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# Think and Explain Physics

AB
the difference between saying 1 qty is proportional to another and saying it is = to anotherproportional - a change in 1 qty implies a corresponding change in the other; when 1 qty is = to the other - identical
If an object has no aceleration, can you conclude that no forces are exerted on it?No, you can conclude no net force is acting on it
A rocket fired from its launching pad not only picks up speed, but its acceleration also increases significantly as firing continues. Why is this so? (Hint - about 90% of the mass of a newly launched rocket is fuel.)Since mass decreases as fuel is expended, acceleration increases
If you push horizontally on your book with a force of 1 N to make the book slide at constant velocity, how much is the force of friction on the book?1 N in the opposite direction to the push
When blocking in flootball, why does a defending lineman often attempt to get his body under that of his opponent and push upward?To reduce ground contact and friction between the opponent's feet and the ground
Why does a sharp knife cut better than a dull knife?Smaller cutting area produces a greater pressure for a given force
An aircraft gains speed during takeoff due to the constant thrust of its engines. When is the accesleration during takeoff greatest, at the beginning of the run along the runway of before the aircraft lifts into the air? Think, then explain.At the beginning of takeoff, when air drag is least
As a sky diver falls faster and faster through the air (before reaching terminal speed)does the net force on her increase, decr4ease or remain unchanged? Does her acceleration increase, decrease or remain unchanged? Defend your answers.Decreases; decreases; resisted by air drag
After she jumps, a sky diver reaches terminal speed after 10 seconds. Does she gain more speed during the first second of fall or the ninth second of fall? Compared with the first second of fall, does she fall a greater of a lesser distance during the ninth second?Maximum gain in speed ocurs during the first second of fall. Since her speed increases, she will be falling faster during the last second of fall, and therefore cover the greatest distance during this time.
A regular tennis ball and another one filled with sand are dropped at the same time from the top of a high building. Your friend says that even though air resisstance is present, both balls should hit the ground at the same time because they are the same size and "plow through" the same amount of air. What do you say?The heavier ball falls through the air with heavier acceleration

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