Gravity and Motion Chapter 2 Section 1 notes

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

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?

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?

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.

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.

The power of the triangle v g t

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?

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?

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!!!

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.

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!

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.

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)

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

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

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.

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!

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”

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

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)