Projects‎ > ‎(1) Discrepant Events‎ > ‎

Motion - Happy & Sad Balls (Joy Burton)


Joy Burton, Science Teacher @ Sutter Middle School

Principle(s) Illustrated

  1. Density
  2. Structure-property relationship of polymers
  3. Effects of heat upon the molecular behavior of objects
  4. Energy: Potential & Kinetic
  5. Conservation of Energy in relation to bouncing balls
  6. Transfer of Energy
  7. Coefficient of friction and restitution
  8. Third Law of Motion

Conservation of Energy and Momentum

2. The laws of conservation of energy and momentum provide a way to predict and describe the movement of objects.

Motion and Forces

1. Newton’s laws predict the motion of most objects. As a basis for understanding this concept:

  1. a.  Students know how to solve problems that involve constant speed and average speed.

  2. b.  Students know that when forces are balanced, no acceleration occurs; thus an object continues to move at a constant speed or stays at rest (Newton’s first law).

  1. d.  Students know that when one object exerts a force on a second object, the second object always exerts a force of equal magnitude and in the opposite direction (Newton’s third law).

  2. e.  Students know the relationship between the universal law of gravitation and the effect of gravity on an object at the surface of Earth.

  3. f.  Students know applying a force to an object perpendicular to the direction of its motion causes the object to change direction but not speed (e.g., Earth’s gravita­ tional force causes a satellite in a circular orbit to change direction but not speed). 

Structure of Matter

3. Each of the more than 100 elements of matter has distinct properties and a distinct atomic structure. All forms of matter are composed of one or more of the elements. As a basis for understanding this concept:

  1. b. Students know that compounds are formed by combining two or more different elements and that compounds have properties that are different from their constituent elements.

  2. c. Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chain polymers.

    d. Students know the states of matter (solid, liquid, gas) depend on molecular motion. 

Investigation and Experimentation

7. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:

  1. a. Select and use appropriate tools and technology ...... to perform tests, collect data, and display data. 

  1. b. Use a variety of print and electronic resources (including the World Wide Web) to collect information and evidence as part of a research project.

  2. c. Communicate the logical connection among hypotheses, science concepts, tests conducted, data collected, and conclusions drawn from the scientific evidence. 

Background Information

Physics behind a bouncing ball:  Ball at surface has potential energy; as the ball falls through air the potential energy changes to kinetic energy.
  • Formula for Gravitational potential energy is PE = mgh (mass,gravitational acceleration, height)
  • Formula for Kinetic energy is KE=1/2 mv2 (mass in kg, velocity in m/sec2)

Law of Conservation of Energy states that energy is neither gained nor lost, only transferred from one form to another. Potential energy is changed to kinetic energy when the ball starts to fall. Some energy is changed to heat energy and/or sound (thud) energy.

Newton's Third Law of Motion states that for every action there is an equal and opposite reaction. Ball pushes on the floor, floor pushes back on the ball. When the unhappy ball hits the floor the ball does not spring back from the floor, whereas the floor pushes back on the happy ball causing it to rebound.

The rubber balls are made from long chains of polymers stretching on impact. Balls that have a lot of bounce like the Happy Balls have tightly linked polymers, therefore most of the energy is transferred back to motion. Whereas the Sad Ball's molecular structure is more loosely arranged which results in its motion being converted to heat energy. If the Sad Ball's temperature is cooled, heated or warmed, the ball will bounce.  Densities of the balls vary slightly, but essentially the Neoprene ball has a density close to that of water while the Norsorex ball is a little more dense.

"Sad Balls" have a low resilience, tend to absorb both kinetic energy of the bounce; and have a thud sound on impact. Norsorex is good for clothing, sound insulation, seals & gaskets, shock absorbers, body armor. "Happy Balls" are softer, bounces well, and have a high coefficient (good bounce). Neoprene is good for swimwear and wet suits (holds heat).

Happy and Sad Balls behave differently, because of their physical properties. They roll at different speeds, they emit sound waves at different decibels, and they bounce at different heights on different surfaces. When the same force is applied on them their compression is different. Since they have different densities, they sink in different solutions at variable rates.

Questioning Script

Prior knowledge & experience:

Middle school students come to class with little or no science background knowledge and experience regarding density, motion and behavior of substances unknown to them.  Students are familiar with objects, such as basketballs, playground balls, baseballs, and softballs. However, many do not have a connection as to "why" balls bounce, their properties and impacts of gravitational forces on the balls.

Root questions:

                    How will each ball behave upon release and impact?

                    Where did the energy go?

                    Explain how this effect would be different if balls were heated for ten minutes, then dropped.

                    How did the energy compare after they hit the floor?

                    What principles were explored in this activity?

                    What properties of each ball could be measured?

Target response:
  • "Sad Balls" have low resilience, tend to absorb both kinetic energy of the bounce; thud sound on impact.
  • "Happy Balls" have a high coefficient (good bounce); bounce well on impact.
Common Misconceptions:
  • A common misconception about Happy and Sad Balls is the perception that the balls will behave the same, since both balls appear very similar in color, texture and "weight".


The Effect of Temperature on a Bouncing Ball