1) average speed is the total distance traveled divided by the time taken to cover the distance. we...
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1) Average speed is the total distance traveled divided by the time taken to cover the distance.
• We use the letters s for speed, d for distance, and t for time whence:
• sav = d/t.• The SI units of speed are
m/s.• Usain Bolt traveled 100 m in
9.63 s. What was his average speed?
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2) A multi-exposure photograph is made in a totally dark room with a strobe and a camera with an open shutter.
• If you travel 40 miles in 2 hours your average speed is sav = 40 mi/2 h = 20 mi/h = 20 mph.
• Average speed tells you nothing about the details of the trip.
• What is happening to the speed of the blue ball?
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3) Instantaneous speed is equal to the average speed over a time interval that is very, very small.
• For practical purposes we need a time interval that is small enough that the average speed does not change very much if we use an even smaller time interval.
• Think of instantaneous speed as what a speedometer says. How does it work?
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4) Velocity is speed and direction. The magnitude of velocity is speed. Quantities that have both magnitude and direction are called vectors.
• Average velocity is displacement divided by time. vav = Dx/t.
• Displacement is a vector quantity whose magnitude is the straight-line distance from A to B and whose direction is the direction from A to B.
• Is the displacement vector ever longer than the distance traveled?
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5) Average acceleration is the change in velocity divided by the time it takes to make that change.
• We use the letters a for acceleration, v for velocity, and t for time whence:
• aav = Dv/Dt.• The SI units of acceleration
are m/s2.• When you travel in a
straight line at a constant speed your acceleration is zero.
• The falling ball accelerates!
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6) According to Aristotle, a heavy physics text should fall significantly faster than a crumpled piece of paper.
• According to Galileo, in a vacuum, the falling speeds of a cannonball and a feather should be equal.
• Astronauts conducted an ultramodern demonstration of this on the Moon where a hammer and a feather fell at the same rate.
• Why does the hammer fall faster on Earth?
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7) As Galileo let balls roll down steeper ramps he discovered the following rule: the acceleration in each case is constant.
• Constant acceleration means the speed changes by the same amount each second.
• In the case of free fall the acceleration due to gravity is about 9.8 m/s2.
• We use the letter g for this acceleration and usually round it off whence:
• g = 10 m/s2.
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8) If you throw a ball vertically upward with an initial speed of 20 m/s, one second later its instantaneous speed will be 10 m/s.
• As the ball continues to rise against gravity it continues to lose speed. It will reach its maximum height in 2 seconds.
• At that moment the instantaneous speed of the ball will be zero.
• Is the ball rising or falling?
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Exercise 58
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Exercise 60
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Exercise 62
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Exercise 64
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Exercise 66
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Exercise 68
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Exercise 70
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Exercise 72
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Exercise 74
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Exercise 76
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Exercise 80