Objectives

  • To investigate free fall
    • Objects dropped from a resting position are in free fall
    • Objects tossed into the air are in free fall
    • Orbiting objects are in free fall

Material for Each Group of Students

  • Awl or other sharp instrument
  • Towels (for cleanup)
  • Two Buckets
  • Two-liter plastic soda bottle
  • Water, enough to fill the bottle

Teacher Background Information

Objects are in free fall when gravity is the only force affecting their motion. Free fall occurs when objects are dropped from a resting position or tossed into the air (disregarding air friction). Orbiting objects are also in free fall because gravity is the only force affecting their motion. In this activity, students demonstrate free fall using water in a bottle.

Procedure

  • Have a volunteer read aloud, then discuss: When objects are dropped from a resting position, we say they are in free fall.
    • How would you describe free fall?
    • What makes objects fall when you drop them?
    • Why is the fall described as free?
Part A: Basic Free Fall
  • Use an awl or other sharp instrument to poke a small hole in the side of the bottle, near the bottom.
  • Put your finger over the hole.
  • Fill the bottle with water.
  • Balance the bottle in the palm of your hand and hold it as high as you can over the buckets. (The further the bottle falls in step F the easier it is to make Observations.)
  • Remove your finger from the hole and observe.
  • Before the water empties, drop the bottle into a bucket.
  • Repeat this process several times and note the results.
Questions
  • What do you observe when you unplug the hole?
  • Describe what happens when you drop the bottle.
  • What is falling in the 5th bullet under Part A: Basic Free Fall?
  • What is falling in the 6th bullet under Part A: Basic Free Fall?
  • How is gravity acting on the bottle and water?
  • How could you prevent the bottle from falling freely after you let it go?
  • Describe a situation in which you have been in free fall. What did it feel like?
Part B: Avanced Study of Free Fall
  • Using the same bottle as in Part A, put your finger over the hole in its side and fill the bottle with water.
  • oss the bottle into the air without spinning it. Do this several times and note the results.
Questions
  • Watch the water stream during the activity. When does it flow?
  • When does it stop flowing?
  • Why does it flow?
  • Why does it stop flowing?
  • Describe the force(s) operating during Part A, 4th and 5th bullets. Illustrate your answer.
  • Orbital motion is a type of free fall. How are the falling bottle and water like an orbiting Space Shuttle and its crew?

Extension

Suspend a small object from a spring scale. Drop the object and scale, and watch what happens to the scale�s needle. If possible, film this process and watch the apparatus fall in slow motion. (The scale shows an object�s weight while it is held, but as soon as it is dropped, the needle returns to zero and stays there for the duration of the fall. During free fall, an object is effectively weightless throughout the fall since its weight cannot pull the scale�s spring.)

Glossary

  • Freefall � occurs when gravity is the only force affecting an object�s motion; for example, when an object is dropped from a resting position, tossed into the air, or is in orbit.
  • Gravity � the attraction between masses. Gravity is one of the four natural forces; it has infinite range and obeys the inverse square law.
  • Weightlessness � a condition in which no object will register any weight on a typical scale. This occurs during any type of free fall, but it does not imply an absence of gravity.