Transcript ppt
Acceleration due to Gravity CCHS Physics Historical Background Aristotle (Greek philosopher/scientist) divided motion into two main classes: Natural Motion -Results from the "nature" of objects Objects strive to get to their proper place EX- clay vs. smoke vs. feather Violent Motion -Results from pushing or pulling forces Externally caused- outside force required EX- pushing a cart, tugging a rope Hypothesis: Objects fall at speeds proportional to their weights SummaryAll motion results from the nature of the moving object or from a sustained push or pull Galileo attempts to prove Aristotle’s hypothesis wrong… • He drops objects of various weights from the Leaning Tower of Pisa and compares the falls. • He thinks that light and heavy objects will fall at the same rate. • We’ll find out who is correct with a demonstration… Let’s See The Real Thing QuickTime™ and a Sorenson Video decompressor are needed to see this picture. So who’s right? • Both- why? • In the absence of air resistance, all objects accelerate towards Earth at the same rate. • We call this “free-fall” acceleration- meaning gravity is the only force acting on an object. • Because we use this value so often we’ll give it a name- g • What do you think g depends on? For an object in free-fall, g depends on… • Mass of the planet object is near • Distance from object to the center of the planet • GM g 2 r (G = universal gravitational constant) • Near the surface of the Earth, g = -9.8 m/s2 Free Fall • Motion of a body when only the force of gravity is acting on the body • The acceleration of an object in free fall is called the acceleration due to gravity, or free fall accleration • Free fall acceleration is denoted by the symbol g Free Fall Movie Handy Chart Velocity and Acceleration Movie Let’s try some examples Estimate the following: • A stone is dropped off a cliff. What is its velocity 5 seconds later? a v f vi t 10 vf 0 5 v f 50 m/s • A ball is tossed straight up at 30 m/s. How long will it take to land? (It returns to same height.) a v f vi t 30 30 10 t6s t Let’s try some more … a little harder though • Ms. Clark throws a ball straight down off a building with an initial speed of 10 m/s. It lands 3 seconds later. How high was the building? 1 2 d at vi t 2 1 d 10 32 10 3 2 d 75 m h 75 m One More Example • Jason hits a volleyball so that it moves with an initial velocity of 6 m/s straight upward. If the volleyball starts from 2 m above the floor, how long will it be in the air before it strikes the floor? – Choose origin to be initial position of ball – The displacement of the ball is -2 m 1 2 d at vi t 2 1 2 9.8 t 2 6t 2 t 1.5s or t 0.27s