Module 6—Work and Energy

Big Picture Big Picture

 

A photograph of a man using a hang-glider and leaping off a platform is shown.

© Drazen Vukelic/shutterstock

Have you ever dreamed of soaring in the sky with the birds? It seems like such an effortless way to spend an afternoon.

 

You can’t just jump into the air, though. You need to climb up to the top of the cliff with the necessary equipment, put on all of your equipment, and take a running leap over the edge. When you think about all of this, you realize that there is considerable effort involved before the fun of floating in the air ever begins.

 

A photograph shows a bird in flight.

© Tony Campbell/shutterstock

Just like you, the bird has to work hard to get up in the sky before it gets to use the air currents to keep it aloft.

 

We’ve all heard the saying “what goes up must come down.” But for a while, both the bird and the person on the hang-glider seem to be breaking that rule. For a time, the hang-glider is breaking the law of gravity as he or she rises and circles far above the rocks at the bottom of the cliff. Something must be acting against the force of gravity, allowing the person with the hang-glider to float and maintain motion high above Earth.

 

Three photos are shown. Top left, an image of a bobsled is taken from a bird’s-eye point of view. Bottom left, a ski jumper descends down the jump. The right photo shows a ski-jump competition site.

left, top: © Steve Broer/shutterstock
left, bottom: © Mag. Alban Egger/shutterstock
right: © Wiliam Mahar/shutterstock

It can be fun to work with gravity too. Can you imagine the adrenaline rush you would feel riding down a run on a bobsled at 125 km/h as you twist and turn and bump against the side of the track? Maybe you’d rather try ski jumping. How fast would you be going just at the edge of the jump? It’s just you—and the abyss—and the 100 km/h gained from sliding down the long ramp toward the edge of the jump. Then, you fly. If you’re really good, you might land 140 m away and win an Olympic gold medal.

 

A crane lifts objects at a construction site in this photograph.

© Juan David Ferrando Subero/shutterstock

There are times when you fight against gravity to rise to higher levels. Think about the bobsled you rode down the track at 125 km/h. Would you want to carry your sled with you as you climb back up to the top of the track? In some cases, it’s just not practical to carry objects up by hand. Think about the building materials lifted by the crane shown in the photo. Would you want to carry these building materials to the top of the building?

 

Climbing up a hill or up a flight of stairs is something people can do, but they often use machines instead. When you are in a mall, do you climb the stairs to the second level, or do you take an escalator or elevator? Is it because you really don’t want to walk up the stairs, or is it because you can’t find any stairs to use?

 

A photograph shows a two crowded escalators and a staircase with only one person in use at a busy mall.

left: © Hannamariah/shutterstock
right: © Yanik Chauvin/shutterstock

 

Perhaps you would like to try a sport where falling is balanced with rising, and it feels like some of the effort you put in is returned.

 

A photograph shows a girl in mid-air after jumping on a trampoline.

© sonya etchison/shutterstock

If so, a trampoline would be to your liking. Each time you fall, it feels like you are being shot back up into the air. The trampoline debuted as an Olympic sport in 2000, and the athletes that compete at this level really do look like they are defying gravity as they effortlessly perform twists, turns, and somersaults in midair.

 

As you work through this module, keep the following questions in mind. They will help you understand the relationship between different forms of energy and work.