Physics

All of us have misconceptions about the way the world works.  Many of these are acquired early in life by inadequate observation and false assumptions, but others are spread by inexact textbooks and movies that do not reflect reality.  For example, movies may show visible laser beams (laser beams are not visible unless reflected by dust or other matter), exploding cars (cars don’t explode on impact), conversations in outer space (sound will not travel in a vacuum), and guns that do not recoil (violation of conservation of momentum).  Most misconceptions can be identified by careful observation and use of critical thinking strategies.  Consider the following misconceptions and how a review of contradictory data is used to identify them.  

misconception: An object at rest has no energy.

• contradictory data: Automobiles at rest can accelerate with no outside force.

• correct explanation: An object at rest has no kinetic energy, but it may have energy in other forms such as thermal, gravitational potential, or, in this case, chemical potential energy in the gasoline. 

• supporting simulation - PHET skate park

misconception: Doubling the speed of an object doubles its kinetic energy.

• contradictory data: The braking distance for a car traveling at 20 miles per hour is 6 meters, while the braking distance at twice the speed (40 miles per hour) is 24 meters, four times as far.

• correct explanation: The kinetic energy of an object increases as the square of the velocity (Ek=1/2 mv2). 

• supporting diagram - stopping distances for vehicles

misconception: Weight is the same thing as mass.

• contradictory data: Astronauts in space are weightless, but still have inertia, a property of mass.

• correct explanation:  Weight is a measure of the force of gravity on mass.  An object can have mass and be weightless if the net force acting upon it is zero. Unfortunately, people confuse the terms and state their “weight” in kilograms (a unit of mass) rather than in newtons (a unit of force).  

• supporting calculator - your weight on different planets

misconception: Acceleration always occurs in the same direction as an object is moving.

• contradictory data:  Newton’s second law states that F=ma, where F represents force,  m represents mass, and a represents acceleration.   The force of friction on tires during braking is opposite the direction of motion, therefore the acceleration must also be in the opposite direction.

• correct explanation:  Acceleration is always in the direction of the net force, which may or may not be in the direction of movement.

• supporting video - centripetal acceleration video

Activity 7.4.1 – Clarifying misconceptions in physics

Note to teacher:  The following is a list of common misconceptions in physics. Give one to each student and instruct them to research  the misconception, explain why it is wrong by reasoning from observed facts and experiences, and provide a correct explanation.  Explanations are found in the answer section in this chapter.

• 1. (example) Electrical charge is used up as it flows through a circuit.  Many analogies are made between plumbing systems and electrical systems, and this may create some confusion if the analogies are taken too far.  In plumbing systems, water is consumed and must be replenished, but in circuits the electrons remain in the wire and cycle when an electromotive force is applied.

  2. Charge flows through circuits at the speed of light.

  3. Red light is most energetic and blue light is least energetic.

  4. We are running out of energy.

  5. Sonic booms occur only the moment when the sound barrier is punctured.

  6. Centrifugal force pushes riders into the walls of turning cars or banking roller coasters.

  7. The sky is blue because it reflects the color of the ocean.

  8. Vacuum cleaners suction (pull) up debris from the carpet.

  9. Warmer objects have more heat than cooler objects.

  10. All rays of sunlight that hit the Earth are parallel.

  11. Electric companies sell electric charge that flows to consumers.

  12. The primary colors of light are red, blue and yellow.

  13. When white light passes through a colored filter, the filter adds color to the light.

  14. Gravity is a strong force.

  15. Infrared light is a kind of heat radiation.

  16. Energy and force are the same thing

  17. Velocity is another word for speed.

  18. If an object has zero velocity it has zero acceleration

  19. The effects of light are instantaneous.

  20. Gamma rays, x-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves are all very different entities.

  21. When two pulses or waves, traveling in opposite directions meet, they bounce off each other.

  22. More massive objects fall faster.

  23. The more mass in a pendulum bob, the faster it swings.

  24. All metals are attracted to a magnet.

As waves move, matter moves along with them.