Elasticity
Zavo Gabriel - Jeff Kalinoski - Jen Singelyn
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Teacher Instructions
Materials that are in your bin:
Superballs
Lacrosse balls
Hackysacks
a stack of copy paper
ziplock sandwich bags
(these we assume you can replenish if necessary)
Materials you or students need to supply:
Eggs
Materials for Part III:
Some ideas are: bubble wrap, sponges, napkins, plastic/wax cups
Activity Outline
Part I: Exploring Elasticity and Collisions
The superball is elastic, because it maintains its original shape. This can be observed by playing with the ball. When the ball is dropped, we see an elastic collision. The ball bounces!
The hacky sack changes shape when it’s pressed on or dropped. This collision is inelastic because the ball deforms, or changes shape. The energy is absorbed during this shape change.
The lacrosse ball, like the superball is elastic and causes an elastic collision. Hopefully the children will be able to predict this before they drop the ball to recognize this.
Part II: Engineering Design Activity: Applying the concepts
The child’s goal is to design something or engineer something from paper that will cause the lacrosse ball or superball to now have an inelastic collision when dropped.
Basically, to do something with the paper so that when the lacrosse ball or superball are dropped they don’t bounce!
There are many possibilities for design here, with the key concept being that the paper structure should somehow deform and change shape, absorbing the energy of the ball.
Part III: Engineering Design Activity: Applying it to the Real World
Now, the child can apply the concepts gathered so far to a situation that is more realistic and meaningful. The idea is to protect the ‘egg man’ during his car crash. Using a variety of materials children can protect the egg by creating an inelastic collision, based on the concepts they’ve gathered from the first two parts of the project.
The Engineering Principles: what the Children will learn
The children will learn the basic concepts of elasticity. They will observe and learn the differences between elastic and inelastic collisions by playing with different balls and creating structures to make an elastic collision inelastic. They will also learn a little bit about energy in order to fully understand these concepts.
Elastic Collision – A collision where total kinetic energy is conserved. Essentially, when two things collide, they bounce off one another. There is no permanent deformation of either item – they spring back to their original shape.
Example: two billiard balls colliding on a pool table.
Another example: a golf club hitting a golf ball
In these examples, you can notice that the balls maintain their original shape.
Inelastic Collision – A collision where total kinetic energy is not conserved. These collisions are often called “hit and stick” collisions, because they often involve the bodies sticking together after they collide. At least one of the bodies will deform and not return to its original shape on its own or possibly ever.
Example: a car crash that is severe enough to damage the car’s body.
In this example, clearly, the car has changed it’s shape, absorbing energy during deformation.
The children will also have a better idea of what engineering is and what engineers do. Engineering can be simplified to mean applied science. They will have the opportunity to apply their science knowledge through the two hands on activities. The children will also have the chance to try something, possibly fail, and try again. Engineering is all about problem solving and seeing what will work the best. We hope that this series of activities will give them some insight into what engineers do.
Specific Instructions:
Separate your class into small groups so that each group will have one superball, one lacrosse ball, and one hackysack. Each group will need a table, desk or some surface to work at.
Part I : Exploring Elasticity and Collisions
Superball & Elastic Collisions
First, have children play with and observe the superball. Have them drop it from different heights and observe that it bounces to almost the original height. Explain that this demonstrates an elastic collision.
Things for you to know and understand about what it means to be elastic that hopefully the children will also grasp:
The superball creates an elastic collision because it is an elastic material. This means that it deforms elastically. This is a materials term that means when it is deformed it returns to its original state. You can demonstrate this by pushing on the ball. When you press on the material, it bounces back to its original shape.
When you drop the ball, because it is elastic, it keeps its shape, losing very little kinetic energy, and is able to bounce.
This brings us to energy. Energy is a difficult concept to grasp, but basically, energy is always conserved, meaning it is never lost. Energy can move from one body to another or change its form, but it is always somewhere. Kinetic energy is the energy of an object in motion. Potential energy is the energy something has when it is not in motion, but ‘wants’ to be.
So, when you hold the ball in the air, it has potential energy, because it ‘wants’ to drop. It wants to drop because gravity is pulling on it. When you let it go, it does drop; the potential energy of the ball in the air becomes the kinetic energy of the falling ball.
When the ball hits the table, or the ground, it does not change its shape and it bounces back up. Its kinetic energy stays that way and keeps it moving. (Actually, it deforms a little when it hits, though it springs back, and some energy is lost because of this.)
When the ball hits the table, it is an elastic collision. Both the table and the ball deform slightly, but because they are elastic, they return to their initial shape. The deformation takes energy from the ball. This is why it doesn’t bounce all the way back up to where it was dropped. The collision isn’t perfectly elastic (it never is), but in our case, it doesn’t matter.
Hacky Sack & Inelastic Collisions
Next, present the hackysack. Ask the students what is different about this ball. Ok, so the size may be slightly different, but what is different about the actual properties? What happens when you press on it like with the superball? What do you think will happen when it’s dropped?
Hopefully students will notice that when you squeeze the hackysack it stays that way. It deforms! It changes shape and doesn’t spring back. This means that the deformation is not elastic. When an object is deformed and remains deformed, it is an inelastic material. A better example is when you dent a car. The material remains dented.
Now, have the students drop the hackysack. You will notice that the ball does not bounce, as students may have predicted. It does not bounce because when it hits the table, it deforms.
The kinetic energy that the hackysack had while falling went into deforming the hacky sack and into the table, rather than making it bounce.
Key concepts for them to understand are that this is inelastic because one of the materials was inelastic (the hacky sack). It was inelastic because it deforms inelastically, meaning that it does not return to its original shape.
Lacrosse Ball
The third ball to have children play with and examine is a lacrosse ball.
Questions to ask…Things you want them to observe…
- Which ball is this more similar to?
- What happens when you press on it? Why?
- What will happen when it drops? Why?
- What kind of collision is this?
Have the student try and squeeze the ball, as done earlier. They will notice that it again maintains its shape, meaning that it is elastic. Hopefully they will predict that the ball and table will have an elastic collision (the ball will bounce). Have the student drop the ball. They will notice that it bounces, but not quite as high as previously. This can be explained because the lacrosse ball is not quite as elastic. Why? Well, there is something about the material and its properties that are different than that of the superball. Even so the collision is elastic.
Part II: Engineering Design Activity: Applying the concepts
The engineering design task for the children to apply what they have learned about collisions and hopefully also learn about problem solving and structure is to design and create, using paper, a structure that will result in an inelastic collision with the lacrosse ball or superball.
Now reiterate to the children what they have learned. This is done best by asking them questions:
- What does it mean when a ball or object is elastic? Inelastic?
- What is special about an inelastic collision?
Reiterate what they learned so far:
- A ball is elastic if the material is elastic, meaning it returns to its original shape.
- If the ball is inelastic it will deform and keep that changed shape.
- Something that deforms when it is hit creates an inelastic collision
- This is because it absorbs the energy
- The energy the ball has when it’s moving goes into making the object deform
Now tell them that they are going to be engineers for the next 30 minutes.
Their job/goal is to design something or engineer something from paper that will cause the lacrosse ball or superball to now have an inelastic collision when dropped.
So, to simplify: Do something with the paper so that when the lacrosse ball or superball are dropped they don’t bounce!
Some students will simply put a stack of paper on the table and drop the ball. This is ineffective because the paper is just like the table in this structure. Does the paper deform or change shape in any way? No…
If they need help: What can you do to the paper so that it will take the energy away from the moving ball?
The key is that paper structure needs to deform in some way..
What can you do to the paper so that it changes shape when the ball is dropped on it??
If children begin thinking about a pillow, that may help. When they put their head on the pillow, their head sinks. Or, if they drop something on a pillow, it will not bounce. This pillow helps create an inelastic collision by absorbing energy and changing it’s shape. So, how can you try to mimic the pillow? There are actually several ways. Encourage them to try and make a ‘pillow-like’ structure out of paper.
If they have difficulty, crumbling paper into a kind of dome works. This ‘mushes’ down on impact, much like a pillow. The paper will crush, absorbing the energy.
Another idea is to create a kind of pocket, or pillow, and put shredded paper inside.
Either way, having an area of air so that the paper can crush is the key.
Away from the pillow idea, folding paper in a way that will crush or bend when hit with an object will also work. Again, the paper will absorb the energy as it deforms.
Rather than building a structure, some students may try to alter the ball, which is a very creative solution. Wrapping crumpled paper around the ball with also prevent it from bouncing, because the paper will absorb the energy.
There are many more possibilities. Encourage the children, to think about what they know – to think about examples of inelastic collisions, where one object changes shape, absorbing the energy. Have them then figure out why that happens and mimic the concept with a paper structure.
Part III: Engineering Design Activity: Applying it to the Real World
Third, to really apply the concepts of elasticity and how to create a collision and engineer something that does what you want, we have another activity. In this activity, we are moving to horizontal motion, rather than vertical motion, but the same concepts still apply. We have a device that launches an egg against a wall. The child’s goal is to protect the egg, who is like a person in a vehicle, from the effects of the crash. We hope that they will apply the concepts from the previous activity to create something, using various materials that will protect the egg and keep it from breaking.
Having the egg move horizontally and crash into a wall is similar to a car crash, something students can see in the real world, or at least on television. It’s something real, more real than dropping balls on paper structures.
They are also trying to actually protect something, as though they were trying to protect a person in a car crash.
Here’s what you have them do:
Reiterate what they learned by engineering a structure that would create an inelastic collision:
- A ball is elastic if the material is elastic, meaning it returns to its original shape.
- If the ball is inelastic it will deform and keep that changed shape.
- Something that deforms when it is hit creates an inelastic collision
- This is because it absorbs the energy
- The energy the ball has when it’s moving goes into making the object deform
Now, explain that we are going to apply these same concepts, in a horizontal manner.
Their goal: Create an inelastic collision between the wall and the egg.
They can use any supplies they have brought in or that you supply. Hopefully they will recognize that they need to make something that will absorb the energy by deforming when hit.
Some possible ideas: bubble wrap, sponges, napkins, plastic/wax cups.
We recommend putting the egg in a ziplock bag, that the air has been released from to prevent it from creating a mess, should their structure not prevent the egg from breaking.
Let them explore and see what they come up with. Encourage them to think about what worked in the previous activity and apply that. Something with air pockets or something ‘squishy’ that will change shape will absorb energy. These are the concepts they need to grasp to be successful in this project.
To test their designs, put an egg, in a bag, on the track provided. Place their structure in front of the wall and run the test to see if their structure provides an inelastic collision, saving the egg.
Enjoy the activity. We hope that you and the students learn something about elasticity and have fun doing it.
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