Waves and Energy

In this lesson, students explore wave properties and how waves transfer energy by observing mechanical waves in water and modeling transverse and longitudinal sound waves in slinky springs.  

• Energy can be transferred in various ways by moving objects or through heat, electric currents, or sound.  
• All waves carry energy from one location to another location, and many waves are caused by a disturbance that travels through a medium (matter that a wave travels through).
• Waves are caused by a disturbance that transfers energy through the medium. In some waves, the disturbance moves perpendicular to the wave itself, while in other waves, the disturbance moves parallel to the direction of the wave.
  
  

• What are some kinds of evidence that indicate when energy has been transferred from one place to another?
• How is energy transferred through waves?
• Why do water waves need a medium?
• How do you know that energy, not matter, is being transferred in the waves?
• How does energy transfer occur in a transverse wave?
• How does energy transfer occur in a longitudinal wave?  
• What is the relationship between vibrating molecules and sound?
• What happens to water molecules after sound waves have passed through?  
  
  

Misconception: As waves move, matter is transferred from one place to another.

Fact: Matter isn’t transferred in waves. Once the energy has been passed on, the molecules return to their original position.

Amplitude:  a measure of the wave’s displacement from its resting position

Sound:  energy that is carried in waves by vibrating molecules

Sound wave:  a pattern of vibrating molecules caused by the movement of sound through a medium

Vibrate:  to move back and forth quickly

Wavelength:  the distance spanned by one cycle of the motion of a wave

Whale Sounds

Whales often leap into the air and then crash back into the water. As they hit the water, they slap their tails and fins on the water’s surface. This behavior is called breaching.

But why do whales do this? That answer has puzzled scientists. It takes a lot of energy to leap into the air. What benefit could they get from this behavior to make it worth the energy?

A team of scientists in Australia wanted to answer this question. They studied 76 groups of humpback whales for more than 200 hours.

They found that breaching was more common when groups of whales were at least 4,000 meters (2.5 miles) apart from each other.

Sound is Energy

The scientists believe the whales breach to communicate over long distances. Crashing into the water produces loud sounds. This is similar to how a drum makes sounds that can be heard from far away.

Understanding how this can happen begins with a basic look at what sound is. Sound is energy that is carried in waves by vibrating molecules. To vibrate means to move back and forth quickly.

When a whale crashes into the water, it transfers kinetic energy from its moving body to the surface of the water. This makes the molecules of water begin to vibrate. They bump into the molecules closest to them. This passes on the energy. Those molecules also begin to vibrate. Then those molecules bump into more particles, and so on. Particles stop vibrating once they have passed on the energy.

Waves Carry Energy

Sound energy moves from one place to another in waves. There are many different kinds of waves. Think of raindrops falling on a still lake. The kinetic energy of the falling rain transfers to the water. This creates water waves. The water waves carry that energy as they ripple outward. Once the ripples die down, the water returns to the same position.

All waves carry energy from one location to another location. Many waves are caused by a disturbance that travels through a medium. A medium is matter that a wave travels through. A whale crashing into the water is one source of a disturbance. Raindrops falling on a lake are another source of a disturbance.

In this lesson, students explore wave properties and how waves transfer energy by observing mechanical waves in water and transverse and longitudinal sound waves in slinky springs. Student teams carry out a three-part investigation into the phenomena of wave energy transfer in which they figure out the relationship between the amount of force applied in the initial disturbance and the resulting amplitude of the wave. Students use the data they gather from their experiments to explain how waves transfer energy, and how their models can be used to explain how sound moves in waves through a medium.