance traveled.
a. If the initial reading is set at zero at the beginning of a trip and the reading is 40 km one-half hour later, what has been your average speed? b. Would it be possible to attain this average speed and never go faster than 80 km/h? |
a. 80 km/h
b. No, not if the trip starts from rest and ends at rest. There are times in which the instantaneous speeds are less than 80 km/h, so the driver must drive at speeds of greater than 80 km/h during one or more time intervals in order to average 80 km/h. |
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A particular car can go from rest to 90 km/h in 10 s. What is its acceleration?
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Its acceleration is 9 km/h*s. Strictly speaking, this would be its average acceleration, for there may have been some variation in its rate of picking up speed.
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In 2.5 s, a car increases its speed from 60 km/h to 65 km/h while a bicycle goes from rest to 5 km/h.
a. Which undergoes the greater acceleration? b. What is the acceleration of each? |
Although the velocities are quite different, the rates of change of velocity are the same; hence, the accelerations of both are 2 km/h*s.
car acceleration = (65 km/h - 60 km/h)/2.5 s = 2 km/h*s bike acceleration = (5 km/h - 0 km/h)/2.5 s = 2 km/h*s |
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deceleration
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A decrease in velocity; the term acceleration applies to decreases as well as to increases in velocity (such as the retarding accelerations produced by the brakes of a car), we often call this ___________. (p.39)
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What is the acceleration of a race car that whizzes past you at a constant velocity of 400 km/h?
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Zero, because its velocity does not change.
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Which has the greater acceleration, an airplane that goes from 1000 km/h to 1005 km/h in 10 seconds or a skateboard that goes from zero to 5 km/h in 1 second?
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Both gain 5 km/h, but the skateboard does so in one-tenth of the time. The skateboard therefore has the greater acceleration - in fact, ten times greater. A little figuring will show that the acceleration of the airplane is 0.5 km/h*s, whereas acceleration of the slower-moving skateboard is 5 km/h*s. Velocity and acceleration are very different concepts, distinguishing between them is very important.
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velocity acquired
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acceleration x time (p.40)
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v=gt
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The instantaneous velocity v of an object falling from rest after a time t. (p.42)
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What would the speedometer reading on a falling rock shown with a beginning velocity of 0 be 5 s after it drops from rest? How about 6 s after it is dropped? How about 6.5 s after it is dropped?
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The speedometer readings would be 50 m/s, 60 m/s, and 65 m/s, respectively.
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A ball is thrown straight upward and leaves your hand at 20 m/s. What predictions can make about the ball? (Please think about this before reading the suggested predictions!)
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First, it will slow by 10 m/s, so it will reach a velocity of 0 at the top of its path 2 seconds after it leaves your hand. Then it will begin to pick up speed at a rate of approximately 10 m/s, so that 4 seconds after leaving your hand it it will hit the ground.
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distance traveled
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d = 1/2 (acceleration x time x time) = 1/2 gt2 (p.43)
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A cat steps off a ledge and drops to the ground in 1/2 second.
a. What is its speed on striking the ground? b. What is its average speed during the 1/2 second? c. How high is the ledge from the ground? |
a. Speed: v = gt = 10 m/s2 x 1/2 s = 5 m/s
b. Average speed: (initial v + final v)/2 = (0 m/s + 5 m/s)/2 = 2.5 m/s c. Distance: d = 2.5 ms x 1/2 s = 1.25 m OR d = 1/2 gt2 = 1/2 x 10 m/s2 x (1/2s)2 = 1.25 m |
