Free Fall

A woman hits her car brakes in a panic stop and decelerates at a rate of 5 m/s 2. The car comes to rest at a distance of 62.5 m from the point where she first applied her brakes. How fast was she traveling when she hit her brakes? A spaceship is traveling at 3000 m/s when its booster rocket is fired accelerating it at a rate of 15 m/s2. What distance does the spaceship travel during its first 4 seconds of acceleration? A dragster is uniformly accelerated from 10 m/s over a time of 4.5 s and a distance of 100 m. What was the acceleration?

It pulls things down towards the Earth

In Space, Gravity keeps the planets moving around the sun.

Aristotle Aristotle’s Theory: Objects fall with a speed that was proportional to the weight, and inversely proportional to the density of the medium the body was falling through. Aristotle (384 BC – 322 BC) was a Greek philosopher, a student of Plato and teacher of Alexander the Great.

What that means is that for the natural motion of falling objects to the Earth, Aristotle asserted that heavy objects fall faster then lighter objects. Aristotle He also thought that there was some acceleration, due to as the body fell its weight increased and it speeded up.

The way Aristotle saw objects to fall on the Earth As we know, objects actually fall like this in a friction free environment (vacuum)

These views of Aristotle did not go unchallenged. Thirty years or so after Aristotle's death, Strato, like Aristotle, believed in close observation of natural phenomena, but he observed much more carefully than Aristotle, and realized that falling bodies usually accelerate. Aristotle

Strato He made two important points: rainwater pouring off a corner of a roof is clearly moving faster when it hits the ground than it was when it left the roof, because a continuous stream can be seen to break into drops which then become spread further apart as they fall towards the ground. His second point was that if you drop something to the ground, it lands with a bigger thud if you drop it from a greater height. One is forced to conclude that falling objects do not usually reach some final speed in a very short time and then fall steadily, which was Aristotle’s picture.

Galileo Galilei Galileo formulated the laws that govern the motion of objects in free fall Also looked at: –Inclined planes –Relative motion –Thermometers –Pendulum

Galileo’s experiment to discover the effects of gravity. Look out Below! All objects fall with the same acceleration regardless of the weight of the object.

Mathematician and Scientist Gravity was discovered by a famous scientist:

Freely Falling Bodies In the absence of air resistance, it is found that all bodies at the same location above the earth fall vertically with the same acceleration. The acceleration rate is still a = 9.81 m/s 2

Freely Falling Bodies In the absence of air resistance, it is found that all bodies at the same location above the earth fall vertically with the same acceleration. Furthermore, if the distance of the fall is small compared to the radius of the earth, the acceleration remains essentially constant throughout the fall.

Freely Falling Bodies In the absence of air resistance, it is found that all bodies at the same location above the earth fall vertically with the same acceleration. Furthermore, if the distance of the fall is small compared to the radius of the earth, the acceleration remains essentially constant throughout the fall. This idealized motion, in which air resistance is neglected and the acceleration is nearly constant everywhere, is known as free-fall.

Freely Falling Bodies What that means is the equations of motion that we have just learned can be used and applied to the motion of freely falling bodies!!! In the absence of air resistance, it is found that all bodies at the same location above the earth fall vertically with the same acceleration. Furthermore, if the distance of the fall is small compared to the radius of the earth, the acceleration remains essentially constant throughout the fall. This idealized motion, in which air resistance is neglected and the acceleration is nearly constant everywhere, is known as free-fall.

PlanetRadius (m)Mass (kg)g (m/s 2 ) Mercury2.43 x x Venus6.073 x x Mars3.38 x x Jupiter6.98 x x Saturn5.82 x x Uranus2.35 x x Neptune2.27 x x Pluto1.15 x x Acceleration Due to Gravity

Taking the uniform acceleration equations and now applying them to free fall means that we now always know that a = g as a given in the problem. The equations change to the form: Acceleration Due to Gravity

Heavy and light objects fall at the same rate

In the absence of air resistance ALL objects, regardless of their mass, accelerate towards the Earth at the same rate when dropped from rest The acceleration rate due to gravity is given symbol g g =  9.81 m/s 2 for all objects close to Earth’s surface Freely Falling Objects

Two Types of Free Fall Problems Object is dropped from rest and falls straight down (1 – D) Object is launched straight up with an initial upward velocity and returns to initial position

Free Fall Summary All objects moving under the influence of gravity only are said to be in free fall –Free fall does not depend on the object’s original motion All objects falling near the earth’s surface fall with a constant acceleration The acceleration is called the acceleration due to gravity, and indicated by g

Acceleration rate due to gravity close to Earth’s surface is constant for ALL objects regardless of their mass. (symbol = g) g = 9.81 m/s 2 (magnitude) and is always directed straight down a = g =  9.81 m/s 2 (constant) will be used in the Kinematic Equations The only forces acting on an object in free fall are its weight and air resistance Neglect air resistance at all times Free Fall Summary

Calculate Height A stone is dropped down a well and it takes 1.6 seconds to reach the bottom. How deep is the well? You may assume the initial speed of the stone is zero.

Calculate Height

Free Fall A stone is dropped from rest from the top of a tall building, as the figure indicates. After 3.0 s of free-fall, a. what is the velocity v of the stone? b. what is the displacement ∆s of the stone?

Free Fall A stone is dropped from rest from the top of a tall building, as the figure indicates. After 3.00 s of free-fall, a. what is the velocity v of the stone? b. what is the displacement ∆s of the stone?

An eagle in majestic flight has an upset tummy, the eagle unloads a surprise from above that has an initial speed of 3.5 m/s and falls freely from the sky. This lands on top of an unsuspecting person after 7.0 seconds? What is the velocity of the surprise when it lands? How far does it fall during this time? Free Fall

An eagle in majestic flight has an upset tummy, the eagle unloads a surprise from above that falls freely from the sky. This lands on top of an unsuspecting person after 7.0 seconds? What is the velocity of the surprise when it lands? How far does it fall during this time? Given:Solution: Free Fall

A cannonball is dropped from the top of the leaning tower of Pisa. If the point of release is 30 m above the ground, what is the velocity of the cannonball just before it strikes the ground? How long does it take to reach the ground? Free Fall

A cannonball is dropped from the top of the leaning tower of Pisa. If the point of release is 30 m above the ground, What is the velocity of the cannonball just before it strikes the ground? How long does it take to reach the ground? Given:Solution: Free Fall

Acceleration due to Gravity Physics Kinematic Motion The End.

QuickLab: Reaction Time Person drops the ruler, holding it just above you hand. Once it drops you try to catch it as quickly as you can. Using the idea of free fall of the meter stick, determine the time it takes for you to close you fingers and catch the meter stick.