"Look, Dad!" Lila said, waving a flyer.
"The amusement park is having a roller coaster design contest.
First prize is two tickets for the first ride on the new coaster!"
Dad leaned back and pretended he was riding a roller coaster.
"I love when I'm pushed back in the seat."
"It only feels like you're being pushed back.
The seat is really pushing you forward."
Dad laughed. "You sure know your stuff!"
Lila started drawing ideas.
First she drew a lift hill.
She knew that roller coasters didn't have motors.
The cars gained speed as gravity pulled them down from the top of the lift hill.
This momentum would carry the cars through the rest of the roller coaster.
"My hill looks too short," Lila thought.
"The cars won't have enough momentum to go through loops or over hills."
She made the hill a bit taller.
If she made it too tall, the cars would gain too much speed.
Then the ride would be uncomfortable or dangerous.
After the lift hill, Lila added loops, hills, and curves.
"Yes!" Lila declared.
She was almost ready to build a model!
As Dad drove to the park, Lila practiced explaining her model.
"Gravity pulls you down at the top of the loop, but the momentum keeps pushing you forward."
"That gives you the weightless feeling?" Dad asked.
"Exactly."
Dad parked.
As Lila started to get out of the car, the box slipped from her hands.
Lila gasped as her model crashed onto the pavement.
"It's ruined! What am I going to do? Judging starts soon!"
Dad helped her pick up the pieces.
"It's not too bad. You'll think of something."
In the waiting room, Lila examined the other contestants' designs.
Some wouldn't work, but others were good.
She stared at the pieces of her model in the box.
"Hey!" she thought suddenly.
"Maybe I can put the pieces together as I explain my model!"
Soon it was Lila's turn. In the judging room, Lila put the lift hill on the table.
"A cable pulls the cars to the top."
Next she put the loop on the table.
"From the lift hill, the cars should have enough speed to go through this loop."
"Why isn't the loop round?" judge number one asked.
"A round loop makes the cars go the same speed all around.
This teardrop shape slows the speed on the sides of the loop to make the ride more comfortable."
The judges nodded.
Lila's plan was working!
But as she took the last piece from the box, her heart sank.
Part of the track was twisted upside down!
"Interesting," judge number two said.
"This isn't how I designed it," Lila admitted.
"I dropped the model in the parking lot."
"Do you think it would work on a real coaster?" judge number one asked.
"Hmm. I think the momentum coming out of the loop would carry the cars through the twist."
Lila sat nervously in the waiting room.
"I couldn't answer the twist question."
"I'm sure you did well," Dad assured her.
Finally the judges called everyone back in.
"The winner is Lila King!" judge number one announced.
"For her good model and her great knowledge of roller coaster physics!"
Judge number two shook Lila's hand and handed her the tickets.
"My company has a summer camp for future engineers," he said.
"You should think about attending."
Lila grinned. "I'll definitely look into it—after I ride the new coaster!"