Bouncing Balls (Energy) Demo

 

                We have a basketball and a superball.  How high will they bounce if I drop them on the floor?  (almost as high as they were dropped from).  (bounce balls at a few different heights).  Notice when I drop one on the ground, it comes back up almost as high as where I dropped it from.  When physicists talk about moving objects, we often talk about energy.  There are different kinds of energy.  One kind is called kinetic energy.  Anything that is moving has kinetic energy, and the faster it is moving, the more kinetic energy it has.  Another kind of energy is potential energy.  We measure potential energy of an object as its height above the ground.  Why do we say a ball being held up has potential energy?  If the ball was dropped, gravity would accelerate it to give it kinetic energy.  So by letting go of the ball, I could change potential energy into kinetic energy.  So the question is:  why does it work that the ball always bounces back up almost to the height it was dropped from.  An important rule that physicists discovered by doing lots of experiments is that energy is conserved, which means that you can't create or destroy energy, but you can change it to different forms of energy.  So let's go through this step by step.  (go through the motions slowly with the ball)  When the ball is held up, it has a lot of potential energy and no kinetic energy.  As it falls, it starts losing it's potential energy and speeds up to get more kinetic energy.  When it hits the floor it has no potential energy, but lots of kinetic energy.  Another interesting thing happens when the ball hits the floor.  Remember that the ball bounces back up to a height lower than it started, so after one bounce it has less potential energy than it started with.  Because energy is conserved, some of that energy must have gone somewhere else.  Can anyone guess what happened to that energy?  (friction)  When the ball hits the ground, it gets squished, which causes friction between the different rubber molecules that make up the ball, and the friction heats the ball up.  In terms of energy, we would say because friction slows it down the ball, it has lost some kinetic energy, and because it has heated up, it has gained some heat or thermal energy.  Thermal energy is just another form of energy that has to do with temperature. The hotter something is, the more thermal energy it has.  So when we see the ball bounce back up almost to its original height, we know that it is always a little bit lower than it started because it has transformed some of that original potential energy to thermal energy.

 

                Let's try a fun experiment.  What happens if I drop one ball on top of the other? (show position, with the superball on top of the basketball).  (Drop balls).  The superball went a lot higher than the height we dropped it from!  It ended up with more potential energy than it started with.  Why did that happen?  (the basketball pushed it / gave it energy).  When the basketball hits the ground it gets pushed back up, and then the superball collides with the basketball.  The basketball pushes the superball away against the basketball that is already moving up, so it's like the superball got the energy from both of the pushes off the ground, making it go a lot faster and higher.  When the superball was pushed by the basketball, it must have taken some of the basketball's kinetic energy.  So here we saw an example of one object giving energy to another object.  Something seems a little strange though.  If the basketball gave some of its energy to the superball, why did it bounce to about the same height as before?  It's because the weight or mass of an object has a lot to do with most how much energy it has.  If the basketball and superball are moving at the same speed, the basketball has a lot more energy than the superball because it has a lot more mass.  This should make sense to us because if you try to throw both balls across the room, it takes a lot more of your energy to throw the basketball than the superball.  So when the basketball gave energy to the superball in our last experiment, the basketball only lost a little bit of speed for the same amount of energy that it took to give the superball a lot of extra speed.

Now I want to try the same experiment, but switch the positions of the balls, so now the basketball is on top of the superball (show position).  Is the basketball going to bounce really high this time?  (have a yes/no vote, then drop the balls).  So the basketball didn't bounce any higher this time.  When the balls collided, the superball didnít even have enough momentum to push the basketball back up, it just slowed the basketball down a little bit before it could hit the ground, so the basketball didnít even bounce up to the height it was dropped from.

 

                Now we know that energy is conserved, it can change to different forms of energy, and that it can be traded between objects.  Has anyone ever wondered how we get the energy to run lights and all the other electrical appliances at home?  They all use the energy from electricity that comes from power plants, but where does that energy come from?  There are lots of different kinds of power plants that use different kinds of energy, but we'll talk about the most common.  Most power plants have big turbines that spin around to change kinetic energy into electrical energy.  Right here I have a hand crank that's connected to a light bulb.  If I turn the crank, I can light up the light bulb.  This is an example of how to make electricity from kinetic energy.  But the big turbines in power plants are too big to spin up by hand.  These power plants burn coal, oil, or gas to heat up water and make a lot of steam.  This steam rises and turns the big fan blades of the turbine, so this process turns thermal energy into kinetic energy.  It doesn't stop there.  Coal, oil, and gas are sometimes called fossil fuels, because they come from the fossils of plants and animals that lived thousands of years ago.  Animals eat plants to get their energy, and the plants get their energy by absorbing sunlight.  So that energy you're using to light up your house came from the sun thousands of years ago, and it was changed into many other types of energy to get to your house.

 

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