Straw Rockets

How does the bounce height of a ball change with the height from which it’s dropped?

As I explained earlier, this activity looks at the interplay between kinetic and potential energy. However, it puts in a bit of a twist by looking at a system with two parts: a bouncy ball and a straw that will pop off when dropped. Both have potential energy at the start.

That potential is transferred to kinetic energy as they drop and then back to potential energy on the rebound. The fact that the system has two parts leads to some interesting dynamics. When the ball and straw system is held above the ground, it has potential energy. Potential energy depends on the height above the ground, the pull of gravity, and the mass of the object. Though both the straw and the ball are at the same height above the ground and feel the same pull of gravity, their potential energy is different. As they start falling, that potential energy is transferred into kinetic energy. Both the straw and the ball fall at the same rate because they are both being pulled by gravity in the same way. The ball shields the straw from the air resistance that would otherwise slow it down. Kinetic energy depends on the mass and the square of the velocity.

Since the mass stays the same and energy doesn’t go away, as the ball and straw fall, they speed up and gain exactly as much kinetic energy as they lost in potential energy. When they hit the ground they are going at their maximum possible speed. The higher the height from which they dropped, the faster they will be going at the end of their fall.

There are some interesting physics going on when the ball hits. It's not important to the activity but interesting nonetheless, so I’ll talk about it briefly. When the bouncy ball hits the ground, it squishes a little bit. The rubber acts like a bunch of tiny little springs. When it’s compressed, the rubber stores the energy as potential energy in a spring. Just like when you compress a spring and then let it go, that potential energy is turned back into kinetic. When they hit the ground, the straw and ball are going at the same speed but they aren’t in contact. The ball hits the ground and bounces. As it bounces, it hits the straw which is still sort of on its way down. At this point, some of the kinetic energy from the bouncy ball is transferred to the straw during the collision. The straw has significantly less mass than the bouncy ball. Since kinetic energy depends on mass and velocity, and since the straw’s mass is lower, the straw gains a whole lot of velocity and shoots into the air. As it goes, its kinetic energy is slowly changed back into potential energy.

Since it has the potential energy it started with when it was first dropped and the kinetic energy that was added to it from the ball, it has even more energy than when it started. This means it will end up going higher than the initial position from which it was dropped. In the first activity, I talked a bit about how some energy is lost to friction in the form of heat.

In this activity, students will be asked to discuss what types of energy are involved in this system. They will probably say that when the ball hits the ground, some energy is lost to heat. There are few other places that energy is lost in the system. There is a large amount of air resistance, so some energy is lost to that. Without air resistance the straw would fly much higher. Energy is also lost to sound when the ball bounces. That bouncing noise you hear takes energy to create. Students will be asked to talk about all the energy in the system and where it goes so it is important not to forget these “hidden” losses of energy.

• Real world connections:
  • Using what you learned in this lab, rank the following points based on the amount of kinetic energy. Then do the same with potential energy.
  • Based on what was discussed about air resistance, why do people who skydive use parachutes to land?

• Suggestions for drawing, illustrating, presenting content in creative ways:
  • Have students write a rap or song about energy and energy transfer based on what they learned in this activity.
• Engineering and design challenges connected to the content:
  • In this lab, students should learn about the relationship between kinetic and potential energy. Using this knowledge, students can be placed in groups and tasked with creating a marble roller coaster using cut-in half pool noodles/foam insulation. Each team wants to meet the requirements set, and also have the marble stay in the track the entire time.
    • Some of the requirements can include: certain amount of turns, certain amount of loops, limit on how much material they can use, etc.
    • Helpful example: https://www.sciencebuddies.org/science-fair-projects/project-ideas/Phys_p036/physics/marble-roller-coaster-loop-the-loop
      

• MS-PS2-2
  Plan an investigation to provide evidence that the change in an object’s motion depends of the sum of the forces on the object and the mass of the object.
• MS-PS4-3-applications
  CCC: Influence of Science, Engineering, and Technology on Society and the Natural World. Technologies extend the measurement, exploration, modeling, and computational capacity of scientific investigations. (MS-PS4-3)
• MS-PS4-3-nature-of-science
  CCC: Science is a Human Endeavor. Advances in technology influence the progress of science and science has influenced advances in technology. (MS-PS4-3)
• MS-PS4-1-empirical-evidence
  SEPs: Scientific Knowledge is Based on Empirical Evidence. Science knowledge is based upon logical and conceptual connections between evidence and explanations. (MS-PS4-1)

Credits