Section 2 review
The term free fall describes any state of motion in which an object has a constant downward acceleration due only to gravity. Near Earth’s surface, the magnitude of that acceleration is 9.8 m/s2 (also known as one “g”). An object can be in free fall even if it is rising. When a rising projectile reaches its maximum height, its vertical velocity is momentarily zero (i.e., it is neither rising nor falling). A falling object reaches terminal velocity when the upward force of air resistance on the object equals the object’s weight. Read the text aloud
free fall, terminal velocity

Review problems and questions

  1. Which of the following objects are in free fall? (Assume that you can ignore air resistance.)
    1. a textbook lying on a desk
    2. a textbook that has been tossed upward and has left the student’s hands but not yet touched any other object or surface
    3. a textbook that has been thrown downward from a tall cabinet
    4. a textbook held tightly by a running student
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  1. Josh tells Amanda that acceleration must be a negative number whenever an object is in free fall. “After all,” he claims, “gravity pulls objects downward, and down is negative.” Amanda replies that the acceleration in a free fall problem can be negative or positive. “You just have to make sure all related quantities, like displacement and velocity, are handled the same way,” she adds. Who is right, and why? (Answer in complete sentences, and cite a specific passage in the text to support your claim.)
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  1. Zorgata drops her ball a distance of 1 m on the surface of planet Zogg. Izqif wants his ball to take twice as much time to fall as Zorgata’s. What height should he drop it from? (Note that gravity is stronger on the surface of Zogg than it is on Earth.)
    1. 2 m
    2. 3 m
    3. 4 m
    4. You cannot say without knowing the value for g on planet Zogg.
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  1. During the Apollo 15 mission, astronaut David Scott performed a scientific experiment on the surface of the Moon. Scott dropped two objects at the same time from the same height. One was a 1.3 kg hammer; the other was a 0.03 kg feather. He dropped the two objects from a height of 1.6 m. The two objects fell, and both hit the Moon’s surface 1.4 s after being released.
    1. What is the acceleration due to gravity on the Moon’s surface, in units of meters per second squared?
    2. Were Scott’s hammer and feather both in free fall?
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