Motion in One Dimension Free Falling Objects

1. Motion in One DimensionFree Falling Objects Chapter 2: Section 3 Source: http://www.physicsclassroom.com

2. Learning Targets • Relate the motion of a freely falling body to motion with constant acceleration • Calculate displacement, velocity, and time at various points in the motion of a freely falling object • Compare the motions of different objects in free fall P2.1D, P2.1g, P2.2C

3. The Big Misconception • "Wouldn't an elephant free-fall faster than a mouse?“ • If we are considering the specific type of falling motion known as free-fall, the objects move under the sole influence of gravity • More massive objects will only fall faster if there is an appreciable amount of air resistance present. • However, objects that are truly in free fall do not encounter air resistance. • Subsequently, all objects free fall at the same rate of acceleration, regardless of their mass.

4. Air Resistance • In the absence of air resistance, all objects dropped near the surface of a planet fall with the same constant acceleration • This is why a feather and an apple fall at the same rate if dropped in a vacuum • In the presence of air resistance, an object will eventually reach terminal velocity

5. What is Free Fall? • A free falling object is one that is falling under the sole influence of gravity. • Any object that is being acted upon only by the force of gravity is said to be in a state of free fall.

6. Characteristics of Free Fall • There are two important characteristics of free falling objects: • Free-falling objects do not encounter air resistance. • All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s/s (m/s2) • The acceleration due to gravity is denoted with the symbols ag or g (on the surface of the Earth)

7. Reviewing Acceleration • Remember that acceleration is the ratio of velocity change to time between any two points in an object's path. • To accelerate at 9.8 m/s/s means to change the velocity by 9.8 m/s each second.

8. An Example of Free Fall • If the velocity and time for a free-falling object being dropped from a position of rest were tabulated, then one would note the following pattern:

9. Graphing Free Fall • Position vs. Time Graph • The slope of any position vs. time graph is the velocity of the object • A curved line on a position versus time graph signifies accelerated motion • The small initial slope indicates a small initial velocity and the large final slope indicates a large final velocity. • The negative slope of the line indicates a negative (i.e., downward) velocity.

10. Graphing Free Fall • Velocity vs. Time Graph • The slope of any velocity vs. time graph is the acceleration of the object • A diagonal line on a velocity versus time graph signifies accelerated motion • Constant negative slope indicates a constant negative acceleration

11. Acceleration is constant during upward and downward motion

12. When we throw an object up in the air, it will continue to move upward for some time, stop momentarily at the peak, and then change direction and begin to fall. • Because the object changes direction, it may seem that the velocity and acceleration are both changing. • Actually objects thrown into the air have a downward acceleration as soon as they are released

14. Velocity and Acceleration During Free Fall • Free falling objects always have the same downward acceleration • When going up, velocity is positive and acceleration is negative (-9.8 m/s2) - the object is slowing down • When falling down, velocity is negative and acceleration is negative (-9.8 m/s2) - the object is speeding up • Remember that when the signs of velocity and acceleration are the same, an object speeds up. When they are opposite, an object is slowing down

15. Calculating Free Fall Velocity • To calculate velocity during free fall, use the velocity with constant acceleration equations vf2 = vi2 + 2a∆y vf = vi+ a∆t *a = -9.8 m/s2 • You can use any of the kinematic equations to solve free fall problems