1. Gravity and Motion Chapter 2 Section 1 notes

2. Objective  Students will be able to describe the effect of gravity and air resistance on falling objects.

3. Demonstration Time  Up front I have a cup of water with a small hole in the bottom.  First, I will show you what happens to the water when I remove my finger covering the hole.  Next, I will show you what happens to the water when I let the cup go as I remove my finger from covering the hole.  What did you notice? Why does this happen?

4. Explanation  In trial 1 we should have noticed the water coming out of the hole in the cup.  During trial 2, if we look closely we should notice that there is no water coming out of the hole in the cup. Why?  Both objects (the water and the cup) are traveling at a constant acceleration due to gravity.  Do we know the rate at which these two objects fall?

5. Gravity and Acceleration  Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects on Earth.  Acceleration is the rate at which velocity changes over time.

6. 9.8 all day every day!  All objects accelerate toward the Earth at a rate of 9.8 m/s 2, or m/s/s.  This means for every second that an object falls, the object’s downward velocity increases by 9.8 m/s.  You can calculate the change in velocity ( v) of a falling object using the following equation:  v= g*t *Remember v= Final Velocity-Initial Velocity  Where g= 9.8 m/s/s and t=time in seconds.

7. The power of the triangle v g t

8. Lets look at an example  A stone at rest is dropped from a cliff, and the stone hits the ground 3 seconds later. What was the stones velocity when it hits the ground?

9. Lets solve for time  A marble at rest is dropped from a tall building. The marble hits the ground with a velocity of 98 m/s. How long was the marble in the air?

10. Think about it  What would hit the ground first, a regular piece of paper or a ball of paper?  Why do these hit the ground at different times if gravity is acting on all objects the same?  Did we just defy physics in room 333?  NO!!!

11. Lets use what we know  If we know all objects accelerate toward the Earth at 9.8 m/s/s then there must be a force acting against the flat piece of paper.  Well what is it?!!  This force is called air resistance  Air resistance is defined as the force that opposes an object’s motion through air.

12. More on Air Resistance  Factors that determine the amount of air resistance acting on an object depend on the size, shape, and speed of the object.  The flat piece of paper had more surface area creating more air resistance which slowed the acceleration due to gravity. This is why the paper ball hit first!

13. Terminal Velocity  As the speed of a falling object increases, so does the amount of air resistance.  Once the force of air resistance is equal to the force of gravity (in the opposite direction) we reach terminal velocity.  Once you reach terminal velocity you don’t accelerate anymore and your speed stays the same.

14. Free…Free Falling  An object is in free fall when gravity is the only force pulling the object down and there are no other forces acting on it.  Because air resistance is a force, free fall can only occur where there is no air. There are only 2 places that have which are in space and in a vacuum. (a vacuum is a place with no matter)

15. Famous NASA video  Physics in Space! Physics in Space!

16. Misconception Alert!!!  True or False…  Astronauts float in their space shuttle because there is no gravity in space.  Astronauts are weightless in space.

17. Orbiting Objects are in Free Fall  It is impossible to be weightless anywhere in the universe!  Weight is a measure of gravitational force. The amount of the force depends on the mass and distances between the two objects.  Astronauts float in orbiting spacecrafts because of free fall.

18. 2 Motions combine to Cause Orbiting  An object is orbiting when it is traveling around another in space.  When a spacecraft orbits Earth it is moving forward as well as free falling. Check out figure 7 on page 40 in your textbook!

19. Orbits and Centripetal Force  Many objects are in orbit in the universe  Ex. Satellites, moons, planets...  When traveling in a circle you are constantly changing direction. An unbalanced force must be present for that to occur.  The unbalanced force that causes objects to move in a circular path is called Centripetal force.  The word centripetal means “toward the center”

20. Projectile Motion and Gravity  Projectile motion is defined as the curved path an object follows when thrown or propelled near the center of the Earth.  Projectile motion has 2 components…  A horizontal motion  A vertical motion  The two components are independent of each other, so they have no effect on each other

21. Projectile motion continued…  When the two motions are combined, they form a curved path. Horizontal motion (force applied) Vertical motion (gravity) Overall path (motions combined)