# pg 56 - 56 Chapter4 Force and Motion i.1 In a freely...

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Unformatted text preview: 56 Chapter4 Force and Motion i .1 In a freely falling elevator on Earth, the book and person seem weightless because they fall with the same acceleration as the elevator. 1.. (5!) Like the elevator in (a), an orbiting spacecraft is falling toward Earth, and because its occupants also fall with the same acceleration, they experience apparent weighﬂessness. (b) ElﬁllllLUll Objects in free fall appear weightless because they all experience the same acceleration. stays put relative to you (Fig. 4.1021). To you, the book seems “weightless,” since it doesn’t seem to fall when you let go of it. And you’re “weightless” too; if you jump off the eleva— tor’s ﬂoor, you ﬂoat to the ceiling rather than falling back down. Of course you, the book, and the elevator are all falling, but because all have the same acceleration that isn’t obvious to you. The gravitational force is still acting; it’s making you fall. So you really do have weight, and your condition is best termed apparent weightlessness. A falling elevator is a dangerous place; your state of apparent weightlessness would end with a deadly smash caused by nongravitational contact forces when you hit the ground. But apparent weightlessness occurs permanently in a state of free fall that doesn’t intersect Earth-—as in an orbiting spacecraft (Fig, 4.1%). It’s not being in outer space that makes as- tronauts seem weightless; it’s that they, like our hapless elevator occupant, are in free fall—moving under the inﬂuence of the gravitational force alone. The condition of appar- ent weightlessness in orbiting spacecraft is sometimes called “microgravity.” M GOT IT? 4.3 A popular children’s book explains the weightlessness astronauts experi- ence by saying there’s no gravity in space; If there were no gravity in space, what would be the motion of a space shuttle, a satellite, or, for that matter, the Moon? 4.5 Using Newton’s Second Law The interesting problems involving Newton’s second law are those where more than one force acts on an object. To apply the second law, we-then need the net force. For an object of constant mass, the second law relates the net force and the acceleration: —. Fnet = m3 Using Newton’s second law with multiple forces becomes much easier if we draw a free- body diagram, a simple diagram that shows only the object of interest and the forces act- ing on it. ...
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